This section describes techniques based on commercial heaters, either to heat liquids prior to being injected into the microsystem , (pre-heated liquids), or the incorporation of commercial components such as Peltier elements. Depending on the final application it is possible to generate either uniform temperature control or temperature gradients, as outlined in the next subsections.
This section describes techniques based on commercial heaters, either to heat liquids prior to being injected into the microsystem , (pre-heated liquids), or the incorporation of commercial components such as Peltier elements. Depending on the final application it is possible to generate either uniform temperature control or temperature gradients, as outlined in the next subsections.
A number of techniques using pre-heated liquids have been reported for microfluidic devices. These methods utilize microheaters such as Peltier elements to establish either a uniform temperature or a constant gradient in a given region. Velve Casquillas and co-workers [36,37] developed a disposable polydimethylsiloxane (PDMS) based microfluidic device consisting of two Peltier stages controlling the temperature of the liquid flowing through a control channel (Figure 1(a)). The virtue of PDMS is its relatively low thermal conductivity (0.15 W/mK typically), which allows efficient heat transfer from the source towards the liquid (minimizing energy losses) [43]. This integrated system is capable of reversibly switching between 5 °C and 45 °C in less than 10 s (Figure 1(b)). Changing the direction of the liquid flow through either a cold or hot Peltier using a syringe pump changes the temperature of cells located underneath the temperature control channel. To characterize the temperature response of the chamber, a thin platinum resistance (50 nm) was bonded to the microchannel block. As the electrical resistance of platinum changes nearly linearly with temperature, the authors could record the temperature inside the cell channels by measuring the resistance in the wire. The previous example shows the potential to exploit external Peltier elements, typically by positioning these elements underneath a microchip. Maltezos and co-workers [2,3] report the use of a microfluidic thermal heat exchanger to cool a Peltier junction and demonstrate rapid heating and cooling of small volumes of solution (typically 0.4 μL). The microfluidic device is able to perform very fast cycling over a temperature range from 22 to 95 °C. The introduction of four parallel Peltier junctions resulted in ramp rates of about 100 °C/s for heating, and 90 °C/s for cooling. In a nutshell, this simple technique represents a miniaturized PCR-on-a-chip system to amplify DNA fragments. More sophisticated set-ups have been described by Khandurina et al. [4] who have developed a device consisting of a compact thermal cycling assembly based on Peltier elements surrounding a microchip gel electrophoresis platform for rapid PCR based analysis. The temperature ramp rates achieved are typically 20&#;30 °C/s. For amplification, the temperature steps are 94 °C, 50 °C and 72 °C with hold times of 30, 20, 25 s, resulting in ~1.25 min/cycle (Figure 2).
Figure 1. (a) A schematic representation of the control device: temperature is set by an external Peltier element; the yeast channel is placed below the temperature control channel (b) Temperature versus time plot showing a heating rate of 4 °C/s. Reprinted from [37], Copyright , with permission from Elsevier.
Figure 2. Temperature versus time for Polymerase Chain Reaction (PCR) amplification. The gray solid line represents the set-point temperature, the black solid line is the temperature
of the bottom Peltier element and the dashed line is the reaction mix temperature. Reprinted with permission from [4]. Copyright American Chemical Society
Along similar lines, Yang et al. [5] used a serpentine shaped thin (0.75 mm) polycarbonate PCR micro reactor and demonstrated its detection sensitivity and specificity in amplification of the E. coli K12-specific gene fragment. During thermal cycling, the PCR device is sandwiched between two Peltier elements (Figure 3). The authors performed 30 cycles in 30 min and were able to amplify the K12-specific gene from 10 cells in the presence of 2% blood. Peltier surface and intra-chamber temperatures are transduced by thermocouples which regulate the temperature cycles. Heating rates of 7&#;8 °C/s and cooling rates of 5&#;6 °C/s can be achieved using this technique.
Figure 3. A picture of the chip included in the heating-cooling device. Reprinted with permission from [5]. Copyright , Royal Society of Chemistry.
Qiu et al. [11] described a new method to perform PCR diagnostics based on plastic microfluidic reactors with relatively large volumes (10 to 100 μL). The device is a portable thermal cycler combined with a compact detector for real-time PCR, which can quantify the amount of amplified DNA during an experiment. The chip is located between the master thermoelectric element and a thermal plate. The system achieves a temperature ramp rate of approximately 4 °C/s for heating and 6 °C/s for cooling, and the temperature of the liquid in the reaction chamber follows the set-point temperature with an accuracy of ±0.1 °C up to a temperature of 94 °C.
Maltezos et al. [2,3] integrated micro-Peltier junctions of size 0.6 × 0.6 × 1 mm3 into their microfluidic device in order to heat and cool nanoliter fluid volumes. These junctions generate a temperature range from &#;3 °C to 120 °C with an accuracy of about 0.2 °C, and good long-term stability. Temperature rates of 106 °C/s for heating and 89 °C/s for cooling were achieved.
Apart from PCR applications, Liu et al. [72] developed a valving mechanism using paraffin, which undergoes solid-liquid phase transition in response to changes in temperature. As shown in Figure 4, a block of paraffin initially blocks the channel. The paraffin is melted by a heater located directly underneath the chip, and moved downstream by pressure coming from an upstream channel. Once the molten paraffin moves out of the heating zone, it begins to solidify on the wall of a wider channel. The opening of the valve is single use and facilitates transportability in a sealed system. However, the response time of devices mentioned above are of the order of 5&#;10 s, which is relatively high compared to other systems which bring into play pressure controlled on-off valves [73,74].
Mahjoob et al. [7] introduced porous inserts with high temperature conductivity to improve heat transfer by providing a large surface area for a given volume. The system is assembled in three layers: the porous medium is located above an impermeable conductive plate and the microchip is placed underneath this plate. An optimized technique is established based on the effects of several parameters (heat exchanger geometry, conductive plate, porous matrix material used. etc.) on the temperature distribution and the power required to circulate the fluid in the heat exchanger. The heating/cooling ramp of the PCR heat exchanger is equal to 150.82 °C/s, which is considerably higher than results reported elsewhere in the literature.
Figure 4. (a&#;b) Schematic illustrations of a close-open paraffin microvalve design. (c&#;e) An open-close-open microvalve design. (f) A photograph of a PCR chamber surrounded by five paraffin-based microvalves: valves 1&#;3 are open-close valves, and valves 4 and 5 are close-open valves. Reprinted with permission from [72]. Copyright American Chemical Society.
It is also possible to generate temperature gradients using the pre-heated liquids approach as reported by Mao et al. [38]. A linear temperature gradient is generated across dozens of parallel microfluidic channels simultaneously, located in between a hot source and a cold sink separated by a straight wall (Figure 5). The device was manufactured using soft lithographic techniques [39] and its dimensions range from 20 × 7 μm² up to 250 × 7 μm². The linear temperature profile of 5.8 °C/mm depicted in Figure 5 was measured in a microfluidic device composed of eight parallel channels located in between the heating and cooling tubes. A thermocouple is placed at different locations giving rise to the plot presented on Figure 5.
Figure 5. A schematic of the device producing a linear temperature gradient. qx is a representation of the heat flux going from the hot source on the left to the cold one on the right. Reprinted with permission from [38]. Copyright American Chemical Society.
In a similar approach, Matsui and co-workers [16] integrated two Peltier elements to generate a temperature gradient, which can achieve temperature gradients of 13.75 °C/mm across a 4 mm gap. The dimensions of the Peltier elements are 20 mm wide, 40 mm in length and 3.4 mm in height. The authors combine a temperature gradient, an applied electric field and a buffer with a temperature-dependent ionic strength in order to focus analytes by balancing their electrophoretic velocities against the bulk velocity of the buffer containing the analytes (TGF). In 45 s, Oregon Green 488 carboxylic acid is concentrated approximately 30 fold by applying a moderate electrical field of 70 V/cm and a temperature gradient of 13.75 °C/mm across a 4 mm gap.
Finally, the generation of temperature gradients using Peltier elements can be applied to map-out solubility phase diagrams. Laval et al. [35] devised a new microfluidic chip that allows the direct and quantitative reading of two-dimensional solubility diagrams (Figure 6). Firstly, droplets containing a solute with a gradual variation of concentration are stocked on the chip. Crystallization is induced in these droplets by rapid cooling, and finally, a temperature gradient is applied to dissolve crystals in droplets at temperatures higher than their solubility temperature. As a result, they directly sample the solubility boundary between droplets with and without crystals, which gives the solubility temperatures at different concentrations (i.e., 2D-readable system: abscissa with temperature, and ordinate with concentration). The temperature field of the chip is controlled by two Peltier elements located underneath a silicon wafer which forms a chip support to optimize thermal transfers, and generates regular temperature gradients of about 0.7 °C/mm along the storage channels. This original technique is simple and cheap and could potentially be used in high throughput studies, given the small amount of reagents needed (around 250 μL).
Figure 6. (a) Design of the microfluidic device (channel width 500microns). Silicone oil is injected in inlet 1 and aqueous solutions in inlets 2 and 3. The two dotted areas indicate the positions of Peltier modules used to apply temperature gradients. The three lines of dots mark the positions of temperature measurements. (b) Example of directly reading out of a solubility diagram. The dotted line surrounding droplets containing crystals gives an estimation of the solubility limit. Reprinted with permission from [35]. Copyright , Royal Society of Chemistry.
