Low-cost and facile implementation of microfluidic colour-changing devices using dry film photoresist-based moulds

Abstract In this work, different microfluidic colour-changing devices are implemented by using dry film photoresist-based moulds instead of standard photolithography moulds. EtertecHT-115T negative dry film photoresist is employed to realise the rapid fabrication of the moulds for colour-changing layers. The major factors that may affect the fidelity of the dry film moulds during fabrication are summarised and analysed, including the optimum exposure times and the appropriate developing times. Especially, the impacts of different concentrations of sodium carbonate (Na2CO3) solution on developing rate are investigated for 1–5 layers (50–250 μm thick) of EtertecHT-115T dry film photoresists by experiments. The created dry film moulds show the advantages of low cost, high manufacturing efficiency and requiring no professional training. Each application of the microfluidic colour-changing devices presents high transparency and good colour-changing effect. The microfluidic colour-changing layers based on dry film moulds can be used in different wearable devices of human, and also can be applied for realising surface camouflage and display functions of soft machines/robotics.


Introduction
Microfluidics is a rapidly expanding field with applications in a number of different markets including biochemical analysis [1,2], drug delivery [3] and medical diagnose [4,5], due to the small sizes, high throughput and low cost of microfluidic systems [6]. In recent years, some studies on the colour-changing function of microfluidic systems are reported. Morin et al. reported a microfluidic network to provide both camouflage/display and movement of a soft machine [7]. In our previous reports, we proposed a microfluidic liquid colour-changing method, which was applied in different optical devices. By circulating colour liquids in microfluidic channels on the colour-changing layers, these devices can realise vision protection [8], surface camouflage and optical filtering [9]. However, the key step to fabricate these microfluidic devices is to make colour-changing layers with different channel structures by using microfluidic moulds, which need complex creating process and require large investments (including clean environment, well trained people, expensive photolithography equipments, etc.), and therefore become the development bottleneck for microfluidic research and application.
Dry film photoresist offers many advantages such as low cost, good uniformity of photoresist distribution, low exposure energy and near vertical sidewalls [10]. It has been most commonly used in the patterning of printed circuit boards (PCBs) [11] and electroplate moulds in Lithographie, Galvanoformung, and Abformung (LIGA)-type process [12]. Recently, dry film photoresist also arises greater interests of the researchers in microfluidic field [13,14]. Smejkal et al. proposed a dry film photoresist microfluidic chip used for analysing the isotachophoresis of lactate in human serum [15]. Horak et al. reported a disposable microfluidic immunochip, which employed photoresist film to biofunctionalize the channels to enhance electrochemical detection of a cardiac biomarker [16]. Weltin et al. presented a flexible microsensor, which was fabricated on a wafer-level including a layer of dry film photoresist for insulation [17]. Qu et al. provided an electrochemical micromachining technology of micro-dimple arrays on cylindrical inner surfaces using GPM200 dry film photoresist [18]. Kling et al. developed a microfluidic platform enabling the electrochemical readout of up to eight enzymelinked assays (ELAs), dry film photoresist served as the channel material in this system because it allowed long storage of preimmobilized assays [19]. However, the microstructures in these devices are directly defined and machined by using dry film photoresists, which are fine for most cases such as liquid/gas transferring and biochemical tests, but impossible for some optical applications, because optical transparency is not considered in these microfluidic systems.
Here, we develop the application of EtertecHT-115T dry film photoresist (50 lm thick) in the moulds implementation for different microfluidic colour-changing devices, including colour-changing glasses, camouflage glasses and colour-changing starfish. The key parameters in the moulds fabrication are analysed and optimised for 1-5 layers (50-250 lm thick) of dry film photoresists by experiments. Different sizes of microstructures for each application can be personal designed and rapidly realised by this low-priced way without need of professional training and expensive equipments. Two or multilayer structures of colour-changing devices can also be easily achieved for more complex functionality by using this method.

Experimental materials
In this paper, 10:1 mixture of sylgard 184 silicone elastomer and curing agent (Dow Corning Corporation, USA) is employed to fabricate the PDMS colour-changing layers. EtertecHT-115T (Eternal Chemical Industry Limited Company, China), a negative dry film photoresist with 50 lm thick, serve as the material of the moulds patterns. In order to improve the flatness and accuracy of the moulds, 10 cm Â10 cm Â2.5 cm mirror stainless steel plates (Xingyang Stainless Steel Company, China) with very smooth surfaces are selected as the substrates of dry film moulds. 365 nm UV lamp (ShunyingPhoto Electricity Limited Company, China) is used for exposure. Anhydrous sodium carbonate, deionised water and nitrogen (Zhongjia Chemical Company, China) are used for developing, cleaning and drying during the experiments. In order to realise irreversible bonding, air plasma cleaner (Wenchang Chip Technology Limited Company, China) is prepared for surfaces treatment.

