HK1136261B - Constructing planar and three-dimensional microstructures with pdms-based conducting composite - Google Patents

Description

construction of planar and three-dimensional microstructures using PDMS-based conductive composites

This application claims U.S. provisional application No. 60/860,713, filed on 2006, 24/11. The disclosures of the above-mentioned provisional applications are hereby incorporated by reference in their entirety.

Technical Field

The subject matter of this patent relates to the synthesis of elastic biocompatible functional microstructures (microstructures) in which the electrical function designed is obtained by mixing conductive nano-to micro-particles with PDMS gels, where the critical volume fraction of solid particles is chosen to ensure good electrical conductivity, reliable mechanical properties and ideal thermal properties. By using this composite material, we developed a method of building planar and three-dimensional microstructures by soft-lithographic technology (soft-lithographic). We have demonstrated applications such as electrodes, conductive strips, two and three dimensional microstructures for wire connections, micro-heaters, micro-heater arrays, flexible thermochromic displays, and applications for microfluidic devices, all of which have demonstrated elastic flexibility and fall-resistance while maintaining their functionality. All results show that the composite material has wide application prospect in micro-manufacturing, particularly in biochips.

Background

In recent years, there have been great advances in the preparation of multifunctional microfluidic devices with the goal of achieving highly integrated lab-on-a-chip [1-3 ]. These advances have benefited from the development of microfabrication techniques, such as soft lithography [4 ]. Polydimethylsiloxane (PDMS) plays an important role in building microstructures, which benefits from its properties such as transparency, biocompatibility, and good flexibility [5 ]. Some complex microdevices (U.S. Pat. No.7,125,510; 6,692,680 and 6,679,471) can be realized by using a simple manufacturing technique such as micro-molding (micro-molding) using a PDMS material. But PDMS is a non-conductive polymer and patterning of metallic structures is very difficult due to the weak adhesion between the metal and the PDMS. Therefore, the integration of conductive structures in PDMS is a critical issue, especially for applications such as electrical micropumps, microsensors, microheaters, ER drivers, etc. that require electrodes for control and signal detection [6-7 ].

Gawron et al [8] first reported embedding fine carbon fibers in PDMS-based microchips for capillary electrophoresis detection. Lee et al [9] reported the transfer and subsequent embedding of thin films of gold patterns into PDMS via silane coupling agent mediated adhesion chemistry. Lim et al [10] developed a method of transferring and stacking metal layers onto PDMS substrates by using sequential selective etching techniques. As shown in U.S. patent No. 6,323,659, an electrode comprising a base material and a filler material is used to determine the presence of water in the material. Wherein the conductive electrode may be formed by depositing carbon black onto the surface of an elastomer by either scraping on a dry powder or exposing the elastomer to a suspension of carbon black in a solvent. In addition, the electrodes may be formed by building an entire layer out of an elastomer doped with a conductive material (i.e., carbon black or finely divided metal particles). However, the incompatibility of PDMS and the intermetallics often leads to failure of the manufacturing process, particularly in the bonding between the two materials. Therefore, the selection of composite materials with good electrical conductivity, reliable mechanical properties, and desirable thermal properties for the construction of microdevices is a very urgent issue. In particular, building microdevices with three-dimensional conductive structures, such as three-dimensional wiring and packaging, has become a challenge in microfabrication. PDMS-based conductive composites can be a promising material for micro-device fabrication.

Disclosure of Invention

The present invention relates generally to microfabrication techniques and PDMS composites. More particularly, the present invention relates to the synthesis of elastic biocompatible functional microstructures, wherein the designed electrical function is made by mixing conductive nano-to micro-particles with PDMS gels, wherein the critical volume fraction of solid particles is chosen to ensure good conductivity, reliable mechanical properties and desired thermal properties. By using such a composite material, we have developed an improved method of building planar and three-dimensional microstructures by soft lithography. The composite material of the present invention can be used to make a variety of useful microstructures. For example, particular embodiments of the present invention may include electrodes, conductive strips, two-dimensional and three-dimensional microstructures for electrical wiring connections, micro-heaters, micro-heater arrays, flexible thermochromic displays, and applications for microfluidic devices. Moreover, structures made using the composite materials and/or methods of the present invention further exhibit elastic flexibility and fall-proof properties while retaining their functionality.

