PROTOCOL TO BUILD 2D TRANSPARENT PDMS MICROMODELS FOR USE IN DRYING EXPERIMENTS

Luis A. Segura, Guillermo M. Badillo, Fabiola V. Villagra and Daniela L. Garrido

Universidad del Bío-Bío, Food Engineering DepartmentAvda Andrés Bello S/N, Chillán, Chile, email: lsegura@ubiobio.cl

Keywords: flexography, transparent micromodels, porous materials, polydimethylsiloxane

ABSTRACT

Shrinkage during drying affects the porous shape and rehydration capability of any given deformable porous structure. Moreover, only a few studies have been carried out to explain the mechanisms that provoke shrinkage during drying. We postulate that shrinkage is mainly produced through changes in capillary pressure during drying in a deformable porous structure. Subsequently, if capillary pressure is mainly responsible for shrinkage in a pore structure, it is then necessary to know the liquid phase distribution inside it as drying occurs. Transparent micromodels can be defined as transparent networks of pores and constrictions that simulate some of the complexities of natural porous media and are a useful tool to observe liquid distribution at pore scale. Micromodels have traditionally been built in glass and resin and have been very useful tools to observe and quantify liquid distribution and transport mechanisms involved in drying. Since they are built in rigid materials, shrinkage can only be indirectly studied. , A new method based on the traditional photolithographic technique has been developed to build deformable micromodels, which is an elastomeric material called polydimethylsiloxane (PDMS). This study reports the protocol to build 2D deformable transparent PDMS micromodels for future applications in the drying process and which is necessary to find evidence about transport properties directly related to shrinkage

INTRODUCTION

Several water transport mechanisms are involved during the drying of a pore structure. These mechanisms are present at different stages of the process and depend on such factors as solid/liquid/gas distribution, topology and morphology of the pore structure, and composition of the wall pores (Laurindo and Prat, 1996, San Martin et al., 2011). The transparent micromodel is a useful tool to observe liquid/gas distribution during drying at pore scale. According to Oyarzún and Segura (2009), “transparent micromodels can be defined as transparent networks of pores and constrictions that simulate some of the complexities of natural porous media (see Buckley, 1991 for a review). More generally, bead packs and single-pore models are micromodels”. It is possible to clarify the water transport mechanisms involved during drying of a pore structure through micromodels because they show complex liquid/gas/solid interactions and the relationship of these interactions with the pore space topology during the process. Although the use of micromodels has produced advances in the comprehension of the phenomena associated with drying processes (Laurindo and Prat, 1996, 1998; Tsimpanogiannis, et al., 1999; Oyarzún and Segura, 2009; Diaz et al., 2011; Segura and Oyarzún, 2012; Vorhauer et al.., In Press), these micromodels have been built

from rigid materials such as glass or plexiglass (resin); therefore, they do not capture some important features of a biological matrix such as flexibility and shrinkage. They also cannot provide evidence of the transport properties directly related to shrinkage.

