1. Introduction
Recently, microfluidic devices have been implemented in a wide variety of applications, ranging from biological analysis to energy harvesting [
1,
2,
3,
4]. These devices offer a number of useful capabilities, such as the ability to precisely control fluid, as well as a reduced consumption of samples or reagents. Microchannel-based devices allow for low fabrication and material costs for polymer-based devices and they provide ultra-high surface-to-volume ratio properties, both of which are desirable characteristics for cost-effective improvement of high-efficiency thermal applications [
5,
6,
7,
8]. Furthermore, the versatility of microchannel designs permits multi-functionality for the devices. For instance, curved channels can create Dean vortices that are used for size-dependent separation of microparticles and enhancements of mass and heat transfer in the channel [
9,
10,
11,
12,
13]. In addition, micro-structures (e.g., micro-pillars or herringbone micro-structures) that are integrated into the microchannel can also dramatically improve heat and mass transfer by disrupting the boundary layers of the flowing fluid.
Despite the aforementioned benefits, the present lack of an adequate method for the mass production and rapid assembly of microfluidic systems for high fluidic flow rates hinders their usage in the industry level. One potential approach for mass production is to fabricate multiple layers of microchannel and later assemble all layers to obtain the final device, however this approach is still time-consuming in all stages of the process i.e., fabrication, alignment, and assembly [
14]. This has led to the development of a manufacturing method known as roll-to-roll (R2R) technology for mass producing the thin-film microfluidic devices. This method works by first imprinting the microchannels into a polymeric film and then laminating another layer of polymeric film onto the first to form the microfluidic device [
15,
16]. However, this method is still expensive, impractical, and rigid (i.e., it makes the fabrication of different designs cumbersome) for small-scale production, especially for prototyping purposes or at the laboratory level. In general, a prototype should be simply and quickly fabricated at a low cost to validate a numerical study or to evaluate the performance of a newly-designed device prior to mass production. The current prototyping approaches reported in the literature for thin-film devices, such as micromachining, xurography, and three-dimensional (3D) printing, can create 3D (out-of-plane) microfluidic networks for facilitating high fluidic flow rates in the device, but still suffer from their low throughput and time-consuming processes [
17,
18,
19]. This has motivated the development of a novel technique for the rapid fabrication of microfluidic device prototypes.
In this paper, we propose a novel and simple fabrication technique to rapidly manufacture a thin-film microfluidic device with 3D (out-of-plane) spiral micro-channels. This technique works by casting a polymeric thin film with patterned microchannels, rolling this thin film around a cylindrical core to close the individual channels and form the 3D spiral structure. Hereafter, the device is referred to as the rolled-up microfluidic device. The chosen material for the film is polydimethylsiloxane (PDMS), so that a simple, fast, and low-cost casting method can be used to pattern the channels, and the device can be assembled by using plasma bonding to avoid the use of adhesive and strong chemicals [
20]. Moreover, PDMS provides advantages including chemical resistance and good thermal stability (up to 450 °C), which allows the devices to work in harsh conditions [
21]. In this work, the rolled-up polymeric devices were developed by focusing on heat-transfer applications i.e., heat sink and heat exchangers. The rolled-up structures provide several potential benefits, such as the highly-compact design of the device allowing for the usage in a confined space, and increased surface area for high efficiency of heat transfer. Moreover, the rolled-up device contains multiple curved channels, which can generate Dean vortices, which in turn aid in enhancing heat and mass transfer within the channel [
11]. Despite the low thermal conductivity of polymer, the thin polymer film would still be beneficial in decreasing thermal resistance, allowing for higher thermal performance [
22,
23,
24].
With the proposed fabrication technique, two different designs of rolled-up devices were made, namely, (i) the single 3D spiral channel device, and (ii) the dual 3D spiral channels device. The single and dual channel rolled-up devices were fabricated from a single thin film or two thin films in order to be employed as a liquid-based heat sink and a liquid-to-liquid heat exchanger, respectively, and for their effectiveness of heat transfer to be characterized.