Fabrication of the microfluidic moulds by using EtertecHT-115T dry film photoresist
The fabrication process of the microfluidic moulds by using EtertecHT-115T negative dry film photoresist is shown in Figure 1. The photomask is prepared by printing the designed micropatterns on a transparency film at 2400 dpi. Compared with conventional glass/metal material, the film photomask can not only meet resolution requirement, but also show the advantages of low-cost, fast and convenient for manufacturing, thus makes the dry film etch technology more accessible to an out-of-clean room environment.
Then, transfer the designed microfluidic patterns from the photomask to the dry film photoresist sheet through the following process: Clean the mirror stainless steel plate with deionised water, dry it in nitrogen stream, and used as the substrate of the mould. Cut the dry film photoresist into 10 cm Â10 cm square. Peel off one of the translucent substrates precoated over the two surfaces of the photoresist (Figure 1a). Laminate the dry film photoresist (the surface with no translucent substrate should be faced down) on the cleaned mirror stainless steel substrate by using an office laminator at 110 C ( Figure 1b). Place the printed photomask on the dry film photoresist and cover it with a transparent Polymethyl Methacrylate (PMMA) plate, fix some clips around the whole structure to avoid light leaking (Figure 1c and 1d). Expose the whole structure under an UV lamp with the power of 5 mW/cm 2 for about 80 s ( Figure 1e). Peel off the other translucent substrate pre-coated over the photoresist and immerse the structure into 1% (mass concentration) sodium carbonate (Na 2 CO 3 ) developer solution at 25 C for about 1 min (Figure 1f and 1g). The 80 s exposure time and 1 min developing time are only good for single-layer (50 um thick) dry film photoresist.
Finally, clean the developed structure with deionised water to wash away the unexposed portion and appear the engraved patterns as designed, then dry it in nitrogen stream (Figure 1h). It can be used as the microfluidic colour-changing mould now.

Fabrication of the microfluidic colour-changing devices using dry film moulds
In this paper, polymer PDMS is selected to make the colour-changing layer of the microfluidic devices due to its characters [20,21], such as high optical transparency in clean environments, less fragile and less expensive. More importantly, it is very convenient to make microfluidic devices with PDMS by using dry film moulds. The fabrication of the microfluidic colour-changing devices is based on soft lithography technology [22][23][24]. It mainly includes two processes： fabrication of the PDMS film with microstructures by using dry film mould and irreversible bonding with substrate [25][26][27].
The fabrication process of the PDMS film by using dry film mould is depicted in Figure 2a. Appropriate (according to the thickness of PDMS film to be made) mixture of liquid PDMS and curing agent at 10:1 in weight is prepared. Pour the mixture over the dry film mould and degas in vacuum environment for 1 h. Then put the whole structure into a vacuum oven at 80 C and cure it for 20 min. Peel off the partial cured PDMS film with microstructures from the dry film mould, and poke two holes at the inlet and outlet of the microfluidic patterns respectively with a 1 mm diameter puncher. One is used as liquid inlet and the other is air outlet.
The fabricated PDMS film with microstructures is bonded on the substrate (glass, resin or plastic) to form closed microfluidic channels for colour liquids circulation. In order to achieve irreversible bonding (strong enough to withstand a high supply pressure), the two surfaces to be bonded are firstly modified by using air plasma cleaner at 80 W for 35 s, which can increase the number of surface oxide sites and improve their hydrophily [28][29][30]. Then place the two treated surfaces together within 1 min and press gently so as to be a full contact. Finally, cure the bonding structure in a vacuum oven at 80 C for 1 h to make sure of that the bonding strength is strong enough. The specific irreversible bonding process is described in Figure 2b.

