Chapter 3: Experimental
3.6 Construction of a Microfulidic Reactor
In this experiment, the transesterification reaction for the production of biodiesel fuel is performed from peanut oil and short-chain alcohol, methanol, using immobilized Candida rugosa lipase obtained with 3:1 molar ratio of methanol to peanut oil.
We investigate the transesterification reaction on the microfluidic platform and detect the new approach of analysis method. This part will be discussed in chapter 4.
3.6-1 PDMS Molding
In this section we will describe the process for polydimethylsiloxane (PDMS) molding. PDMS is the choice of material for optical transparency, elasticity, durability, flexibility and bio-compatibility for microfluidic reactor with biological sample.
Besides that, PDMS can be reproduced with high fidelity on the micro scale by replica molding and the optically transparent down to 280nm so it can be used for detection schemes such as UV-Vis detector and fluorescence. [61] Because PDMS is elastomeric, it will conform to smooth and andrelease from delicate features of a mold without damaging itself.
Following are the process for PDMS molding.
Step1. The PDMS is polymerized by mixing 10:1 (w/w) ratio Sylgard 184 with curing agent and stirred thoroughly.
Step2. Put in the vacuum pump to degas PDMS mixture and then pour the PDMS
Step3. Cure PDMS in hotplate until PDMS is solidified, and the temperature is 110 °C.
Figure 3-15 Illustration of PDMS mother mold.
The mold is made of aluminum, and the size is 10 cm by 10 cm square containing 36 cylinders. Each cylinder is 5 mm of the diameter and of 5 mm of the height. We can make lots of reaction chamber at the same time by replica molding.
The reaction chamber is open-head in one side; therefore the volume of the chamber can be calculated. The volume of reactor chamber is 0.0981 cm3.
After preparing the PDMS stamp, we imprint the needle shape to transfer a channel in the both sides of chamber under 220 °C temperature and 353.28 kg/m2 pressure for 6 hours shown in figure 3-16.
Figure 3-16 Example of the microreactor made of PDMS. (a) the top view of the channel by optic microscope, (b) the side view of the channel by optic microscope, and (c) the scheme of the PDMS chamber and microchannel.
3.6-2 Adhesion of PDMS Elastomer to Substrate
One of the most important issues is in the selection of appropriate materials for a device. PDMS belongs to group of siloxanes shown in figure 3-17. [62]
Untreated PDMS presents hydrophobic surface, which makes the microchannels difficult to wet with aqueous solutions and easily generate bubbles. For some time it has been known that exposing PDMS to various energy sources can alter its surface properties. Energy sources such as oxygen plasmas have been the most popular way in PDMS chip making. The surface Si-CH3 groups along the PDMS backbone are transformed into Si-OH groups by the reactive oxygen species in the plasma. Then condensation of silanols (Si-OH) with appropriate groups, such as OH, COOH on another surface leads to the formation of the Si-O-Si structure when the two layers contact together. [62] This is usual way of surface chemistry for PDMS and silicon substrate. For PDMS and glass, this condensation reaction yields Si-O-Si bonds after loss of water. However, in this experiment the reactant of transesterification reaction we used is oil and alcohol. Exposing PDMS to O2 plasma can not bond well for adhesive PDMS with silicon and glass, even cause the overflowing.
As mentioned first, PDMS belongs to group of siloxanes and it has the similar group of materials which is called silicoketones or silicones. Furthermore, the adhesion behavior of PDMS elastomers in contact with substrates functionalized with a number of chemical groups that are capable of reactions with silicone elastomers. [63]
We try to bond PDMS and substrate by silicone sealant for adhesion and this way would not overflow when pumping chemical reactant into PDMS reactor. However, the transparent silicone sealent becomes fold and less transparent when completely drying. This may result non-uniform binding between PDMS and substrate and affect the transparency of PDMS and the optical analysis.