Peltier elements are widely used to create hot/cold zones, and are able to generate a spatial distribution of temperature with impressive accuracy. However, for many techniques, these elements are not considered as an integral part of the microfluidic chip because of their size, which is typically several millimeters. However, methods have been developed to integrate heating or cooling functionalities directly into microfluidic systems. These approaches are presented in the following sections.
We now turn to integrated techniques, from which heat diffuses from/to the integrated heating/cooling source. The first example we present derives from the use of a chemical reaction. In , Guijt et al. [65] made use of endothermic and exothermic processes to locally regulate temperature in a microchannel. This method is fully integrated and cost effective with channels of typical dimensions: 54 μm wide and 19 μm deep. For cooling, the evaporation of acetone (Reagent 1) in the air (Reagent 2) is used as an endothermic process. For heating, the dissolution of 97 wt% H2SO4 (Reagent 1) in water (Reagent 2) is used as an exothermic reaction. The central channel (represented in red on Figure 7) is filled with a solution of 1 μM Rhodamine B in water so that the fluorescence gives a direct measurement of the temperature inside the microchannel. Note that heating experiments were conducted in glass-glass channels whereas cooling trials were carried out in PDMS-glass systems. By tuning the flow rate ratio between the two reagents, the authors demonstrate control over the intensity of the reaction and hence the temperature. This approach can achieve temperatures ranging from &#;3 °C up to 76 °C with ramps about 1 °C/s.
Figure 7. Two reactant channels merging into a temperature control channel, running parallel to the working channel.
This kind of approach was optimized by Maltezos et al. in [66] for cooling. The authors compared a range of different solvents and angles (&#; in the schematic) of the Y-junction, evaporated in a N2 flux. They concluded that the most efficient solvent they tested was di-ethyl ether with an angle of 10°, which offers the possibility to cool down to &#;20°C with a steady state for several minutes. This method is again cheap and clearly suited for microfluidic applications but requires further refinement of the heating control to work efficiently in PDMS channels.
The following section concerns the most widely reported technique in the literature based on Joule heating temperature control approaches [20,21,28&#;30,41&#;46]. The technique relies on a simple physical property of conducting metals or liquids. Whichever technique is used to embed heating resistors in a microfluidic system, a linear relationship can be demonstrated between the dissipated power (given by the applied potential and the resistance of the heater) and the heated flux. A stationary temperature profile (Figure 8) can be achieved either by the addition of a heat sink, or by feedback control requiring the integration of a sensor (this point is critical for all techniques in which power is applied&#;increasing the mean temperature&#;as opposed to imposing a temperature). In addition, due to the small size of the heaters, the required heating power generated is in the range of 1 W by applying only a few Volts.
Figure 8. (a) Calibration curves: plot of the resistance R versus temperature T for the three microheaters. Reprinted from [42], Copyright , with permission from Elsevier. (b) Temperature increase as a function of power supply. Reprinted from [45], with kind permission from Springer Science+Business Media.
Thermal actuation of microfluidic valves by generating a heating pulse has recently been reported. Pitchaimani et al. [75] used a PDMS based microfluidic chip to control fluid flow in microchannels. The authors took advantage of constrained deformation in PDMS to develop a thermally actuated plastic microfluidic valve. The fluid flow is controlled through the deflection of a thin elastomeric film, actuated by a temperature-sensitive fluid located inside the valve. Heaters are manufactured by depositing a 100 nm thick gold film onto a cleaned plastic film by sputtering. Depending on the heater power used, the local channel temperature was 10 to 19 °C above the room temperature, enabling control of flow rates from 0.33 to 4.7 μL/min in a 110 μm wide and 45 μm deep microchannel.
Similarly, Gu et al. [76] used a PDMS based three-layer structure to control the opening/closing of a microchannel (Figure 9). This technique is also applicable to polymethylmethacrylate (PMMA). The valve-containing device can withstand about 700 kPa without delamination, and the PDMS/PMMA bonding strength reaches a plateau when the temperature is higher than 70 °C.
Finally, different temperature profiles may be required: either homogeneous as in PCR applications, or gradient-like for TGF or droplet actuation techniques. In both cases, it may be crucial to perform a temperature profile with the best achievable accuracy, although some applications do not require a sharp control. In order to meet such stringent requirements, different heating techniques and geometries of heaters have been investigated: the use of ionic liquids, in situ fabrication of wires and surface patterning of metal resistors using classical microelectronic techniques. These techniques are summarized in two larger categories: the generation of a homogeneous temperature profile and generation of a temperature gradient.
The next two subsections are dedicated to spatial control of the temperature.
Figure 9. Process flow of bonding a thermoplastic substrate with a polydimethylsiloxane (PDMS) layer (a&#;d), followed by additional steps for valve fabrication (e&#;g). Reprinted with permission from [76]. Copyright American Chemical Society.
To our knowledge, the only reported work using a conductive liquid is from De Mello et al. [41]. The authors present a microfluidic device incorporating working channels (sample) with a serpentine-like geometry and parallel channels (Figure 10) in which ionic liquids are Joule heated with an ac current (up to 3.75 kV, f = 50 Hz and P = 1 W). Consequently, the internal temperature can be easily and directly controlled. Temperature measurements were performed using three thermocouples. The ionic liquids used in this experiment were [BMIM][PF6] and [BMIM][Tf2N]. Devices can be heated rapidly or slowly, depending on the applied voltage, and temperatures ranging from 50 °C to 90 °C can be set to within ±0.2 °C.
Figure 10. Sketch of the device composed of a working channel (depicted in black) together with parallel channels containing the conductive liquid (depicted in gray). Crosses stand for the position of thermocouples. Reprinted with permission from [41]. Copyright , Royal Society of Chemistry.
The serpentine-like geometry was also studied by Lao et al. [44] with integrated platinum heaters and sensors (Figure 11(a)), thermally isolated and digitally feedback controlled allowing a temperature control of ±1 °C and rapid heating/cooling processes: (heating rate of 20 °C/s and cooling rate of 10 °C/s, response time of approximately 5 s). A feedback control, based on a gain scheduling control algorithm, is used to have an improved temperature response inside the chamber. The maximum power required to maintain a 20 μL glycerol solution at 90 °C is 2.2 W. Figure 11(b) shows a good agreement between the chamber temperature and the set point over one cycle, demonstrating a good control of the overshoot.
Figure 11. (a) Integration of platinum heaters (serpentine-like geometry) together with the integration of sensors. (b) Temperature response of the reaction chamber for different fluids, showing the gain scheduling control algorithm efficiency. Reprinted from [44], Copyright , with permission from Elsevier.
Based on the same heater geometry, Mavraki et al. [42] developed a simple microfluidic chip made of Pyralux&#; with a double-sided Cu-clad polyimide (PI) 136 μm thick substrate. PCR, with a fast DNA amplification rate, is performed. The DNA sample flows through the different thermal zones required to perform PCR (denaturation at 95 °C, annealing at 60 °C and extension at 72 °C, see Section 3.2) in a 150 μm wide and 30 μm deep microchannel, completing 25 thermal cycles and resulting in a 225 multiplication factor of DNA. Each thermal zone is about 25 mm × 10 mm. This study shows a characterization of the microheaters used through the resistance versus temperature plot (Figure 8(a)).
Temperature control can be performed using platinum thin layers as heaters and as temperature sensors. Dinca et al. [8] presented a micro PCR reactor device using this type of heater. For the fastest experiment, 32 cycles were successfully carried out in less than 25 min, with temperature ramps of 7.7 °C/s for heating and 6.2 °C/s for cooling. Lien et al. [9] presented an integrated microfluidic system capable of performing RT-PCR (Reverse Transcription of RNA to DNA previously to PCR: 70 °C during 10 min, 48 °C during 1 h and 95 °C during 15 min) processes for multiple simultaneous detections of four major types of aquaculture disease markers. Bloc platinum resistors are chosen as the material for the micro heaters and the temperature sensors, and gold (Au) metallization is used for the electrical connectors of both the micro temperature sensors and the array-type micro heaters (heating rate 20 °C/s and cooling rate 10 °C/s).
Hsieh et al. [12] performed a rational approach by comparing the temperature response for different geometries of microheaters (Figure 12): two-blocks, two-blocks with additional side heaters, and an array with additional side heaters. Experiments show a temperature homogeneity improvement while increasing the number of heating sources for a given spatial region. An interesting matter raised by the authors is the level of accuracy while stating that the temperature is homogeneous on a whole cavity. As shown in Figure 12(c), it is obvious that a sensor placed at different locations (represented by gray lines) returns an average temperature smoothing the fluctuations along the sensor. Hence these experiments underline that stating a homogeneous temperature requires temperature mapping over the whole region of interest.
Figure 12. Infrared images of each microthermal cycler without heat sinks at the denaturing temperature. (a) 2-D temperature profile of the block-type microheaters. (b) 2-D temperature profile of the block-type microheaters with additional side heaters by applying an AC field. (c) 2-D temperature profile of the array-type microheaters with AC units. The dotted line shows the location of the PCR reaction chamber. The dimensions of each block in (a) are 2,900 μm × 6,000 μm, which are divided into grids (100 μm × 100 μm) with a spacing of 100 μm in (c). Reprinted from [12], Copyright , with permission from Elsevier.