Fidelity of the microstructures on dry film moulds
The fidelity of the microstructures on dry film moulds can directly affect the performances of the microfluidic colour-changing devices after fabrication. The major factors that affect the fidelity of the dry film moulds are investigated, including bubble, exposure time, developing time, and developer concentration.
When laminating dry film photoresist on the substrate, bubbles will be easily formed between the photoresist and substrate, resulting in irregular channel boundaries. Figure 3a shows two formed bubbles between single-layer EtertecHT-115T dry film photoresist and mirror stainless steel substrate. One of the bubbles is located on the microfluidic channel exactly, thus irregular channel boundary is formed, as demonstrated in Figure 3b. So, bubble examination after lamination is very necessary. In this research, bubble examination is carried out through micro-observations by using light microscope XSP-63B (Shanghai Optical Instrument Company, China). If the measured bubble diameter is less than 1/5 of the minimum channel width, the bubble influence can be neglected. Otherwise, dry film lamination and bubble examination must be repeated.
Among the factors that influence the moulds fidelity, exposure time is most crucial. Figure 4 presents the microscope images of the microfluidic channels with different photoresist thickness under overexposure, underexposure and appropriate exposure, respectively. It is obvious that overexposure makes the extra photoresist around the channels unwashable, resulting in wider channel structures than mask graphics, whereas underexposure makes the photoresist unable to be polymerised completely and easily to be washed off, resulting in obscurer and narrower channel structures than mask graphics. Through a large number of experiments, the general exposure time ranges for 1-5 layers (50 lm-250 lm thick) of EtertecHT-115T dry film photoresists are obtained. Another six measurements are conducted for each obtained time range, then the optimum exposure time ranges for different thickness dry film photoresists are summarised and proposed, as illustrated in Figure 5a.
Developing process is also an important part in the mould fabrication with dry film photoresist. Underdevelopment leads to insufficient depths of channels, overdevelopment makes the dry film microstructures weak and easily to be destroyed. Commonly, 1% (mass concentration) Na 2 CO 3 solution is used for development. In this paper, in order to investigate the influence of developer concentration on developing rate, different concentrations of Na 2 CO 3 solutions are tested by experiments, as shown in Figure 5b. Figure 5b indicates that the developing rate is not stable for the same concentration of developer solution, the longer the developing time lasts, the slower the developing rate becomes. The mainly reason is that the photoresist dissolved in the developer gradually increases as the growth of the developing time, resulting in the decreases of the developer effective concentration. Figure 5b also provides the developing times for various dry film thickness by using different concentrations of Na 2 CO 3 solutions. When the thickness is 100 lm, the developing time of 1% Na 2 CO 3 is 102 s. For 0.5%, 2% and 5% Na 2 CO 3 , they are 184 s, 111 s and 105 s, respectively. And when the thickness is 250 lm, the developing time of 1% Na 2 CO 3 is 344 s. For 0.5%, 2% and 5% Na 2 CO 3 , they are 490 s, 270 s and 351 s, respectively. Therefore, it is not that the higher the concentration is, the faster the developing rate becomes. The experimental results indicate that when the dry film thickness is less the 100 lm, 1% Na 2 CO 3 solution has almost the same developing rate with 2% and 5% Na 2 CO 3 , this is the reason why it is widely used in the industry. However, when the thickness is greater than 100 lm, the developing rate of 2% Na 2 CO 3 solution is obviously faster than 1% and 5% Na 2 CO 3 . Table 1 presents the time-consuming comparison by using different moulds making methods. Compared with conventional photolithography way, the moulds fabrication based on dry film photoresist shows high efficiency, and no expensive equipments and professionals are needed. However, the channels resolution is lower for dry film photoresist moulds, which are suitable for wider channels (>20 mm) prototyping or beginners in this field.
3.2. The applications of the colour-changing devices based on dry film photoresist moulds Figure 6 presents the microfluidic colour-changing devices with different dimensions fabricated by using dry film photoresist moulds, including colour-changing glasses, camouflage glasses and colour-changing starfish. Figure 6a depicts the application effects, Figure  6b and 6c reveal the specific designs of the microstructures for each application. The microfluidic colour-changing glasses can be used to comfort and protect human eyes in an environment with strong lights by circulating colour liquids through the colour-changing layer. By designing disruptive microstructures on the lens and filling with proper colour liquids matched to the background environment, simple camouflage effect of the aStandard photolithography: mainly consider the time spent in the steps (a)-(h) of Figure 1. bDry film photoresist: mainly consider the time spent in pre-baking, irradiation and post-baking, without including the cooling time after baking. glasses can be achieved for concealing needs in the wilderness. Inspired by some coloured animals with special colour-changing organisms [31] (such as chameleon), a colourchanging device similar to starfish is designed in this paper. With the simple liquid colour-changing layers matched to the environment around, the starfishes can camouflage and protect themselves from hunting. Each application presents good colour-changing effect and high transparency. Other colour-changing and camouflage effects can be realised by changing the designs of microstructure on the dry film moulds. Two or multilayer structures of colour-changing devices can also be easily fabricated by this method to meet different requirements. Through targeted designing and making, the microfluidic colour-changing layers based on dry film moulds can be used in other various wearable devices for human. Meantime, they also can be applied for realising surface camouflage and display functions of soft machines/robotics, which have widespread usage in military.

Conclusions
From the fabrication and applications of the microfluidic colour-changing systems based on the dry film photoresists moulds, it can be concluded that the microfluidic moulds for different colour-changing devices can be rapidly realised by using EtertecHT-115T dry film photoresist and high-resolution masks. Proper selection of exposure times, developing time and concentration of developer solution for different dry film thickness can achieve good moulds performances. Different designs of the micropatterns and multilayer structures can be easily and quickly implemented by using dry film moulds. Each design and application of the microfluidic colour-changing device shows good colour-changing effect and high optical transparency. This fabrication and colour-changing ways also present new appearance modifying opportunities for other wearable devices of human and machines.

Disclosure statement
The authors declared no competing financial interests.

Author contributions
S.L. conceived and designed the research. M.Z. and S.L. wrote the main manuscript text. M.Z did all the experiments and prepared all the figures. All the authors contributed to the discussion of the results and reviewed the manuscript.