Patent Application
20150243404
This invention relates to a polydimethylsiloxane (PDMS)-based composite and the synthesis method thereof. In particular, the present invention relates to a PDMS-based conducting and magnetic composite.
In recent years, there has been considerable progress on fabricating devices by using PDMS-based conducting and/or magnetic composite. One of the major applications of such composite is fabricating the electrodes, micro-sensors or micro-heaters, etc. in the microfluidics regime [1-2]. PDMS has played a very important role for building micro-devices because of the properties like transparency, biocompatibility and good flexibility [3]. However, PDMS itself is a non-conducting and non-magnetic polymer. It is very difficult to be patterned with the metallic structure due to weak adhesion between metal and PDMS. Therefore, techniques of mixing the conducting particles with PDMS gel in the integration of microfluidic chip are very essential.
Starting from the first report of mixing carbon fibers into PDMS by Graron et al. in 2001 [4], many different PDMS-based composites are invented for different uses: Cu+PDMS composite [5] for mechanical applications; Ag+PDMS composite [6] for conducting purpose; Ni+PDMS composite [5] and Fe3O4@Ag composite [7] for the magnetoresistive and piezoresistive purposes; Fe(CO)5+PDMS [8] for magnetic valves in microfluidic devices; and Fe+PDMS composite [9], CoFe2O4+PDMS composite [10] and Fe2-xCoSmxO4+PDMS [11] composite for magnetorheological (MR) elastomers.
Among the aforesaid PDMS-based composites, Ni+PDMS composite is commonly used as a pressure sensor because of its obvious effect of piezoresistivity. Recent work suggested the resistivity of Ni+PDMS samples reduces dramatically about 10 orders when high pressure is applied. However, the resistivity of the compressed sample is still too high to be considered as electrical conductor. On the other hand, Ag+PDMS composite is a promising material for low resistivity and therefore suitable for any electrical applications. However, due to the high concentration of Ag in Ag+PDMS, uniformity is a great concern. For the conducting and magnetic application, Fe3O4 coated with Ag particle (Fe3O4@Ag) is one of the solutions to make the conducting particles magnetic but the fabrication process of particles is complicated and not easy to control.
Most of the aforesaid PDMS-based composites are synthesized by direct mixing the corresponding particles with the PDMS gel, i.e., by simply pouring solid particles in PDMS gel and stirring until the gel become uniform. It is believed that the above process is not a good method as there is so little solvent (PDMS) to dissolve all the particles when a high concentration of solid particles in PDMS composite is required. Furthermore, stirring is difficult because of the high viscosity of the particles+PDMS gel. For instance, it is difficult to fabricate 88% wt (weight percentage) Ag+PDMS composite by mixing 17.6 g of Ag power with only 2.4 g of PDMS gel uniformly. It should also be noted that some of the aforesaid complex particles, for instance like Fe3O4@Ag, CoFe2O4 or Fe2-xCoSmxO4, are not readily available which are difficult to synthesize.
In the light of the foregoing background, it is an object of the present invention to provide an alternative method of synthesizing PDMS-based composite.
Accordingly, the present invention, in one aspect, is a method of synthesizing PDMS-based composite which comprises the steps of (1) wetting a plurality of conducting particles using a low flash point solvent; (2) mixing the conducting particles with a PDMS gel; (3) evaporating the low flash point solvent to form a mixture; and curing the mixture to form the PDMS-based composite.
In one embodiment, the conducting particles are selected from a group consisting of silver, nickel, iron and gold; and the average size of the conducting particles range from 1.2 μm to 5 μm.
In another embodiment, the low flash point solvent is selected from a group consisting of acetone, hexane and heptane.
In another embodiment, the evaporating step is conducted under room temperature for 24 hours.
In another embodiment, the curing step further comprises a step of heating the mixture at 60° C. for 48 hours.
In one embodiment, the method of synthesizing PDMS-based composite further comprises a step of sonicating the plurality of conducting particles; wherein the duration of the sonicating step is 15 minutes.
According to another aspect of the present invention, a PDMS-based composite is disclosed. The PDMS-based composite comprises a PDMS gel and a plurality of conducting particles distributed within the PDMS gel; wherein the weight percentage of the conducting particles in the PDMS-based composite is in the range of 86% to 91%.
In one exemplary embodiment, the conducting particles are selected from a group consisting of silver, nickel, iron and gold and the average size of the conducting particles ranges from 1.2 μm to 5 μm.
In an exemplary embodiment, silver is used as the conducting particle and the weight percentage of the silver in the PDMS-based composites is in the range of 88% to 90% and conductivity of the corresponding PDMS-based composite is in the range of 10−2 Sm−1 to 104 Sm−1.