The authors went deeper into their study by investigating other geometries such as serpentine-shape and self-compensated array-type heaters [13]. The aim of the study was to improve the temperature uniformity for PCR applications. Indeed, a homogeneous heater pattern cannot lead to a homogeneous temperature due to side effects, where thermal losses are higher than in the central zone of interest. The authors use electron-beam evaporation and standard lift-off processes to pattern thin-film heaters (90 nm Pt/15 nm Ti), a temperature sensor (90 nm Pt/15 nm Ti) and electrical leads (180 nm Au/20 nm Ti). Results show that a regular array gives a better homogeneity than two-blocks or serpentine, however this can be improved by a self-compensation: the heaters placed at the edges are smaller in order to counter-balance the side effects. The authors tested different self-compensations configurations. The self-compensated heaters happen to give the best uniformity on a selective region, with percentages of the uniform area of 90.3, 99.9 and 96.8 % at 94, 55 and 72 °C respectively, within thermal variation of 1 °C. This approach has been valued for PCR amplification by flowing reagents from one region, with a set temperature of 55 °C, to a warmer one (set temperature: 75 °C). The microfluidic system contains three heating regions of different temperatures together with microfluidic channels. The temperature cycling is achieved by making a loop on the three regions. In , Wang et al. [15] designed a microchip based on this principle. As shown in Figure 13, they designed three reaction open chambers (5 mm diameter) connected with microfluidic channels. Underneath, three array-type microheaters (Figure 12(c)) are patterned and delivered a homogeneous temperature profile. The liquid is displaced thanks to peristaltic valves [73] in approximately 2 s. A cycle is performed in 110 s. The main advantage of this method is the ease of temperature calibration and thus its precision.
Figure 13. IR images of the device showing the three different temperature zones (5, 72 and 94 °C). Reprinted from [15], Copyright , with permission from Elsevier.
In the same spirit of shape optimization, Selva et al. [29,45] provided shape optimization of heating resistors in order to generate different temperature profiles. Shape optimization was carried out on the heating resistor shape, coupling two numerical tools: a genetic algorithm (NSGAII) [77,78], and a finite element study of the thermal response of the heaters. The resistors are made of chromium 15 nm thick. The typical heating power required is of the order of hundreds of mW. A 600 μm × 600 μm square region is heated at 49 °C with a transient regime of 2.2 ± 0.1 s to reach an asymptotic state (90% of the asymptotic value is reached in about 1 s, which is much faster than a Peltier heater), see Figure 14(a,b) for which it is clear that side effects have to be compensated by thinner resistors at the Diagnostics , 3 47 edges. The cycling temperature was demonstrated as having good stability over time, provided a heat sink is placed below the cavity (Figure 14(c)).
By patterning the substrate with an optimized resistor, it is possible to generate a homogeneous temperature within a cavity with great accuracy and with short response times (standard deviation below 1 °C and asymptotic regime reached after 2.2 s).
Figure 14. (a) Experimental temperature distribution resulting from the optimized resistor, providing a mean temperature in the cavity of 49 °C; (b) Experimental temperature distribution in the non-optimized case (i.e., with constant-width elements), for a mean temperature into the cavity of 51 °C. (c) Experimental mean temperature versus time for cycles with a 10 s period and an acquisition frequency of 25 Hz. The transient state lasts approximately 2 s. Reprinted from [45], with kind permission from Springer Science+Business Media.
Figure 15. Temperature in the working channel as a function of the squared input voltage. The three insets are IR images illustrating the spatial distribution. Reprinted with permission from [67]. Copyright , American Institute of Physics.
The last reported technique is the integration of metal wires. Wu et al. [67] designed a microheater and also thermal sensor directly by injecting silver paint (or other conductive materials) into a PDMS microchannel. In this study, they use SPI silver paste diluted by SPI thinner (ratio 1:3) followed by an ultrasonic bath treatment. The paste is injected in the channel and then heated to vaporize the solvent in three steps: 60 °C, 100 °C and 150 °C. The calibration curve Resistance vs. Temperature is done with an IR camera and reveals a good spatial homogeneity in the middle of the serpentine. It also shows a good linearity in the 45&#;105 °C range. They achieved a heating rate of 20 °C/s and a steady state error of about ±0.5 °C. With an applied voltage varying from 0.9 to 2.2 V, the authors obtained a temperature from 45 to 110 °C (Figure 15). Moreover, by measuring the resistance of a thinner wire, they could deduce its temperature. Finally, by designing a double serpentine (a large one for heating and a thin one for sensing), they created a microheater and a thermal sensor. Adding air-cooling channels, LabView voltage and air pressure controls (with a PID module), they finally designed a 25 × 25 mm² temperature controller that can be bonded under a micro-chip. One of the advantages of this technique is the low cost of the device.
For given applications (e.g., droplet actuation, Soret effect, TGF, etc.) it is necessary to generate temperature gradients, either in a controlled way (controlled shape of the temperature profile) or not.
In the field of droplet-based microfluidics, a first application is focused on the displacement of a droplet in a capillary (1D geometry). Nguyen et al. [22] presented both theoretical and experimental results of thermocapillary effects of a liquid plug in a long capillary, subject to a transient temperature gradient generated by a resistive heater. The transient temperature gradient spreads in the capillary wall much slower than the droplet itself. Consequently, the plug moves out of the high-gradient region and decelerates. Jiao et al. [23] reported the reciprocating thermocapillary motion of a liquid plug located in a capillary and positioned between two heaters. The model shows the coupling effect between the surface tension driven movement of the plug and the heat transfer in the capillary wall. The temperature gradients, generated by the two heaters, cause a liquid motion. Finally, Shen et al. [14] investigated the physical mechanisms affecting migration of droplets due to thermocapillarity. A constant thermal gradient (up to 4.21 °C/mm) is generated by powering a metal heater stripe at one edge of the chamber, and cooling at the opposite edge by circulating coolant through a brass heat sink. The results of this study shed light on the critical role of mechanical or chemical hysteresis, and highlight the need to minimize power requirements in microfluidic devices.
Figure 16. (a) A schematic view of the microfluidic device (dimensions in μm). (b) Variation of the delay distance d with temperature. Reprinted with permission from [18]. Copyright , Institute of Physics.
Another 1D droplet handling can be performed using the integration of a serpentine-like micro-heater which locally generates a temperature gradient together with a local decrease in the continuous phase viscosity. Considering such an integration in a 1D geometry, it is possible to control the breakup or switching of a droplet arriving in a T-junction as reported by Yap et al. [18,19]. The authors present a thermal control technique for microdroplets at a bifurcation, using an integrated microheater which induces simultaneously thermocapillarity and a reduction in fluidic resistance in one of the branches (Figure 16(a)). Droplet breakup and switching are demonstrated within a temperature range of 25&#;38 °C (Figure 16(b)), which enables dealing with biological samples.
Jiao et al. [20,21] presented a device with four integrated heaters providing temperature gradients for droplet-based microfluidic systems (Figure 17). The heaters are structured on a glass wafer of a 10 mm × 10 mm square region and are made of thin-film titanium and platinum. The maximum heating power of each heater is equal to 0.5 W.
Figure 17. Top and lateral views of the device showing four heaters placed along a square, generating temperature gradients, for which both shape and magnitude influence the heating power of each single heater. Reprinted with permission from [21]. Copyright , Institute of Physics.
In such a configuration, it is possible to drive droplets by imposing a succession of different temperature gradients along the 2D substrate. The four microheaters actuated independently generate variable surface tension gradients. The droplet can be positioned anywhere in the channel depending on the strength of individual heaters (Figure 18).
At a more integrated level, Darhuber et al. [24&#;27] developed a system with thin Ti metallic microheaters (thickness 100 nm, length 3 mm and 0.8 mm width, and 500 nm SiO2 layer deposited for electrical heaters isolation ) coupled with a chemical patterned glass substrate and electronic actuation. The typical range of applied power for a single microheater is 5&#;200 W (maximum output voltage 10 V; maximum output current 90 mA). Based on thermocapillary actuation, they controlled, with a great accuracy, the formation, 2D displacement, coalescence and break-up of droplet on demand [27] (Figure 19). The initial volume of liquid is 3&#;16 μL.
Selva et al. [29] also reported shape optimization on resistors (chromium 15 nm thick, connected by gold wires 150 nm thick) to generate a linear temperature profile, as sketched in Figure 20. Applying a power ranging from 200 to 500 mW, an intense temperature gradient (up to 11 °C/mm with a standard deviation of approximately 1%) is generated (Figure 20). The transient regime of application of the gradient lasts about 250 ms.
Figure 18. Succession of droplet positions by spatially varying the temperature gradient in time, (duration 80 s). Reprinted with permission from [21]. Copyright , Institute of Physics.
Figure 19. (a&#;e) Thermally induced splitting of a dodecane drop on a partially wetting stripe (w = 1,000 μm). The voltage applied to the microheater (155 Ω) was 2.5 V. The images were recorded at t = 0, 6.0, 7.5, 8.0, and 8.5 s. (f&#;i) Dodecane drop propelled through an intersection outlined by the dark gray pattern (w = 1,000 μm, time lapse 104 s). (j&#;l) Dodecane drop turning a 90° corner (time lapse 164 s). Reprinted with permission from [24]. Copyright , American Institute of Physics.
Figure 20. Top view of an output resistor geometry obtained performing shape optimization, made of chromium resistors (in gray) and gold connectors (in yellow). The experimental temperature profile along the cavity shows a linear dependence of the temperature with the x-axis.