One embodiment of the present invention relates to a prepared planar structure, three-dimensional structure or a combination thereof, said structure comprising at least one PDMS based conductive composite, wherein said structure provides a pre-designed electrical conductivity and mechanical properties. A further embodiment of the present invention relates to a prepared planar structure, three-dimensional structure or a combination thereof, wherein the at least one PDMS based conductive composite comprises (a) silver + PDMS, (b) carbon black (C) + PDMS, or (C) a combination thereof. In one embodiment of the present invention, the at least one PDMS based conductive composite comprises silver and PDMS, wherein the weight concentration ratio of silver to PDMS ranges from about 83% to about 90%. In a more preferred embodiment, the silver/PDMS weight concentration ratio is between about 84% and about 87%. Another embodiment of the present invention relates to a prepared planar structure, three-dimensional structure or a combination thereof, wherein the at least one PDMS based conductive composite comprises C + PDMS, wherein the carbon black/PDMS weight concentration ratio is from about 10% to about 30%. In a more preferred embodiment, the carbon black/PDMS weight concentration ratio is about 15% to 27%. In another embodiment of the present invention, the Ag + PDMS composite comprises Ag particles having an average size of about 1.0 micron to about 2.2 microns. In another embodiment, the C + PDMS composite comprises carbon black particles having an average size of about 30 to 100 nanometers.

Another embodiment of the present invention relates to a fabricated planar structure, a three-dimensional structure, or a combination thereof, wherein the fabricated structure is a rod array (rod array), a multi-layer wiring co-junction (co-junction), or a cross bridge (cross bridge), comprising pre-designed electrical and mechanical properties. In one embodiment of the invention, the fabricated structures or pre-designed patterns are fabricated by soft lithography. In another embodiment of the present invention, the prepared structure is embedded into a PDMS bulk material (bulk material) by molding into designed shapes and patterns. In another embodiment of the present invention, the prepared structure comprises at least one conductive wiring structure having a minimum dimension of 10 microns. In a preferred embodiment of the invention, the prepared structure is mechanically elastic and flexible while maintaining the designed electrical conductivity. In another preferred embodiment of the invention, the produced structure is fall-proof.

One embodiment of the present invention relates to the use of the composite material prepared according to the present invention as a microheater, or a device containing a microheater. In a particular embodiment of the invention, the microheater or device containing the microheater contains a heating strip that is at least 25 microns wide or long. In another embodiment of the invention, the maximum local temperature generated by the heating strip may be up to 250 ℃ from ambient temperature. In a further embodiment of the invention, the microheater or device containing the microheater has (a) a monolithic structure that is rich in mechanical elasticity and flexibility while maintaining local heating function, (b) a crash-proof monolithic structure, or (c) a combination of (a) and (b).

Another embodiment of the invention relates to the use of the composite material prepared by the invention as a thermal array. In particular embodiments of the present invention, the present thermal array comprises a temperature sensing mechanism that optionally controls the conductivity in the heating strip. In a further embodiment of the invention, the thermal array further comprises a temperature sensing mechanism comprising at least one thermochromic micro-color bar, the color of which can be sensed optically. In a further embodiment of the invention, the thermal array comprises a temperature sensing mechanism comprising at least one thermochromic micro-colour bar, the colour of which can be sensed optically, and wherein the detection of the colour of the at least one thermochromic micro-colour bar is monitored optically, the conductivity through the heating bar subsequently being controlled by an electro-optical feedback system, wherein the electro-optical feedback system stops heating when a desired thermochromic micro-colour bar is activated by a desired threshold temperature.

Another embodiment of the present invention is directed to the use of the composite material prepared using the present invention as a thermally activated display. In one embodiment of the invention, the thermally activated display comprises: (a) a thermochromic composite, and (b) a silver + PDMS composite; wherein the prepared structure is thermochromic, electrically conductive, and flexible. In another embodiment of the present invention, the thermally activated display comprises: (a) a thermochromic composite layer in contact with (b) the silver + PDMS composite layer. In a further embodiment of the invention, the thermally activated display comprises a prepared silver + PDMS structure with a pattern of conductors embedded therein, the pattern corresponding to the pre-designed pattern of the display.