As previously mentioned, micromodels built of glass and resin have been very useful tools to observe and quantify liquid distribution and transport mechanisms involved in drying; however, since they are built of rigid materials, shrinkage can only be indirectly studied. , A new method based on the photolithographic technique has been developed to build deformable micromodels, which is an elastomeric material called polydimethylsiloxane (PDMS). With this kind of materials it is possible to obtain micromodels with a pore radius value greater than 20 m over a short period of time. Another important advantage of this kind of micromodel as compared with those built of glass or resin is the possibility of building 3D micromodels (Anderson et al., 2000). The method consists in modifying the classical photolithographic technique. Duffy et al. (1998) were the first to develop the technique; some papers have been published since then (Mc. Donald et al., 2000; Jo et al., 2000; Anderson et al., 2000, and Kim et al., 2009) that describe the method in detail. According to McDonald et al. (2000) “… PDMS is an excellent material for the fabrication of microchannel systems for use with biological samples in aqueous solutions for a number of reasons (i) features on the micron scale can be reproduced with high fidelity in PDMS by replica molding; (ii) it is optically transparent down to 280 nm so it can be used for a number of detection schemes (e.g. UV/Vis absorbance and fluorescence); (iii) it cures at low temperature; (iv) it is nontoxic; (v) it can be deformed reversibly; (vi) because it is elastomeric, it will conform to smooth, nonplanar surfaces, and it releases from delicate features of a mold without damaging them or itself…”. Polydimethylsiloxane micromodels have been applied in different fields such as chemistry (Mogensen et al., 2003), biomedicine (Ki-Hum et al., 2006), physics (Studer et al., 2004), nanotechnology (Ryu et al., 2004), micromolds (Parashar et al., 2002; Xia et al., 1996), microvalves (Yoo et al., 2006; Abhinkar, 2007), microfluidic systems (Anderson et al, 2000; Mcdonald and Whitesides, 2002), chemical sensors (Lowder et al., 2007; Wessling et al., 2001), biochip cells (Baudoin et al., 2007; Marquette and Blue, 2004), gas transport systems (Stroock et al., 2002), and chemical and microchemical analysis systems (Bruin, 2000). In each of these applications the PDMS manufacturing process structures vary depending on the concentration of the base and curing agent in the mixture (Ren et al., 2006), or the curing time modification proposed by the manufacturer (Chronis et al., 2003; Jeong et al., 2004). Figure 1 shows the general outline of the manufacturing process of the 2D PDMS microfluidic system with the technique proposed by Duffy et al. (1998); Xia and Whitesides, (1998), and Qin et al. (1998).

Although PDMS systems have been applied to different disciplines, there is no evidence about building network systems to study shrinkage during the drying process. This study aims to develop a protocol for building 2D PDMS micromodels of deformable pore networks. This kind of micromodel can be a useful tool to find evidence about the transport properties involved in the shrinkage phenomenon of a given porous matrix.

Figure 1. (A) Rapid prototyping is the generation of a high-resolution transparency to be used as a photomask. (B) Fabrication of the master involves spin-coating photoresist on a silicon wafer and developing the photoresist through a photomask. (C) Replica molding involves pouring a mixture of PDMS prepolymer and curing agent on the master, curing at 70 °C for 1 h, and peeling the replica off the master. (Adapted from Jessamine et al., 2002).

MATERIALS AND METHODS

The following materials were used to construct the micromodels: photopolymer plate (TOYOBO model KM43GS), Sylgar 184 Silicone Elastomer Base (Dow Corning), Sylgar 184 Silicone Elastomer Curing Agent (Dow Corning), Silica Gel, Acetate sheeting, commercial soda plate glass (4 mm), and glass and plastic materials.

The following equipment was used: a flexo station consisting of an electric oven (model Nex TO-16) adapted with a fan and an ultraviolet light projection camera. A temperature-controlled oven (Shel-lab 1375 FX), Close Focus Microscope monocular 45x (Model 03098-00), Harrick Plasma Cleaner (Harrick Plasma Company), Flowmeter (Model 032-15-N), vacuum pump (Trivac D 2,5 E Oerlikon Leybold Vacuum, SA.), Analytical Balance (Precisa, model XB 320M), and a Canon printer (Pixma, IP4910) were also used. Other handmade equipment such as a curing chamber, a vacuum chamber, and a metal press were used.

The first step to manufacture the PDMS micromodels was to develop the pattern design (matrix) with a pore size distribution of a selected material (e.g., apple, carrot, and potato). The next

step was to obtain the photopolymer plate mold. The summary of the step-by-step procedure is as follows: The photopolymer was covered with the pattern (negative) of a porous system before being exposed to ultraviolet light (UV). The photopolymer which did not react with the ultraviolet light was then removed using water. Water selectively dissolves the areas that were not exposed to ultraviolet light and leaves a porous structure printed in the photopolymer material. Next, the mold was filled with Sylgard 184 and cured at room temperature to obtain a layer of PMDS porous structure. Finally, a second plane layer (without pores) of Sylgard 184 was developed, joined, and bound through air plasma to obtain the 2D PDMS micromodel. The manufacturing stages of the deformable PDMS micromodel, along with modifications to the protocol developed by Duffy et al. (1998); Xia and Whitesides, (1998) and Qin et al. (1998), are described in detail below.