Using this resistor pattern, another phenomenon has been emphasized by Selva et al. [28]: thermomechanical effects due to PDMS dilation with increasing temperature. The authors present studies of pancake-like shaped bubbles in a Hele-Shaw cell, submitted to a temperature gradient [29]. Under such a confinement, there are mainly two competing mechanisms arising from the temperature gradient: thermocapillarity, and the thermal dilation of the PDMS cavity (Figure 21(a)). A theoretical model predicts the cavity dilation to be the dominant effect, which happens to be in excellent agreement with experimental results, inducing a bubble motion toward the cold region of the cavity. According to this study, Selva et al. [30] report a method for bubble and droplet displacement, switching (Figure 21(b)) and trapping based on a thermomechanical effect. This technique presents a high level of integration with low applied voltage (~10 V) and low power consumption (<0.4 W). This work clearly highlights for the first time competing phenomena involved in microfluidics when changing the temperature.
Figure 21. (a) Sketch of the competition between thermocapillary and thermomechanical effects on bubble migration. Reprinted with permission from [28]; Copyright , American Institute of Physics. (b) Images of a 300 μm diameter bubbles inside a switching device: (left) without actuation, and (right) with a 4 °C/mm temperature gradient. Reprinted with permission from [30]. Copyright , Royal Society of Chemistry.
In order to generate a temperature gradient, copper blocks can also be integrated within a microsystem. Ross et al. [17] described such a system in which a temperature gradient is generated for TGF purposes (see Section 5). The device consists of two copper blocks set to different temperatures in order to generate a temperature gradient across a 2 mm gap microfluidic channel (Figure 22). The Diagnostics , 3 52 system is based on TGF, where temperature gradients of 25 °C/mm are produced by thermally anchoring a thin polycarbonate microchannel chip to alternately heated or cooled copper blocks. The technique is demonstrated for a large variety of analytes (fluorescent dyes, amino acids, DNA, proteins, etc.) and is capable of more than -fold concentration of a dilute analyte.
Figure 22. Schematic drawing of the Temperature Gradient Focusing apparatus. Reprinted with permission from [46]. Copyright American Chemical Society.
An interesting technique of embedded heaters is reported by Vigolo et al. [43]. The authors used a silver-filled epoxy (Epo-Tek_H20S, Epoxy Technology) that can be injected and solidified in a microfluidic chip, in parallel channels geometry. Applying an input current, both sides of a microchannel were heated by Joule effect. Depending on the geometry of the channels, either the control of a temperature gradient (Figure 23(a)) or the maintenance of a constant temperature (Figure 23(b)) can be achieved. This approach presents a fully embedded technique to control temperature, and permits working continuously from 25 °C to 75 °C in a PDMS based microfluidic (accuracy ±2&#;3 °C). In the transient regime, the temperature increases within 10&#;20 s and reaches a stable value in less than one minute. A thermocouple in contact with a thin glass cover slip was used to measure the temperature. Authors could finally obtain the temperature of the strip by taking into account the thermal conductivity, thickness and cross-sectional area of the glass slide.
Figure 23. (a) Plot of the temperature along the channel surrounded by channels filled with an epoxy. (b) A schematic view of a device that is able to create a constant temperature. Reprinted with permission from [43]. Copyright , Royal Society of Chemistry.
This technique can be combined with the pre-heated liquid technique as reported by Vigolo et al. [79] for thermophoresis studies (see Section 5). The authors describe a method for selective driving of particles towards either the hot or the cold side by adding specific electrolytes to their initial solution. The authors used a microfluidic device where temperature gradients were established by combining pre-heated liquid or epoxy resistors on either sides of the microchannel. Experiments bring into play the use of polystyrene beads of 477 nm in diameter in the presence of 100 mM NaCl with a flow rate of 0.01 μL/min, and show the accumulation of particles on the cold side by fluorescence measurements. Temperature gradient can also be used to generate natural convection for mixing purposes. Rapid and homogeneous mixing is difficult to achieve in microscale. Indeed, even if diffusion processes are favored in miniature fluidic systems, a pure diffusion-based mixing can be very inefficient, especially in solutions where macromolecules have a diffusion coefficient several orders of magnitude lower than that of most liquids. However, micromixing in chambers remains challenging even though many in-line micromixers have been developed and successfully demonstrated [32,34]. Kim et al. [33] presented an effective technique that enables micromixing in a microfluidic chamber without using a pump. By using natural convection in conjunction with alternating heating of two heaters (Figure 24), efficient micromixing is achieved. Heaters are made in a Ti/Pt alloy formed by a lift-off process, whose dimensions are typically 20 nm/100 nm in thickness. Fluorescent microbeads of 8-μm diameter were used as flow tracers to measure the flow speed at steady state. Standard deviation was used to determine the degree of mixing in the chamber, where I is the normalized intensity of each pixel.
A concentration gradient Pilot Pack has been assembled using Elveflow instruments and could be used to create temperature gradients.
Figure 24. Natural convection-driven flows in a chamber. (a&#;d) Flow trajectories of fluorescent microparticles taken for 35 s. Measured maximum temperatures, Tmax, in (a) to
(d) are 52, 51, 46, and 50 °C, respectively. The yellow arrows indicate the gravity direction, and the white arrows depict the flow direction of the individual fluorescent particles of 8 μm
diameter. Scale bar, 1 mm. Reprinted with permission from [33]. Copyright American Chemical Society.
MHI MC-GAXP-30 Spiral Microheater C (OD=D2= 1&#;) -Maximum temperature of element.
Ceramic Base Included with the MC-GAXP-30. Note: the one and two-inch microheaters do not have a ceramic recess. The picture shown is generic.
Approximately 250 Watts [10V @ 27A (RMS)].
D1 and D2 are separated by 0.1&#; T is 0.25,&#; and H is 2.5&#;. D1 is the OD.
** Power reported under free radiating/ no-load condition with heater resting on the provided ceramic plate. Horizontal.
Connectors comprise One Set of Appropriately rated connecting screw clips.
Note that limits cannot be exceeded on current, voltage, and element temperature.
Note that the current will be different if the heater is covered (insulated).
For more information, please visit our GAXP Spiral Microheaters page
For controls, choose either
Feedback controls are BPAN-O-PLUS-120T and Transformer.
For customers who do not have a 120V wall supply and purchase the power controller, a 220V to 120V transformer will be added at no extra cost when the BPAN control system is purchased.
or the
Open Controls with no feedback comprise the DC Power Supply and a TC Read. Please also choose the T/C and TC extension wire.
For more information on the BPAN-O-PLUS controller, please click on the Power and Temperature Control Panel Pages.
The Alumina ceramic is rated to C. Leads should be able to carry the current, e.g., 10 AWG wire.
*Packaging & Handling fee shown is for one (1) Spiral Microheater additional heaters will increase pricing.
*The weight will be calculated with the control panel option when checking out.
*The correct weight and shipping price will be shown in the final acknowledgment from MHI.
Note that the maximum temperature is at the center of the spiral region unless asymmetric power delivery is required
In the diagram in the specifications tab D1 and D2 are separated by 0.1&#; T is 0.25&#; and H is 2.5&#;. D1 is the OD.
For obtaining maximum temperature MHI controls are highly recommended. Large diameter GAXP® microheaters could develop some springiness from induced magnetic forces. Potting in ceramic paste is allowed as long as care is taken to factor-in the loss of any thermocouple response in the measured temperature. Note element temperature will be higher and maximum element temperature should not be exceeded. In the free radiation mode the power can be maximized.
The objective of the use for free radiating devices is to maximize the radiative POWER transfer from the heating elements. If the heating element is covered with insulation, the objective of the user changes to obtain a certain temperature inside the insulated region. The control of temperature inside a chamber, or power maximization in a free radiating mode can be made with MHI temperature and/or power controllers. When covered the power and current will be lower and the element temperature can be exceeded quickly if not monitored and controlled. For either objective, one cannot exceed the rated temperature or maximum current of the heating element. Power output will be lower in enclosed radiation mode with a temperature control feature as opposed to or in addition to the power control.
In the diagram D1 and D2 are separated by 0.1&#; T is 0.25&#; and H is 2.5&#;. D1 is the OD=1&#;.
There is no hole for a TC per se, however the ceramic is easily bored at a point of your choosing and for your T/C bead diameter. Please note that the only correct way to measure temperature for such a configuration is with an optical pyrometer. One will be able to calibrate the current with the temperature or power output. A current (amps) vs pyrometric measured temperature is provided below &#; when a TC is placed at any position and the current is monitored &#; one can then calibrate the TC reading (at the t/c placement location) with the actual pyrometric temperature &#; within reasonable error. The curve below should be use cautiously (only for general guidance) as it pertains to free radiating conditions only. Note that very often the TC reading that you measure will show a number well below the actual pyrometric temperature. Calibration is required when using t/C.
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&#;We have used the heater, it is working well. Do you have calibration chart (Temperature v/s Voltage) for this MC170 microheater. &#;&#;&#;&#;&#;..Since we do not have temperature measuring tool for such a high temperature (C)&#;&#;&#;&#;..We are planing to order one more heater&#;&#;&#;&#;&#;&#;&#;.&#;
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MHI &#;&#;. the only heating elements that work well for us in these studies; when we publish our results &#; I will be sure to send you the reference, I see your on your website that you link to work utilizing MHI products.
Product: THM and Robust Radiator..