In an exemplary embodiment, nickel is used as the conducting particle and the weight percentage of the nickel in the PDMS-based composites is in the range of 87% to 90% and the resistivity of the corresponding PDMS-based composite drops in more than 7 orders of magnitude when a compressive stress applied is increased from 0 kPa to 250 kPa.
There are many advantages to the present invention. First of all, the synthesis method presented can increase the solubility of metal particles in PDMS and reduce the electrical resistivity of resulting PDMS-based composite. Moreover, it can also make the spreading of metal particles in the composite more uniform.
Another advantage of the presented method is that no heating is required in order to evaporate the solvent due to its low flash point property. Such method can also minimize the insulating effect of the PDMS-based composite.
a to 3c show the electrical and mechanical characteristics cylindrical sample (diameter: 52.7 mm, height: 23.8 mm) of 90% wt Ni+PDMS composite according to one embodiment of the present invention. In particular,
a to 4b show the comparison of electrical characteristics of the first and second compressions on the 90% wt Ni+PDMS cylindrical sample (separated by 10 min) according to one embodiment of the present invention. In particular,
a to 5b show the electrical characteristics and the Young's modulus of Ag+PDMS composites according to one embodiment of the present invention with different concentrations of Ag. In particular,
a to 6e show the schematic illustration of the photolithography technique according to one embodiment of the present invention.
a to 9b show the set-up for polymerase chain reaction (PCR) according to an embodiment of the present invention. In particular,
a to 10b show the thermal cycles of PCR and the PCR result according to the same embodiment as of
a to 11b show the set-up for testing the bond strength according to an embodiment of the present invention. In particular,
a to 13b show the process of plasma activated bonding. In particular,
a to 14j show 2-D circuit boards of different sizes according to different embodiments of the present invention. In particular,
a shows the top view a 50×50 mm2 two-layered conducting circuit board with the cross section of channels with dimension of 500×800 μm2.
a shows the setup of cell culture. Cell culture chip was maintained at 37° C. by underneath microheater and in a 5%-CO2-humidified chamber. HeLa cells were cultured, in Minimum Essential Medium supplemented with 10% fetal bovine serum (FBS), 100 unit/mL penicillin and 100 mg/mL streptomycin.
As used herein and in the claims, “comprising” means including the following elements but not excluding others.
As used herein an in the claims, the word “simple” refers to the particle powder could be readily purchased in any chemicals supplier. It is not necessary to synthesize the particle powder in-house.
As used herein and in the claims, “PDMS-based composite” refers to a composite chemical structure comprising at least one conducting (and magnetic) particle component that imparts low electrical resistivity (and magnetic property) to all or portion of the entire structure; wherein “conducting (and magnetic) particle component” refers to a micro-sized particle component that is electrically conductive with or without magnetic property and the phrase “low electrical resistivity” refers to the resistivity in the order of 10−3 Ωm or below.
As used herein and in the claims, the phrase “solvent with low flash point” or “low flash point solvent” refers to a solvent that has a flash point below 0° C.
Referring now to
In one embodiment, the conducting particles are selected from a group consisting of nickel and silver. In yet another embodiment, the conducting particles are micro-sized metal particle with an average size ranging from 1.2 μm to 5 μm. In one embodiment, the low flash point solvent is selected from a group consisting of acetone, hexane and heptane.
In one embodiment, the fourth step 26 is conducted under room temperature for 24 hours in a fume hood. In another embodiment, heat curing is used in step 28. In a particular implementation of the present method, the heat curing is conducted by heating the mixture at 60° C. for 48 hours in an oven.
According to another aspect of the present invention, a PDMS-based composite is disclosed. The PDMS-based composite comprises a PDMS gel and a plurality of conducting particles distributed within the PDMS gel. The weight percentage of the conducting particles in the PDMS-based composite is in the range of 86% to 91%.
In one embodiment, the conducting particles are selected from a group consisting of nickel and silver. In yet another embodiment, the conducting particles are micro-sized metal particles with an average size ranging from 1.2 μm to 5 μm.
In one embodiment, silver micro-particles are selected as the conducting particles used in the PDMS-based composite. Accordingly, the weight percentage of silver particles in the resulting PDMS-based (Ag+PDMS) composite is in the range of 88% to 90%. Furthermore, the conductivity and Young's modulus of the resulting Ag+PDMS composite are in the range of 10−2 Sm−1 to 104 Sm−1 and 0.9 MPa to 2.1 MPa respectively.