1) Pattern design

To obtain micromodels that can capture the main geometrical and topological features of some deformable materials (Karathanos et al., 1996; Díaz et al., 2011), a Log-Normal pore size distribution function with porous radii ranging between 60 and 360 m was selected (Figure 2)

A code in Visual Basic (version 6.0) was used (Oyarzún and Segura, 2009) to generate pore radii values that obey a Log-Normal distribution function. The code was fed with statistical values from the selected materials, i.e., mean value of 140 m, standard deviation of 40 m, maximum radius of 330 m, and minimum radius of 60 m with 50 x 50 pores in accordance with the procedure described by Oyarzún (2008).

Figure 2. Log-Normal pore size distribution function

Once the data from the pore size distribution function were generated, they were drawn with the AutoCAD software (version 16.1, Autodesk Inc.). A graduated throat with values ranging from a minimum radius of 60 m to a maximum radius of 360 m was added and connected to the network pattern to measure the effect of shrinkage as a function of the capillary pressure and humidity saturation of the network during a given drying process experiment. The photomask dimension was 8 x 6 cm, which is smaller than the photopolymer plate, but which allowed cutting the edges of the PDMS layer to obtain regular borders and straight edges. Figure 3 shows a positive pattern of 50 × 50 pores with pores throat radii distributed according to a Log-Normal distribution function. Pore throats (channels) are represented in white and the solid phase in black.

Figure 3. Network pattern generated in AutoCAD (50x50 pores, node-to node distance of 1 mm, and throat radius of log-Normal between 60 and 360 µm) with a graduated throat with values ranging between the

lowest and highest radius.

2) Developing photopolymer matrix

A 13 x 11 cm photopolymer plate (TOYOBO model KM43GS) was used to build matrix support for the PDMS polymer. The photopolymer plate was aligned with the photomask shown in Figure 3 and placed in the UV chamber for 4.5 minutes at a wavelength of 365 nm. Next, the photopolymer plate was washed with a sponge and clean water at room temperature for 3 min and the matrix was exposed 2 min to UV light to strengthen the structure.

Figure 4a shows the developed photopolymer matrix and Figure 4b shows a glass frame (cuvette) where Sylgar 184 must then be poured.

(a) (b)Figure 4.(a) Matrix developed in the photopolymer ; b) Matrix developed with the glass frame (cuvette)

3) Conditioning of glass plates

This procedure is similar to the one described in a previous study (Díaz et al., 2011). “Glass edges to the cuvette were buffed with sandpaper (400 grit sandpaper to water) to remove sharp edges and eliminate scratches on the glass surface. Next, they were immersed in a solution containing 2% of detergent.

The glass plates were cleared on both sides with a cotton swab and rinsed with abundant running water to remove residual detergent.

The plates were then washed with distilled water to remove residual salts present in drinking water. The plates were immediately transferred to an oven, where they were dried for 30 minutes at 150 °C. To avoid contamination of the plate surfaces, they were attached with metal clips, which were held to the oven rack.”

4) Fabrication of PDMS Micromodel Device

The following procedure is described to develop the PDMS device (deformable micromodel): 25 ml of Sylgard 184 (base and curing agent) in a 10:1 ratio, respectively, was prepared in a Petri dish. Subsequently, the mixture was shaken to ensure homogeneity between the base and the curing agent and a vacuum was applied for 10 min to remove bubbles produced by agitating the mixture. It was then poured into the cuvette (Figure 4b) and cured for 48 h at room temperature. Finally, the mixture was cured at 145 °C for 15 min in an oven at a controlled temperature.

(a) (b)

Figure 5. (a) PDMS poured in the photopolymer matrix; (b) Porous PDMS layer after curing was finished (48 hours).

The procedure shown in Figure 5 was carried out to obtain the second plane layer (without pores) of PDMS, but instead of using the photopolymer revealed matrix, a commercial soda glass was used with the same dimensions of the photopolymer plate (13x11 cm).

5) Micromodel Bonding

The bonding process is described as follows: The two layers of PDMS (with pores and without pores) were immersed in ethanol and then dried in a vacuum at room temperature in a desiccator for 1 h. Next, both PDMS layers were put in the Harrick Plasma Cleaner equipment (Figures 6a and 6b) for 25 min at low pressure (200-300 mtorr) with a controlled airflow of 12 ml/min and at a maximum power of 26.9 Watt. The plasma process changes the surface properties of elastomer by adding free radicals on the surface of the PDMS layers; theses are nitrogen and oxygen in the case of air plasma.