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Product: RR and Furnace
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&#;I wanted to let you know that we were able to connect the heaters successfully with the help of your video. &#;The heaters are heating up like they should as far as we can tell&#;&#; &#;
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&#;&#;.. , I just received the PH-G4-3.5 I ordered and it looks great. &#;..we&#;d like to purchase a controller for it as well.&#;
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A number of techniques using pre-heated liquids have been reported for microfluidic devices. These methods utilize microheaters such as Peltier elements to establish either a uniform temperature or a constant gradient in a given region. Velve Casquillas and co-workers [36,37] developed a disposable polydimethylsiloxane (PDMS) based microfluidic device consisting of two Peltier stages controlling the temperature of the liquid flowing through a control channel (Figure 1(a)). The virtue of PDMS is its relatively low thermal conductivity (0.15 W/mK typically), which allows efficient heat transfer from the source towards the liquid (minimizing energy losses) [43]. This integrated system is capable of reversibly switching between 5 °C and 45 °C in less than 10 s (Figure 1(b)). Changing the direction of the liquid flow through either a cold or hot Peltier using a syringe pump changes the temperature of cells located underneath the temperature control channel. To characterize the temperature response of the chamber, a thin platinum resistance (50 nm) was bonded to the microchannel block. As the electrical resistance of platinum changes nearly linearly with temperature, the authors could record the temperature inside the cell channels by measuring the resistance in the wire. The previous example shows the potential to exploit external Peltier elements, typically by positioning these elements underneath a microchip. Maltezos and co-workers [2,3] report the use of a microfluidic thermal heat exchanger to cool a Peltier junction and demonstrate rapid heating and cooling of small volumes of solution (typically 0.4 μL). The microfluidic device is able to perform very fast cycling over a temperature range from 22 to 95 °C. The introduction of four parallel Peltier junctions resulted in ramp rates of about 100 °C/s for heating, and 90 °C/s for cooling. In a nutshell, this simple technique represents a miniaturized PCR-on-a-chip system to amplify DNA fragments. More sophisticated set-ups have been described by Khandurina et al. [4] who have developed a device consisting of a compact thermal cycling assembly based on Peltier elements surrounding a microchip gel electrophoresis platform for rapid PCR based analysis. The temperature ramp rates achieved are typically 20&#;30 °C/s. For amplification, the temperature steps are 94 °C, 50 °C and 72 °C with hold times of 30, 20, 25 s, resulting in ~1.25 min/cycle (Figure 2).
Figure 1. (a) A schematic representation of the control device: temperature is set by an external Peltier element; the yeast channel is placed below the temperature control channel (b) Temperature versus time plot showing a heating rate of 4 °C/s. Reprinted from [37], Copyright , with permission from Elsevier.
Figure 2. Temperature versus time for Polymerase Chain Reaction (PCR) amplification. The gray solid line represents the set-point temperature, the black solid line is the temperature
of the bottom Peltier element and the dashed line is the reaction mix temperature. Reprinted with permission from [4]. Copyright American Chemical Society
Along similar lines, Yang et al. [5] used a serpentine shaped thin (0.75 mm) polycarbonate PCR micro reactor and demonstrated its detection sensitivity and specificity in amplification of the E. coli K12-specific gene fragment. During thermal cycling, the PCR device is sandwiched between two Peltier elements (Figure 3). The authors performed 30 cycles in 30 min and were able to amplify the K12-specific gene from 10 cells in the presence of 2% blood. Peltier surface and intra-chamber temperatures are transduced by thermocouples which regulate the temperature cycles. Heating rates of 7&#;8 °C/s and cooling rates of 5&#;6 °C/s can be achieved using this technique.
Figure 3. A picture of the chip included in the heating-cooling device. Reprinted with permission from [5]. Copyright , Royal Society of Chemistry.
Qiu et al. [11] described a new method to perform PCR diagnostics based on plastic microfluidic reactors with relatively large volumes (10 to 100 μL). The device is a portable thermal cycler combined with a compact detector for real-time PCR, which can quantify the amount of amplified DNA during an experiment. The chip is located between the master thermoelectric element and a thermal plate. The system achieves a temperature ramp rate of approximately 4 °C/s for heating and 6 °C/s for cooling, and the temperature of the liquid in the reaction chamber follows the set-point temperature with an accuracy of ±0.1 °C up to a temperature of 94 °C.
Maltezos et al. [2,3] integrated micro-Peltier junctions of size 0.6 × 0.6 × 1 mm3 into their microfluidic device in order to heat and cool nanoliter fluid volumes. These junctions generate a temperature range from &#;3 °C to 120 °C with an accuracy of about 0.2 °C, and good long-term stability. Temperature rates of 106 °C/s for heating and 89 °C/s for cooling were achieved.
Apart from PCR applications, Liu et al. [72] developed a valving mechanism using paraffin, which undergoes solid-liquid phase transition in response to changes in temperature. As shown in Figure 4, a block of paraffin initially blocks the channel. The paraffin is melted by a heater located directly underneath the chip, and moved downstream by pressure coming from an upstream channel. Once the molten paraffin moves out of the heating zone, it begins to solidify on the wall of a wider channel. The opening of the valve is single use and facilitates transportability in a sealed system. However, the response time of devices mentioned above are of the order of 5&#;10 s, which is relatively high compared to other systems which bring into play pressure controlled on-off valves [73,74].
Mahjoob et al. [7] introduced porous inserts with high temperature conductivity to improve heat transfer by providing a large surface area for a given volume. The system is assembled in three layers: the porous medium is located above an impermeable conductive plate and the microchip is placed underneath this plate. An optimized technique is established based on the effects of several parameters (heat exchanger geometry, conductive plate, porous matrix material used. etc.) on the temperature distribution and the power required to circulate the fluid in the heat exchanger. The heating/cooling ramp of the PCR heat exchanger is equal to 150.82 °C/s, which is considerably higher than results reported elsewhere in the literature.
Figure 4. (a&#;b) Schematic illustrations of a close-open paraffin microvalve design. (c&#;e) An open-close-open microvalve design. (f) A photograph of a PCR chamber surrounded by five paraffin-based microvalves: valves 1&#;3 are open-close valves, and valves 4 and 5 are close-open valves. Reprinted with permission from [72]. Copyright American Chemical Society.
It is also possible to generate temperature gradients using the pre-heated liquids approach as reported by Mao et al. [38]. A linear temperature gradient is generated across dozens of parallel microfluidic channels simultaneously, located in between a hot source and a cold sink separated by a straight wall (Figure 5). The device was manufactured using soft lithographic techniques [39] and its dimensions range from 20 × 7 μm² up to 250 × 7 μm². The linear temperature profile of 5.8 °C/mm depicted in Figure 5 was measured in a microfluidic device composed of eight parallel channels located in between the heating and cooling tubes. A thermocouple is placed at different locations giving rise to the plot presented on Figure 5.
Figure 5. A schematic of the device producing a linear temperature gradient. qx is a representation of the heat flux going from the hot source on the left to the cold one on the right. Reprinted with permission from [38]. Copyright American Chemical Society.
In a similar approach, Matsui and co-workers [16] integrated two Peltier elements to generate a temperature gradient, which can achieve temperature gradients of 13.75 °C/mm across a 4 mm gap. The dimensions of the Peltier elements are 20 mm wide, 40 mm in length and 3.4 mm in height. The authors combine a temperature gradient, an applied electric field and a buffer with a temperature-dependent ionic strength in order to focus analytes by balancing their electrophoretic velocities against the bulk velocity of the buffer containing the analytes (TGF). In 45 s, Oregon Green 488 carboxylic acid is concentrated approximately 30 fold by applying a moderate electrical field of 70 V/cm and a temperature gradient of 13.75 °C/mm across a 4 mm gap.
Finally, the generation of temperature gradients using Peltier elements can be applied to map-out solubility phase diagrams. Laval et al. [35] devised a new microfluidic chip that allows the direct and quantitative reading of two-dimensional solubility diagrams (Figure 6). Firstly, droplets containing a solute with a gradual variation of concentration are stocked on the chip. Crystallization is induced in these droplets by rapid cooling, and finally, a temperature gradient is applied to dissolve crystals in droplets at temperatures higher than their solubility temperature. As a result, they directly sample the solubility boundary between droplets with and without crystals, which gives the solubility temperatures at different concentrations (i.e., 2D-readable system: abscissa with temperature, and ordinate with concentration). The temperature field of the chip is controlled by two Peltier elements located underneath a silicon wafer which forms a chip support to optimize thermal transfers, and generates regular temperature gradients of about 0.7 °C/mm along the storage channels. This original technique is simple and cheap and could potentially be used in high throughput studies, given the small amount of reagents needed (around 250 μL).
Figure 6. (a) Design of the microfluidic device (channel width 500microns). Silicone oil is injected in inlet 1 and aqueous solutions in inlets 2 and 3. The two dotted areas indicate the positions of Peltier modules used to apply temperature gradients. The three lines of dots mark the positions of temperature measurements. (b) Example of directly reading out of a solubility diagram. The dotted line surrounding droplets containing crystals gives an estimation of the solubility limit. Reprinted with permission from [35]. Copyright , Royal Society of Chemistry.
Peltier elements are widely used to create hot/cold zones, and are able to generate a spatial distribution of temperature with impressive accuracy. However, for many techniques, these elements are not considered as an integral part of the microfluidic chip because of their size, which is typically several millimeters. However, methods have been developed to integrate heating or cooling functionalities directly into microfluidic systems. These approaches are presented in the following sections.