To date, only one prior rolled-up microfluidic device by which the microchannels were made by rolling PDMS-parylene thin films has been reported in the literature [
25]. However, the method used was a chemical vapor deposition technique (CVD), which would not be suitable for large scale microfluidic systems or mass production of the devices, as the size of the CVD machine will limit the scale of the device. Said microchannels was also employed solely as a particle-separator, not as a heat sink or exchanger. As such, this current work would be the first report on a scalable microfluidic prototype of 3D spiral microchannels fabricated by rolling up polymeric thin film(s). This novel fabrication approach could represent an initial step in making a pioneering prototype for a polymer-based roll-to-roll processing, allowing for the mass production of the polymer-based microchannels in form of thin films. Our work was developed by focusing on the usage of polymer-based thin film for further development of R2R technology, especially for cases where rapid and simple prototyping of the device is required to validate transport phenomena, such as heat and mass transfer, in the new design of a microfluidic network. For instance, in our previous work, simulations were performed to optimize the dimensions of a microchannel rolled-up heat exchanger that is made from thin film polymers, demonstrating that the effects of curved microchannels can highly improve heat transfer performance when compared to an equivalent straight channel [
25]. This current work would then be a promising approach for fabricating a prototyping device in order to validate that kind of numerical study. In addition, regarding the device itself, Dean vortices are generated along the curved channel, which can be exploited for the size-dependent separation of microparticles as well as the prevention of fouling in a microchannel [
12,
26]. These rolled-up devices thus can be used for diverse applications, such as heat sinks, heat exchangers, micromixers, micro separators, and desalination equipment [
22,
23,
24,
25,
26,
27,
28].
3. Results and Discussion
3.1. Preliminary Test of Thermal Performance of Rolled-Up Device as a Heat Sink
The results of the testing of the rolled-up device employed as a heat sink are shown in
Figure 3. The single-channel experimental results demonstrate that when DI water flows through parallel curved channels along the rolled-up device, the heat from the hot water reservoir is harvested, resulting in a temperature change between the inlet and outlet of up to 15 °C (see
Figure 3A). This temperature change increases with rising flow rate–steeply at Re < 50 and more slowly after that—until it reaches a maximum (around 15 °C) at approximately Re = 125, after which it stabilized at that value. As would be expected, the pressure drop increases proportionally with flow rate, as shown in
Figure 3B. The lack of sudden drops or peaks in this measurement, combined with the absence of bubbles of other visual signs of water loss, verify that the rolled-up device can be assumed to free of leakage. Much like for the pressure drop, the Nusselt number (Nu) also increases with the flow rate, with no observable maximum value within the parameter space explored, unlike for the temperature change (see
Figure 3C). As both the pressure drop and the Nusselt number form roughly linear relationships with Reynolds number, so too does the thermal performance factor (TPF), as demonstrated in
Figure 3D. This being the case, these experimental results verify that, despite the low thermal conductivity of the material employed, our rolled-up device can provide a prototype that features good thermal performance, which is suitable for heat-recovery applications.
The rolled-up devices were also tested to verify the limits of their leakage-free behaviour. In this study, it was found that the single-layer device can withstand high pressures of up to 80,000 Pa before internal leakage between microchannels, which is revealed via a sudden drop or rise of the pressure inside each channel, as monitored in real-time by LabVIEW. In the future, tougher materials would be able to significantly improve this limit.
In a review of the available literature, only a few reports were found demonstrating the thermal performance of a microchannel-based device constructed from polymer. As would be expected, when compared with a metal-based heat sink [
30], the thermal performance of our polymer-based thin film device is significantly lower, due to the fact that the thermal conductivity of copper is 3000 times higher than that of PDMS. These metal-based devices also operate at notably higher flow rates. However, when compared with another device made from the same material as our device and with comparable microchannel dimensions [
31], the thermal performance of our heat sink (maximum Nu of approximately 1.2) is comparable to the heat sink described in that work (maximum Nu of approximately 1.7).
3.2. Preliminary Test of Thermal Performance of Dual-Channel Rolled-Up Device as a Heat Exchanger
The results of the testing of the rolled-up device employed as a heat exchanger are shown in
Figure 4. For the preliminary test of the dual-channel device, the flow rate in the cold water loop was keep constant in order to observe the effect of the flow rate of the hot loop on the thermal performance of the device. As can be seen in
Figure 4A, at a given constant cold loop flow rate, the water in the hot loop can be cooled down by up to 8 °C between inlet and outlet. Unlike for the heat sink experiments, there was no maximum temperature change or levelling-off observed in this test, though this could possibly be due to the lower flow rates that were employed (required due to the increased pressure from the peristaltic pumps and smaller channel size). The relatively linear plot of pressure drop against the Reynolds number in
Figure 4B again shows no evidence of water leakage within the device, verifying that the dual-channel rolled-up device is also leakage-free. This preliminary result shows that heat transfer can efficiently occur between hot and cold channels, despite the low thermal conductivity of the material employed, thus suggesting that the dual-channel rolled-up device prototype made via our approach can also be used effectively for heat-transfer applications.