After the plasma process, the two PDMS layers were joined and placed in a metal press (Figure 6c) where a pressure of 3 Kgf was applied for 45 min at 75 °C. Applying these conditions produces an irreversible bonding of the two PDMS sheets to obtain a 2D transparent PDMS micromodel.

(a) (b)

(d)Figure 6. (a) Plasma Cleaner Chamber; (b) PDMS layer inside the plasma chamber; (c) Irreversible

sealing of PDMS sandwich at a pressure of 3 Kgf in an oven at 75 °C for 45 min.

RESULTS AND DISCUSSION

From a subroutine in Visual Basic (version 6.0) and available statistical data of a given porous material (maximum and minimum throat radius, mean, and standard deviation), pore size distributions and network images were generated using the AutoCAD software. Figures 7a, b, and c show a 2D PDMS micromodel built in the present study. The micromodel was developed with a Log-Normal pore size distribution function with porous values ranging between 60 and 360 μm. Figures 7d, e, f, and g show images of micromodels built from different materials such as glass and resin in order to compare the results obtained in the present study with those available in the specialized literature. Figure 7d shows a glass micromodel obtained by Zamorano (2007), Figures 7e, and f show a resin micromodel obtained by Diaz et al., (2011), and Figure 7g shows a resin micromodel developed by Bonnet and Lenormand, 1977.

Comparing the micromodel built in the present study with glass micromodels, the PDMS micromodel displays channels (pore throats) with better pore straight cuts and better resolution than the others (Oyarzún and Segura 2009). By comparing images from Figure 7 it is possible to see that the glass micromodel (Figure 7d) has irregular channels produced by acid attack; this is compared with the high resolution achieved in the channels produced with PDMS (Figure 7c). Figure 7f shows a resin micromodel developed by Diaz et al., (2011) where a good resolution was also obtained. When comparing the three materials, it can be seen that the resolution was improved for PDMS micromodels because the micromodel pattern obtained in the present study has a square

configuration, while those obtained by Bonnet and Lenormand (1977) had an almost completely circular shape.

Another important achievement of the present study is a constant throat depth compared with other micromodel manufacturing techniques. The throat depth of the micromodels obtained in the present study was 0.233 mm.

(a) (b) (c)

(d) (e) (f)

(g)

Figure 7. a) 2D PDMS micromodel; b) deformability of the micromodel developed with the protocol in this study; c) zoom of the pore structure d) Glass micromodel (Zamorano, 2007); e) and f) Resin micromodel

developed by Diaz et al,,(2011); g) micromodel built by Bonnet and Lenormand (1977).

The most important feature of the PDMS micromodel built in the present study as compared with the others shown in Figure 7 is its capacity to be deformed when subjected to a stress condition, such as shrinkage, during a drying process. The micromodel that was built contains a graduated channel (Figure 3) that can measure variations with capillary pressure as drying occurs.

Finally, there is no evidence in the specialized literature related to the use of this kind of PDMS micromodels to study the drying process, but there is evidence of the use of these materials in the development of microfluidic systems (see Jessamine et al., 2002 for further review). The general protocol for the manufacture of micromodels used in the present study is detailed as follows:

Figure 8. Diagram of the technique developed for manufacturing the deformable micromodel

CONCLUSIONS

A protocol to obtain 2D PDMS micromodels based on the modified protocol developed by Diaz et al., (2011), Duffy et al., (1998), Xia and Whitesides, (1998) and Qin et al., (1998) was implemented. The micromodels have the following characteristics: straight cuts of the solid matrix, throat depth of 232 m, and node to node distance of 1 mm. Additionally, deformable 2D PDMS micromodel captures the main feature of the biological material when subjected to drying, which is contraction (shrinkage effect) . It is also possible to extend this use to other natural materials. The micromodel that was built contains a graduated channel that can measure variations with capillary pressure as drying occurs. Therefore, future research will be focused on studying the effects of capillary pressure on shrinkage during drying of a pore structure using this kind of micromodel.

ACKNOWLEDGEMENTS

This study was funded by FONDECYT Grant #1120347.

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