We now turn to integrated techniques, from which heat diffuses from/to the integrated heating/cooling source. The first example we present derives from the use of a chemical reaction. In , Guijt et al. [65] made use of endothermic and exothermic processes to locally regulate temperature in a microchannel. This method is fully integrated and cost effective with channels of typical dimensions: 54 μm wide and 19 μm deep. For cooling, the evaporation of acetone (Reagent 1) in the air (Reagent 2) is used as an endothermic process. For heating, the dissolution of 97 wt% H2SO4 (Reagent 1) in water (Reagent 2) is used as an exothermic reaction. The central channel (represented in red on Figure 7) is filled with a solution of 1 μM Rhodamine B in water so that the fluorescence gives a direct measurement of the temperature inside the microchannel. Note that heating experiments were conducted in glass-glass channels whereas cooling trials were carried out in PDMS-glass systems. By tuning the flow rate ratio between the two reagents, the authors demonstrate control over the intensity of the reaction and hence the temperature. This approach can achieve temperatures ranging from &#;3 °C up to 76 °C with ramps about 1 °C/s.
Figure 7. Two reactant channels merging into a temperature control channel, running parallel to the working channel.
This kind of approach was optimized by Maltezos et al. in [66] for cooling. The authors compared a range of different solvents and angles (&#; in the schematic) of the Y-junction, evaporated in a N2 flux. They concluded that the most efficient solvent they tested was di-ethyl ether with an angle of 10°, which offers the possibility to cool down to &#;20°C with a steady state for several minutes. This method is again cheap and clearly suited for microfluidic applications but requires further refinement of the heating control to work efficiently in PDMS channels.
The following section concerns the most widely reported technique in the literature based on Joule heating temperature control approaches [20,21,28&#;30,41&#;46]. The technique relies on a simple physical property of conducting metals or liquids. Whichever technique is used to embed heating resistors in a microfluidic system, a linear relationship can be demonstrated between the dissipated power (given by the applied potential and the resistance of the heater) and the heated flux. A stationary temperature profile (Figure 8) can be achieved either by the addition of a heat sink, or by feedback control requiring the integration of a sensor (this point is critical for all techniques in which power is applied&#;increasing the mean temperature&#;as opposed to imposing a temperature). In addition, due to the small size of the heaters, the required heating power generated is in the range of 1 W by applying only a few Volts.
Figure 8. (a) Calibration curves: plot of the resistance R versus temperature T for the three microheaters. Reprinted from [42], Copyright , with permission from Elsevier. (b) Temperature increase as a function of power supply. Reprinted from [45], with kind permission from Springer Science+Business Media.
Thermal actuation of microfluidic valves by generating a heating pulse has recently been reported. Pitchaimani et al. [75] used a PDMS based microfluidic chip to control fluid flow in microchannels. The authors took advantage of constrained deformation in PDMS to develop a thermally actuated plastic microfluidic valve. The fluid flow is controlled through the deflection of a thin elastomeric film, actuated by a temperature-sensitive fluid located inside the valve. Heaters are manufactured by depositing a 100 nm thick gold film onto a cleaned plastic film by sputtering. Depending on the heater power used, the local channel temperature was 10 to 19 °C above the room temperature, enabling control of flow rates from 0.33 to 4.7 μL/min in a 110 μm wide and 45 μm deep microchannel.
Similarly, Gu et al. [76] used a PDMS based three-layer structure to control the opening/closing of a microchannel (Figure 9). This technique is also applicable to polymethylmethacrylate (PMMA). The valve-containing device can withstand about 700 kPa without delamination, and the PDMS/PMMA bonding strength reaches a plateau when the temperature is higher than 70 °C.
Finally, different temperature profiles may be required: either homogeneous as in PCR applications, or gradient-like for TGF or droplet actuation techniques. In both cases, it may be crucial to perform a temperature profile with the best achievable accuracy, although some applications do not require a sharp control. In order to meet such stringent requirements, different heating techniques and geometries of heaters have been investigated: the use of ionic liquids, in situ fabrication of wires and surface patterning of metal resistors using classical microelectronic techniques. These techniques are summarized in two larger categories: the generation of a homogeneous temperature profile and generation of a temperature gradient.
The next two subsections are dedicated to spatial control of the temperature.
Figure 9. Process flow of bonding a thermoplastic substrate with a polydimethylsiloxane (PDMS) layer (a&#;d), followed by additional steps for valve fabrication (e&#;g). Reprinted with permission from [76]. Copyright American Chemical Society.
To our knowledge, the only reported work using a conductive liquid is from De Mello et al. [41]. The authors present a microfluidic device incorporating working channels (sample) with a serpentine-like geometry and parallel channels (Figure 10) in which ionic liquids are Joule heated with an ac current (up to 3.75 kV, f = 50 Hz and P = 1 W). Consequently, the internal temperature can be easily and directly controlled. Temperature measurements were performed using three thermocouples. The ionic liquids used in this experiment were [BMIM][PF6] and [BMIM][Tf2N]. Devices can be heated rapidly or slowly, depending on the applied voltage, and temperatures ranging from 50 °C to 90 °C can be set to within ±0.2 °C.
Figure 10. Sketch of the device composed of a working channel (depicted in black) together with parallel channels containing the conductive liquid (depicted in gray). Crosses stand for the position of thermocouples. Reprinted with permission from [41]. Copyright , Royal Society of Chemistry.
The serpentine-like geometry was also studied by Lao et al. [44] with integrated platinum heaters and sensors (Figure 11(a)), thermally isolated and digitally feedback controlled allowing a temperature control of ±1 °C and rapid heating/cooling processes: (heating rate of 20 °C/s and cooling rate of 10 °C/s, response time of approximately 5 s). A feedback control, based on a gain scheduling control algorithm, is used to have an improved temperature response inside the chamber. The maximum power required to maintain a 20 μL glycerol solution at 90 °C is 2.2 W. Figure 11(b) shows a good agreement between the chamber temperature and the set point over one cycle, demonstrating a good control of the overshoot.
Figure 11. (a) Integration of platinum heaters (serpentine-like geometry) together with the integration of sensors. (b) Temperature response of the reaction chamber for different fluids, showing the gain scheduling control algorithm efficiency. Reprinted from [44], Copyright , with permission from Elsevier.
Based on the same heater geometry, Mavraki et al. [42] developed a simple microfluidic chip made of Pyralux&#; with a double-sided Cu-clad polyimide (PI) 136 μm thick substrate. PCR, with a fast DNA amplification rate, is performed. The DNA sample flows through the different thermal zones required to perform PCR (denaturation at 95 °C, annealing at 60 °C and extension at 72 °C, see Section 3.2) in a 150 μm wide and 30 μm deep microchannel, completing 25 thermal cycles and resulting in a 225 multiplication factor of DNA. Each thermal zone is about 25 mm × 10 mm. This study shows a characterization of the microheaters used through the resistance versus temperature plot (Figure 8(a)).
Temperature control can be performed using platinum thin layers as heaters and as temperature sensors. Dinca et al. [8] presented a micro PCR reactor device using this type of heater. For the fastest experiment, 32 cycles were successfully carried out in less than 25 min, with temperature ramps of 7.7 °C/s for heating and 6.2 °C/s for cooling. Lien et al. [9] presented an integrated microfluidic system capable of performing RT-PCR (Reverse Transcription of RNA to DNA previously to PCR: 70 °C during 10 min, 48 °C during 1 h and 95 °C during 15 min) processes for multiple simultaneous detections of four major types of aquaculture disease markers. Bloc platinum resistors are chosen as the material for the micro heaters and the temperature sensors, and gold (Au) metallization is used for the electrical connectors of both the micro temperature sensors and the array-type micro heaters (heating rate 20 °C/s and cooling rate 10 °C/s).
Hsieh et al. [12] performed a rational approach by comparing the temperature response for different geometries of microheaters (Figure 12): two-blocks, two-blocks with additional side heaters, and an array with additional side heaters. Experiments show a temperature homogeneity improvement while increasing the number of heating sources for a given spatial region. An interesting matter raised by the authors is the level of accuracy while stating that the temperature is homogeneous on a whole cavity. As shown in Figure 12(c), it is obvious that a sensor placed at different locations (represented by gray lines) returns an average temperature smoothing the fluctuations along the sensor. Hence these experiments underline that stating a homogeneous temperature requires temperature mapping over the whole region of interest.
Figure 12. Infrared images of each microthermal cycler without heat sinks at the denaturing temperature. (a) 2-D temperature profile of the block-type microheaters. (b) 2-D temperature profile of the block-type microheaters with additional side heaters by applying an AC field. (c) 2-D temperature profile of the array-type microheaters with AC units. The dotted line shows the location of the PCR reaction chamber. The dimensions of each block in (a) are 2,900 μm × 6,000 μm, which are divided into grids (100 μm × 100 μm) with a spacing of 100 μm in (c). Reprinted from [12], Copyright , with permission from Elsevier.
The authors went deeper into their study by investigating other geometries such as serpentine-shape and self-compensated array-type heaters [13]. The aim of the study was to improve the temperature uniformity for PCR applications. Indeed, a homogeneous heater pattern cannot lead to a homogeneous temperature due to side effects, where thermal losses are higher than in the central zone of interest. The authors use electron-beam evaporation and standard lift-off processes to pattern thin-film heaters (90 nm Pt/15 nm Ti), a temperature sensor (90 nm Pt/15 nm Ti) and electrical leads (180 nm Au/20 nm Ti). Results show that a regular array gives a better homogeneity than two-blocks or serpentine, however this can be improved by a self-compensation: the heaters placed at the edges are smaller in order to counter-balance the side effects. The authors tested different self-compensations configurations. The self-compensated heaters happen to give the best uniformity on a selective region, with percentages of the uniform area of 90.3, 99.9 and 96.8 % at 94, 55 and 72 °C respectively, within thermal variation of 1 °C. This approach has been valued for PCR amplification by flowing reagents from one region, with a set temperature of 55 °C, to a warmer one (set temperature: 75 °C). The microfluidic system contains three heating regions of different temperatures together with microfluidic channels. The temperature cycling is achieved by making a loop on the three regions. In , Wang et al. [15] designed a microchip based on this principle. As shown in Figure 13, they designed three reaction open chambers (5 mm diameter) connected with microfluidic channels. Underneath, three array-type microheaters (Figure 12(c)) are patterned and delivered a homogeneous temperature profile. The liquid is displaced thanks to peristaltic valves [73] in approximately 2 s. A cycle is performed in 110 s. The main advantage of this method is the ease of temperature calibration and thus its precision.