In yet another embodiment, nickel micro-particles are selected as the conducting particles used in the PDMS-based composite. The resistivity of the resulting PDMS-based (Ni+PDMS) composite is less than 10−3 Ωm when a compressive stress is applied on the resulting Ni+PDMS composite. Moreover, the resistivity of the resulting Ni+PDMS composite drops in more than 7 orders of magnitude when the applied compressive stress applied increases from 0 kPa to 250 kPa.
In order to demonstrate the method as claimed, an illustrative example of synthesizing Ni+PDMS is discussed below. First of all, a given amount of Ni powders (for example: Sigma Aldrich, powder size <5 μm) is mixed with acetone (flash point: −17° C.) to wet the particles completely. Sonication was carried out for 15 minutes to separate any aggregated particles. This procedure makes the particle size return to the size when it was produced in factory and it would make the particles spread in the PDMS+acetone solution more thoroughly. PDMS (for example: Dow Corning, SYLGARD 184) comprises “elastomer” and “hardener”. The elastomer and hardener are used in 10:1 weight ratio in this illustrative example. A suitable amount of PDMS elastomer is added to the Ni+acetone solution and stirred gently until the mixture becomes a homogenous solution. After that, a suitable amount of PDMS hardener is added into the homogenous solution and stirred thoroughly. In one particular implementation, 19.8 g Ni powders, 2 g PDMS elastomer and 0.2 g PDMS hardener are utilized to synthesize 22 g 90% wt Ni+PDMS composite.
The final solution (Ni+acetone+PDMS elastomer+hardener) is put in the fume hood for a period of time such that all acetone has been evaporated. In one embodiment, the aforesaid period is about 24 hours. The main reason of using a solvent with low flash point is that no heating is required to force the solvent out before curing. Heating before curing does not favor the synthesis process because when impurity is present in the solvent, such heating step makes the PDMS mixture starts to cure and the molecules of the solvent will be trapped within the PDMS mixture. It will then be hard for the solvent to be vaporized completely. Moreover, voids form even the molecules can escape such that the resistivity of the PDMS-based composite will be increased.
Furthermore, evaporation of the solvent with low flash point takes away the heat from the solution and decreases the temperature of the solution which further reduces the rate of curing when the impurity is present. Also, since the mixture is not yet cured while acetone is evaporating, the Ni particles and PDMS molecules can replace the voids that are originally filled with acetone. Therefore, Ni particles are more densely packed in the composite and so resistivity of the resulting composite is reduced.
After the evaporation of acetone, the solution is heated in the oven at 60° C. for 48 hours. No cracks are found in the synthesized samples. Furthermore, the resulting samples are sponge-like, which are compressible and flexible.
In another illustrative example, Ag powders (for example: Unist Business Corp. (Shanghi), powder size ˜1.2-2.2 μm) are used as the conducting particles. The synthesizing steps as described in the foregoing illustrative example can be applied to synthesize Ag+PDMS composite.
Characterization of PDMS-Based (Ni+PDMS) Composite (without Patterning)
The characteristics of the Ni+PDMS composite obtained according to the forgoing illustrative example is now discussed. Referring to
Relationships between resistivity and strain with the change of compressive stress on the 90% wt Ni+PDMS composite were also examined. For the resistivity and strain measurement, a cylindrical sample of 90% wt Ni+PDMS composite (with diameter of 52.7 mm and height about 23.8 mm) was prepared for a pushing system made in-house (comprising a Chatillon digital force gauge, THK LM guide actuator KR). By compressing the sample, the applied force and displacement compressed could be obtained by the pushing system. Resistance of the sample was measured by a multimeter (Agilent 34410A 6½ digital multimeter). The compressive stress, resistivity and strain were then calculated. The results are plotted in
a to 4b show that the resistivity of the 90 wt % Ni+PDMS composite of the first and second compression (separated by 10 minutes). It is clearly shown that the resistivity of low compressive stress is significantly reduced in the second compression since the 90% wt Ni+PDMS composite is less reversible. However, the resistivity in the second compression can also reach 1.1×10−3 Ωm, which is about the same as that in the first compression (9.8×10−4 Ωm) (referring to
The conductivity of Ag+PDMS composite with different concentrations of Ag is shown in
The mechanical reliability of Ag+PDMS composite under deformation processing was also examined. The samples prepared were 25×1×1 mm3 strips of Ag+PDMS composites with different concentrations of Ag for the experiment in a pulling system (MTS, Alliance RT/5). The Young's modulus is calculated from the tensile stress—strain curve obtained by the system. The Young's modulus of the Ag+PDMS composite with different concentrations of Ag is shown in
Combining the techniques of photolithography, laser cutting and soft-lithography, many devices can be fabricated using the PDMS-based composites of the present invention, for instance micro-heaters, 2-D and 3-D conducting circuit boards applied in electronic and bio-system regimes.