Like for the single-layer device, the two-layer device was also tested for the maximum conditions that it could remain leakage-free. For this device, the thin film was able to withstand up to 90,000 Pa before suffering internal leakage between hot and cold channels. As for the heat sink, stronger polymers would be able to increase this maximum survivable pressure.
In order to assess the thermal performance of our two-layer device employed as a heat-exchanger when compared to the literature, another device that is made from the same material and with comparable microchannel dimensions must be utilised. In this way, the heat transfer efficiency of the dual-channel thin film (maximum Nu of approximately 0.2) is found to again be similar to the performance of the two-layer heat exchanger from the same study used previously to compare the heat sink (maximum Nu of approximately 0.3) [
31].
3.3. Fabrication Techniques and Implications
As discussed, our fabrication approach of microfluidic system by rolling up polymeric thin films can provide several advantages in many aspects. First, our proposed fabrication technique allows for a simple and fast manufacturing process, starting from casting of the thin film, to alignment, and finally assembly of the device. In the conventional approach, the multilayers of microfluidic samples are separately made prior to layer-by-layer assembly of the device. In our approach, the device can be made from a single step of rolling up the large microfluidic thin film. This is significantly less time-consuming than the conventional assembly method. Moreover, the manufacturing cost of polymeric thin films in term of a labor cost (manufacturing time) and the cost of materials is lower than conventional materials (i.e., metal), allowing for further development of the cost-effective mass production of the microfluidic system in the future. As compared with metallic materials, some properties of polymeric materials, such as biological and chemical resistance, light-weight, and transparency, have beneficial qualities. For instance, polymers can be used for the fabrication of devices that are used in harsh conditions, such as environments in contact with high acidic or basis solutions, or seawaters. Additionally, in the aerospace or offshore industries, equipment with highly-compact design, low-footprint area and low weight are strongly advantageous. Thin-film polymer-based devices would be promising for these applications due to the material properties of polymers. Furthermore, the transparency of the polymer can allow for optical observation of the fluid inside the device, for instance, permitting straightforward observation of clogging or fouling inside the device.
In terms of thermal transport performance, when the thin film is rolled up around the center core and the multiple curved channels are created, generation of secondary flow in the form of Dean vortices will occur along the microchannel. In addition, the microchannel design can dramatically increase the surface area per volume ratio of the fluid, and thus the thin film of the polymer can provide decreased thermal resistance along the film, despite the low thermal conductivity of the polymer. These characteristics are all greatly beneficial for heat transfer. In this way, the thin-film microfluidic system is well suited to be employed for heat transfer applications, such as heat sinks and heat exchangers. For example, Heng et al., [
26] proposed a polymer-based heat exchanger made from polymeric thin films. They optimized the parameters of the device (i.e., the thickness of the film, dimensions and configuration of the microchannels) to achieve highly-efficient heat transfer in the microchannel by using computational fluid dynamic simulations in the software FLUENT. Our proposed fabrication approach could be applied to produce the rapid prototype of that system to validate experimentally that numerical work.
Lastly, via our fabrication approach, the rolled-up device can be manufactured from multilayers of polymeric thin film. The multilayered rolled-up microfluidic device containing multiple parallel curved microchannels allows for high fluidic flow through the device while maintaining reduced pressure drop along the system. This principle would be promising for further development of the rolled-up microfluidic system for industrial level usage. As mentioned previously, the rolled-up microfluidic system consists of multiple curved channels that generate Dean vortices, boosting the mass and heat transfer in the microchannel. This principle vortices-induced enhancement of mass and heat transfer in the curved channel has been well-explained and applied for numerous applications, such as mixers, microsorters, waste heat recovery, and heat exchange. In this work, the devices made by using this rolling-up technique were employed as a heat sink to achieve heat recovery and a heat exchanger to achieve thermal transport. For future work, the rolled-up device could be developed and employed for other applications, including micromixers and cell/microparticle separation.