Figure 13. IR images of the device showing the three different temperature zones (5, 72 and 94 °C). Reprinted from [15], Copyright , with permission from Elsevier.
In the same spirit of shape optimization, Selva et al. [29,45] provided shape optimization of heating resistors in order to generate different temperature profiles. Shape optimization was carried out on the heating resistor shape, coupling two numerical tools: a genetic algorithm (NSGAII) [77,78], and a finite element study of the thermal response of the heaters. The resistors are made of chromium 15 nm thick. The typical heating power required is of the order of hundreds of mW. A 600 μm × 600 μm square region is heated at 49 °C with a transient regime of 2.2 ± 0.1 s to reach an asymptotic state (90% of the asymptotic value is reached in about 1 s, which is much faster than a Peltier heater), see Figure 14(a,b) for which it is clear that side effects have to be compensated by thinner resistors at the Diagnostics , 3 47 edges. The cycling temperature was demonstrated as having good stability over time, provided a heat sink is placed below the cavity (Figure 14(c)).
By patterning the substrate with an optimized resistor, it is possible to generate a homogeneous temperature within a cavity with great accuracy and with short response times (standard deviation below 1 °C and asymptotic regime reached after 2.2 s).
Figure 14. (a) Experimental temperature distribution resulting from the optimized resistor, providing a mean temperature in the cavity of 49 °C; (b) Experimental temperature distribution in the non-optimized case (i.e., with constant-width elements), for a mean temperature into the cavity of 51 °C. (c) Experimental mean temperature versus time for cycles with a 10 s period and an acquisition frequency of 25 Hz. The transient state lasts approximately 2 s. Reprinted from [45], with kind permission from Springer Science+Business Media.
Figure 15. Temperature in the working channel as a function of the squared input voltage. The three insets are IR images illustrating the spatial distribution. Reprinted with permission from [67]. Copyright , American Institute of Physics.
The last reported technique is the integration of metal wires. Wu et al. [67] designed a microheater and also thermal sensor directly by injecting silver paint (or other conductive materials) into a PDMS microchannel. In this study, they use SPI silver paste diluted by SPI thinner (ratio 1:3) followed by an ultrasonic bath treatment. The paste is injected in the channel and then heated to vaporize the solvent in three steps: 60 °C, 100 °C and 150 °C. The calibration curve Resistance vs. Temperature is done with an IR camera and reveals a good spatial homogeneity in the middle of the serpentine. It also shows a good linearity in the 45&#;105 °C range. They achieved a heating rate of 20 °C/s and a steady state error of about ±0.5 °C. With an applied voltage varying from 0.9 to 2.2 V, the authors obtained a temperature from 45 to 110 °C (Figure 15). Moreover, by measuring the resistance of a thinner wire, they could deduce its temperature. Finally, by designing a double serpentine (a large one for heating and a thin one for sensing), they created a microheater and a thermal sensor. Adding air-cooling channels, LabView voltage and air pressure controls (with a PID module), they finally designed a 25 × 25 mm² temperature controller that can be bonded under a micro-chip. One of the advantages of this technique is the low cost of the device.
For given applications (e.g., droplet actuation, Soret effect, TGF, etc.) it is necessary to generate temperature gradients, either in a controlled way (controlled shape of the temperature profile) or not.
In the field of droplet-based microfluidics, a first application is focused on the displacement of a droplet in a capillary (1D geometry). Nguyen et al. [22] presented both theoretical and experimental results of thermocapillary effects of a liquid plug in a long capillary, subject to a transient temperature gradient generated by a resistive heater. The transient temperature gradient spreads in the capillary wall much slower than the droplet itself. Consequently, the plug moves out of the high-gradient region and decelerates. Jiao et al. [23] reported the reciprocating thermocapillary motion of a liquid plug located in a capillary and positioned between two heaters. The model shows the coupling effect between the surface tension driven movement of the plug and the heat transfer in the capillary wall. The temperature gradients, generated by the two heaters, cause a liquid motion. Finally, Shen et al. [14] investigated the physical mechanisms affecting migration of droplets due to thermocapillarity. A constant thermal gradient (up to 4.21 °C/mm) is generated by powering a metal heater stripe at one edge of the chamber, and cooling at the opposite edge by circulating coolant through a brass heat sink. The results of this study shed light on the critical role of mechanical or chemical hysteresis, and highlight the need to minimize power requirements in microfluidic devices.
Figure 16. (a) A schematic view of the microfluidic device (dimensions in μm). (b) Variation of the delay distance d with temperature. Reprinted with permission from [18]. Copyright , Institute of Physics.
Another 1D droplet handling can be performed using the integration of a serpentine-like micro-heater which locally generates a temperature gradient together with a local decrease in the continuous phase viscosity. Considering such an integration in a 1D geometry, it is possible to control the breakup or switching of a droplet arriving in a T-junction as reported by Yap et al. [18,19]. The authors present a thermal control technique for microdroplets at a bifurcation, using an integrated microheater which induces simultaneously thermocapillarity and a reduction in fluidic resistance in one of the branches (Figure 16(a)). Droplet breakup and switching are demonstrated within a temperature range of 25&#;38 °C (Figure 16(b)), which enables dealing with biological samples.
Jiao et al. [20,21] presented a device with four integrated heaters providing temperature gradients for droplet-based microfluidic systems (Figure 17). The heaters are structured on a glass wafer of a 10 mm × 10 mm square region and are made of thin-film titanium and platinum. The maximum heating power of each heater is equal to 0.5 W.
Figure 17. Top and lateral views of the device showing four heaters placed along a square, generating temperature gradients, for which both shape and magnitude influence the heating power of each single heater. Reprinted with permission from [21]. Copyright , Institute of Physics.
In such a configuration, it is possible to drive droplets by imposing a succession of different temperature gradients along the 2D substrate. The four microheaters actuated independently generate variable surface tension gradients. The droplet can be positioned anywhere in the channel depending on the strength of individual heaters (Figure 18).
At a more integrated level, Darhuber et al. [24&#;27] developed a system with thin Ti metallic microheaters (thickness 100 nm, length 3 mm and 0.8 mm width, and 500 nm SiO2 layer deposited for electrical heaters isolation ) coupled with a chemical patterned glass substrate and electronic actuation. The typical range of applied power for a single microheater is 5&#;200 W (maximum output voltage 10 V; maximum output current 90 mA). Based on thermocapillary actuation, they controlled, with a great accuracy, the formation, 2D displacement, coalescence and break-up of droplet on demand [27] (Figure 19). The initial volume of liquid is 3&#;16 μL.
Selva et al. [29] also reported shape optimization on resistors (chromium 15 nm thick, connected by gold wires 150 nm thick) to generate a linear temperature profile, as sketched in Figure 20. Applying a power ranging from 200 to 500 mW, an intense temperature gradient (up to 11 °C/mm with a standard deviation of approximately 1%) is generated (Figure 20). The transient regime of application of the gradient lasts about 250 ms.
Figure 18. Succession of droplet positions by spatially varying the temperature gradient in time, (duration 80 s). Reprinted with permission from [21]. Copyright , Institute of Physics.
Figure 19. (a&#;e) Thermally induced splitting of a dodecane drop on a partially wetting stripe (w = 1,000 μm). The voltage applied to the microheater (155 Ω) was 2.5 V. The images were recorded at t = 0, 6.0, 7.5, 8.0, and 8.5 s. (f&#;i) Dodecane drop propelled through an intersection outlined by the dark gray pattern (w = 1,000 μm, time lapse 104 s). (j&#;l) Dodecane drop turning a 90° corner (time lapse 164 s). Reprinted with permission from [24]. Copyright , American Institute of Physics.
Figure 20. Top view of an output resistor geometry obtained performing shape optimization, made of chromium resistors (in gray) and gold connectors (in yellow). The experimental temperature profile along the cavity shows a linear dependence of the temperature with the x-axis.
Using this resistor pattern, another phenomenon has been emphasized by Selva et al. [28]: thermomechanical effects due to PDMS dilation with increasing temperature. The authors present studies of pancake-like shaped bubbles in a Hele-Shaw cell, submitted to a temperature gradient [29]. Under such a confinement, there are mainly two competing mechanisms arising from the temperature gradient: thermocapillarity, and the thermal dilation of the PDMS cavity (Figure 21(a)). A theoretical model predicts the cavity dilation to be the dominant effect, which happens to be in excellent agreement with experimental results, inducing a bubble motion toward the cold region of the cavity. According to this study, Selva et al. [30] report a method for bubble and droplet displacement, switching (Figure 21(b)) and trapping based on a thermomechanical effect. This technique presents a high level of integration with low applied voltage (~10 V) and low power consumption (<0.4 W). This work clearly highlights for the first time competing phenomena involved in microfluidics when changing the temperature.