The method of fabricating micro-heaters with channel width smaller than 300 μm, i.e. photolithography technique, is shown in
In one implementation, the photoresist is AZ4620; the substrate used could be selected from 100×100 mm2 glass and 4″ silicon wafer. In another implementation, the developer used to treat the substrate 30 is FHD-5. In yet another implementation, the solvent used to remove the photoresist 32a and 32b are acetone and propan-2-ol.
a shows the channel width of the patterned micro-heater can be as narrow as 10 μm.
For fabrication of micro-heaters with channel width larger than 300 μm, laser-engraving technique is used instead of photolithography technique. A piece of plastics (for example: polymethylmethacrylate (PMMA)) is engraved by a laser-cutting system (Universal Laser System VLS3.50) to form a mold to pattern the PDMS-based composite. The PMMA mold is dipped in a solvent (for example: ethanol or propan-2-ol) to remove the plastic particles left on the mold and then rinsed by DI water. Ag+PDMS gel is then plastered on the mold. Unnecessary portion of the gel (which is outside the pattern) is removed by rubbing the plastic mold on a piece of clean paper. The gel is then cured. To support Ag+PDMS conducting channels, a thick layer of pure PDMS gel is poured on the substrate. After heating, the micro-heater can be easily peeled off from the plastic mold. Experiments show that the bonding between the conducting micro-structure and bulk PDMS was perfect and no de-bonding or cracks were found in the fabricated micro-heaters.
In order to examine the heating capability of the micro-heaters, an infrared (IR) camera (FLIR Systems trademark, model Prism DS) was employed to detect both the heat images and thermal characteristics when the micro-heater was subjected to different applied voltages. By using infrared sensing technique, accurate temperature readings as well as comprehensive thermal distribution patterns could be obtained. In the experiment, the relationship between temperature and applied voltage was determined by focusing the IR camera on the central helical range of the micro-heater. In
Multi-target heating of the aforementioned micro-heater is also demonstrated by a set-up of biological reaction—polymerase chain reaction (PCR). PCR is a well-known DNA amplification technique. This method repeats thermal cycling involving two or three temperature steps. The micro-heater 38 is connected into a custom-designed temperature control box 40 as shown in
In order to build up complicated devices based on PDMS-based composite of the present invention, it is important to study the direct PDMS-based composite to pure PDMS bonding techniques, both reversible and irreversible. Three different bonding techniques are examined, namely: (a) natural bonding, (b) plasma activated bonding and (c) half-cured bonding. In natural bonding, two pieces of cured PDMS composites will bind together without any treatment on the corresponding surface. This bonding is reversible, i.e. they can be separated and bound together as many times as needed. For plasma activated bonding, the two pieces of cured PDMS composites will be treated in plasma ambience (Harrick Plasma Cleaner PDC-001/002) and then bound together. Half-cured bonding is a technique in which one piece of the cured PDMS composites and another piece of the half cured PDMS composite are bound together and then the complex is cured thoroughly. The last two bonding techniques are irreversible and forced separation will damage the surface of the pieces of PDMS composites. In the experiment of bond strength testing, two pieces of PDMS-based composites with the size of 8.5×8.5×2 mm3 are prepared. One of the pieces comprises different PDMS-based composites (layer A) including pure PDMS, 10% wt C+PDMS composite, 20% wt C+PDMS composite, 40% wt Ag+PDMS composite and 80% wt Ag+PDMS composite. Carbon black powder may be sourced from, for example, Vulcan XC72-R, Cabot Inc., USA. A hole with diameter about 1 mm was pinned in layer A. Another piece (layer B) is pure PDMS. The procedure described above or any other procedures known in the art can be used in preparing the PDMS-based composites for the bond strength testing except the time of curing and plasma activation are different. Details of curing and plasma activation times are listed in table 1.
iii) Fabrication of Two-Dimensional (2D) Circuit Board Structures
A 2D circuit board may be fabricated by the photolithography technique as shown in
3D circuit boards may be fabricated by the similar process to 2D circuit boards.
After the fabrication of the upper layer 64a and the lower layer 64b, the corresponding vertical channels are aligned and the upper layer 64a and the lower layer 64b are stacked face-to-face towards each other as shown in
a demonstrates the control of temperature on biomicrofluidic chip 82 by a small 2D circuit board (micro-heater) 84 fabricated according to the aforesaid procedures for cell culture. In this experiment, HeLa cells, as shown in
The exemplary embodiments of the present invention are thus fully described. Although the description referred to particular embodiments, it will be clear to one skilled in the art that the present invention may be practiced with variation of these specific details. Hence this invention should not be construed as limited to the embodiments set forth herein.