An improved method with an adhesive layer can leave selectively coating a patterned substrate and a very thin layer of uniform adhesion. [64]
Step 2: Place a thin film of liquid PDMS on the solidified PDMS stamp. Then,
Step 1: Prepare the liquid PDMS by mixing 10:1 (w/w) ratio with curing agent.
put the stamp onto the substrate (silicon or glass).
Step 3: Placed them into an oven at 60 °C for 20-30 min to cure the PDMS prepolymer.
Because the liquid PDMS is elastic, it can flat well after PDMS and substrate are brought into contact. Therefore, the liquid PDMS can form uniform layer of adhesive and bond very strongly to avoid the overflowing shown in figure 3-18. This way is simple and fast for the bonding of the substrates to form enclosed microfluidic networks. Also this method retains the layer of their original material and can maintain their desired functions essentially unaffected by the adhesive.
Figure 3-18 Schematic view of the adhesion between PDMS molding and glass substrate. (a) and (c) show it bonds the PDMS and glass by silicone sealant adhesive.
(b) and (d) show an improved method that bonds the PDMS and glass by liquid PDMS adhesive. Here we use the glass as the substrate in order to observe the
3.6-3 Fabrication and Integration of the Microfluidic Reactor
Some steps of fabrication procedure are discussed before. In this section, we would like to introduce the details of the whole fabrication process and then integration the microfluidic system.
Step 1: Prepare the PDMS molding. (This is discussed in 3.6-1)
Step 2: Screw through the PDMS stamp to make tunnels in both sides and vertically down the channel.
Step 3: Adhesion of PDMS stamp to the substrate.
Step 4: Put hollow needles through the vertically tunnels and then with Teflon tube. The needle is 0.6mm with outside diameter (O.D.) and Teflon tube is 0.56mm with I.D. and 1.16mm with O.D.
The fabrication process displays in the figure 3-19. This microfluidic platform provides a setup of fluidic operations, which are designed for easy combination within a well defined and low cost fabrication technology. The platform use minute volumes of samples and can allow saving of reagents and speeding up the analysis time. Due to tendency to use easily fabricate as well as low cost approaches, microfluidic reactor made of PDMS are more and more popular in various analytical devices. Additionally, the surface-to-sample-volume ratio becomes larger. Therefore, it increases the influence of material properties on the measurement.
Figure 3-19 Schematic diagrams of fabrication and integrations in microfluidic reactor.
The microfluidic system contains the microchannels and the reaction chambers.
The scheme of microfluidic system and syringe pump (Model 270 Series, KD Scientific Inc., Holliston, USA) is showed in figure 3-20. The volume of rector chamber is 0.0981 cm3, and we set the parameters of syringe pump is 0.06 ml with cut-off volume and flow rate is 0.67 ml/min for fast and repeat pumping in order to mixing the reactant of oil and alcohol.
Figure 3-20 Schematic of the testing apparatus used for microfluidic system with syringe pump. (a) is microfluidic reactor with textured surface by TMAH, and (b) is textured surface of solar cell.
We investigate the lipase-immobilized on anisotropic texturing surface by of TMAH/IPA solution and detect the transesterification reaction in the microfluidic reactor platform for analysis. In the study, we find that there are some absorbance responses of UV-Vis spectroscopy by detecting the biocatalysis reaction. According to this phenomenon, the optical detection can be connected with the spectroscopic methods of NMR in order to more specific identify the structure of triglyceride and esters. The microfluidic reactor platform for the detection of UV/Vis-NMR system is shown in figure 3-20 (a). Under the same phenomenon of UV-Vis spectroscopy, the
optical response can be connected with the photoelectric devices or solar cell. Since random pyramidal texturing of silicon substrates allows increasing the short circuit current of the device, and it is usually achieved, in commercial solar cells. [34][36][37]
The microfluidic reactor platform for the detection of UV/Vis-photodetector system is shown in figure 3-20 (b).