Figure 21. (a) Sketch of the competition between thermocapillary and thermomechanical effects on bubble migration. Reprinted with permission from [28]; Copyright , American Institute of Physics. (b) Images of a 300 μm diameter bubbles inside a switching device: (left) without actuation, and (right) with a 4 °C/mm temperature gradient. Reprinted with permission from [30]. Copyright , Royal Society of Chemistry.
In order to generate a temperature gradient, copper blocks can also be integrated within a microsystem. Ross et al. [17] described such a system in which a temperature gradient is generated for TGF purposes (see Section 5). The device consists of two copper blocks set to different temperatures in order to generate a temperature gradient across a 2 mm gap microfluidic channel (Figure 22). The Diagnostics , 3 52 system is based on TGF, where temperature gradients of 25 °C/mm are produced by thermally anchoring a thin polycarbonate microchannel chip to alternately heated or cooled copper blocks. The technique is demonstrated for a large variety of analytes (fluorescent dyes, amino acids, DNA, proteins, etc.) and is capable of more than -fold concentration of a dilute analyte.
Figure 22. Schematic drawing of the Temperature Gradient Focusing apparatus. Reprinted with permission from [46]. Copyright American Chemical Society.
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An interesting technique of embedded heaters is reported by Vigolo et al. [43]. The authors used a silver-filled epoxy (Epo-Tek_H20S, Epoxy Technology) that can be injected and solidified in a microfluidic chip, in parallel channels geometry. Applying an input current, both sides of a microchannel were heated by Joule effect. Depending on the geometry of the channels, either the control of a temperature gradient (Figure 23(a)) or the maintenance of a constant temperature (Figure 23(b)) can be achieved. This approach presents a fully embedded technique to control temperature, and permits working continuously from 25 °C to 75 °C in a PDMS based microfluidic (accuracy ±2&#;3 °C). In the transient regime, the temperature increases within 10&#;20 s and reaches a stable value in less than one minute. A thermocouple in contact with a thin glass cover slip was used to measure the temperature. Authors could finally obtain the temperature of the strip by taking into account the thermal conductivity, thickness and cross-sectional area of the glass slide.
Figure 23. (a) Plot of the temperature along the channel surrounded by channels filled with an epoxy. (b) A schematic view of a device that is able to create a constant temperature. Reprinted with permission from [43]. Copyright , Royal Society of Chemistry.
This technique can be combined with the pre-heated liquid technique as reported by Vigolo et al. [79] for thermophoresis studies (see Section 5). The authors describe a method for selective driving of particles towards either the hot or the cold side by adding specific electrolytes to their initial solution. The authors used a microfluidic device where temperature gradients were established by combining pre-heated liquid or epoxy resistors on either sides of the microchannel. Experiments bring into play the use of polystyrene beads of 477 nm in diameter in the presence of 100 mM NaCl with a flow rate of 0.01 μL/min, and show the accumulation of particles on the cold side by fluorescence measurements. Temperature gradient can also be used to generate natural convection for mixing purposes. Rapid and homogeneous mixing is difficult to achieve in microscale. Indeed, even if diffusion processes are favored in miniature fluidic systems, a pure diffusion-based mixing can be very inefficient, especially in solutions where macromolecules have a diffusion coefficient several orders of magnitude lower than that of most liquids. However, micromixing in chambers remains challenging even though many in-line micromixers have been developed and successfully demonstrated [32,34]. Kim et al. [33] presented an effective technique that enables micromixing in a microfluidic chamber without using a pump. By using natural convection in conjunction with alternating heating of two heaters (Figure 24), efficient micromixing is achieved. Heaters are made in a Ti/Pt alloy formed by a lift-off process, whose dimensions are typically 20 nm/100 nm in thickness. Fluorescent microbeads of 8-μm diameter were used as flow tracers to measure the flow speed at steady state. Standard deviation was used to determine the degree of mixing in the chamber, where I is the normalized intensity of each pixel.
A concentration gradient Pilot Pack has been assembled using Elveflow instruments and could be used to create temperature gradients.
Figure 24. Natural convection-driven flows in a chamber. (a&#;d) Flow trajectories of fluorescent microparticles taken for 35 s. Measured maximum temperatures, Tmax, in (a) to
(d) are 52, 51, 46, and 50 °C, respectively. The yellow arrows indicate the gravity direction, and the white arrows depict the flow direction of the individual fluorescent particles of 8 μm
diameter. Scale bar, 1 mm. Reprinted with permission from [33]. Copyright American Chemical Society.
MHI MC-GAXP-30 Spiral Microheater C (OD=D2= 1&#;) -Maximum temperature of element.
Ceramic Base Included with the MC-GAXP-30. Note: the one and two-inch microheaters do not have a ceramic recess. The picture shown is generic.
Approximately 250 Watts [10V @ 27A (RMS)].
D1 and D2 are separated by 0.1&#; T is 0.25,&#; and H is 2.5&#;. D1 is the OD.
** Power reported under free radiating/ no-load condition with heater resting on the provided ceramic plate. Horizontal.
Connectors comprise One Set of Appropriately rated connecting screw clips.
Note that limits cannot be exceeded on current, voltage, and element temperature.
Note that the current will be different if the heater is covered (insulated).
For more information, please visit our GAXP Spiral Microheaters page
For controls, choose either
Feedback controls are BPAN-O-PLUS-120T and Transformer.
For customers who do not have a 120V wall supply and purchase the power controller, a 220V to 120V transformer will be added at no extra cost when the BPAN control system is purchased.
or the
Open Controls with no feedback comprise the DC Power Supply and a TC Read. Please also choose the T/C and TC extension wire.
For more information on the BPAN-O-PLUS controller, please click on the Power and Temperature Control Panel Pages.
The Alumina ceramic is rated to C. Leads should be able to carry the current, e.g., 10 AWG wire.
*Packaging & Handling fee shown is for one (1) Spiral Microheater additional heaters will increase pricing.
*The weight will be calculated with the control panel option when checking out.
*The correct weight and shipping price will be shown in the final acknowledgment from MHI.
Note that the maximum temperature is at the center of the spiral region unless asymmetric power delivery is required
In the diagram in the specifications tab D1 and D2 are separated by 0.1&#; T is 0.25&#; and H is 2.5&#;. D1 is the OD.
For obtaining maximum temperature MHI controls are highly recommended. Large diameter GAXP® microheaters could develop some springiness from induced magnetic forces. Potting in ceramic paste is allowed as long as care is taken to factor-in the loss of any thermocouple response in the measured temperature. Note element temperature will be higher and maximum element temperature should not be exceeded. In the free radiation mode the power can be maximized.
The objective of the use for free radiating devices is to maximize the radiative POWER transfer from the heating elements. If the heating element is covered with insulation, the objective of the user changes to obtain a certain temperature inside the insulated region. The control of temperature inside a chamber, or power maximization in a free radiating mode can be made with MHI temperature and/or power controllers. When covered the power and current will be lower and the element temperature can be exceeded quickly if not monitored and controlled. For either objective, one cannot exceed the rated temperature or maximum current of the heating element. Power output will be lower in enclosed radiation mode with a temperature control feature as opposed to or in addition to the power control.
In the diagram D1 and D2 are separated by 0.1&#; T is 0.25&#; and H is 2.5&#;. D1 is the OD=1&#;.
There is no hole for a TC per se, however the ceramic is easily bored at a point of your choosing and for your T/C bead diameter. Please note that the only correct way to measure temperature for such a configuration is with an optical pyrometer. One will be able to calibrate the current with the temperature or power output. A current (amps) vs pyrometric measured temperature is provided below &#; when a TC is placed at any position and the current is monitored &#; one can then calibrate the TC reading (at the t/c placement location) with the actual pyrometric temperature &#; within reasonable error. The curve below should be use cautiously (only for general guidance) as it pertains to free radiating conditions only. Note that very often the TC reading that you measure will show a number well below the actual pyrometric temperature. Calibration is required when using t/C.
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National Laboratory, USA
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&#;We have used the heater, it is working well. Do you have calibration chart (Temperature v/s Voltage) for this MC170 microheater. &#;&#;&#;&#;&#;..Since we do not have temperature measuring tool for such a high temperature (C)&#;&#;&#;&#;..We are planing to order one more heater&#;&#;&#;&#;&#;&#;&#;.&#;
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&#;I wanted to let you know that we were able to connect the heaters successfully with the help of your video. &#;The heaters are heating up like they should as far as we can tell&#;&#; &#;
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&#;We have received the Ultra Coil&#; the piece seems very nice once again. Thank you.&#;
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&#;&#;the equipment arrived very quickly and in good order.&#;
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&#;Thank you for your . We tested the microheater last week. It worked well&#;&#;
R. Sweden
&#;&#;I always give you my best recommendation for the prompt service and customer minded attitude&#;&#;
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&#;&#;.. the customer service to date has been exceptional. Calls and e-mails are returned promptly, and your customers are well taken care of. We will definitely work with MHI again in the future.&#;
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&#;&#;.. , I just received the PH-G4-3.5 I ordered and it looks great. &#;..we&#;d like to purchase a controller for it as well.&#;
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&#;The heater looks great, and in tact so far!&#;&#;&#;..&#;We received the ceramic holder today. Thank you very much. We appreciate your spontaneous enabling help I hope we can try the heater soon.&#;
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&#;..&#;This should get me right back on track with this project &#; which has gone very well thanks to your assistance&#;.
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