A further embodiment of the invention relates to the use of the composite material prepared according to the invention as a thermally activated display embedded with a plurality of individual conductor patterns in a matrix-like array of individual pixels; wherein each pixel may independently display the same or different color as an adjacent pixel based on the degree of heating supplied by the wire to each individual pixel. In one embodiment of the invention, a thermally activated display comprises: (a) a thermochromic composite layer in contact with (b) the silver + PDMS composite layer: wherein the conductor pattern is embedded in the Ag + PDMS layer. In another embodiment of the present invention, a thermally activated display comprises silver + PDMS, wherein the silver/PDMS weight concentration is from about 84% to about 88%. In another embodiment of the present invention, a thermally activated display comprises microencapsulated thermochromic powder as the thermochromic composite material.

Another embodiment of the present invention is directed to a composite prepared using the present invention in a method of preparing a thermally activated display, comprising: (a) mixing the microencapsulated thermochromic powder with PDMS to obtain a particle concentration of 20% (W/W); (b) mixing silver powder and PDMS, wherein the Ag/PDMS weight concentration is from about 84 wt% to about 88 wt%, to form a gel-like mixture, (c) embedding at least one pattern of conductive lines in the silver + PDMS mixture; (d) applying the layer of (a) to a gel-like mixture of silver + PDMS; and (e) curing the layered composite.

Drawings

FIG. 1: SEM photographs of the cured conductive composite and powder: (a) silver + PDMS (84 wt%); (b) c + PDMS (28 wt%).

FIG. 2: (a) the conductivity is related to the change of the weight concentration of the powder; (b) the conductivity varies with temperature.

FIG. 3: 26 wt% C + PDMS strips (25X 2X 1 mm)³) And 86 wt% Ag + PDMS strips (25X 1 mm)³) The conductivity changes upon stretching. (a) And (b): for C + PDMS and Ag + PDMS, quasi-static stretching and recovery was at a rate of 1.5 mm/min. (c) Dynamic tensile properties of the C + PDMS samples, peak-to-peak amplitude of 1mm, frequency 50 Hz. (d) Dynamic tensile properties of the Ag + PDMS samples, peak-to-peak amplitude of 0.5mm, 50 Hz.

FIG. 4: process flow diagram showing the patterning of conductive PDMS using soft lithography. (a) Micropatterning of conductive PDMS, (b) - (d) show SEM photographs of various prepared conductive patterns.

FIG. 5: patterning and bonding of multilayer and three-dimensional conductive PDMS. (a) A schematic diagram of a designed three-dimensional wire; (b) a microfabrication process; (c) reversely combining the two halves (reverse bonding) into a plate through a jumper wire; (d) test circuitry with LEDs to show the function of the combined board.

FIG. 6: schematic of a representative microheater. The three-dimensional spiral patterned structure is made of a silver microparticle-PDMS composite material. Illustration is shown: SEM photograph of microheater with line width 25 microns.

FIG. 7: temperature of the central heating portion of the micro-heater as a function of input voltage. The two insets are infrared photographs of the thermal profile at a particular applied voltage. The bright spot in the right photograph is a high temperature zone at-250 ℃.

FIG. 8: schematic representation of a representative display structure. The mark-shaped patterned lead is formed by soft lithography of a silver microparticle PDMS composite material. The conductor pattern is embedded in the thermochromic patch. The right inset shows top and bottom views of the prepared device.

FIG. 9: the degree of display varies with applied voltage. The left five curves correspond to stepped function voltages of different magnitudes, while the right curve corresponds to the voltage off condition. The illustrations show the identification image at different display levels. The image edges in the inset (c) are blurred due to overheating.

FIG. 10: and displaying the power consumption of the display at different T/T ratio heating pulse trains. The duty cycle was fixed at 50 Hz. The table gives the optimum voltage values (in order to obtain an accurate image) associated with different T/T ratios. The solid line is calculated from the expression given in the text. The solid blocks are measurement data.

FIG. 11: the function of the display is not affected by mechanical deformations. Here, the display is wrapped around the post. (a) Showing the display film without an input signal; (b) it is shown that the logo image is correctly displayed when the voltage heating pulse train is applied.

FIG. 12: schematic diagram of three-dimensional layered structure of PDMS micro-reaction chip. Thermochromic color bars and micro-heaters are located on the lower layer, while microfluidic channels for chemical reactions are located on the upper layer. The lower left inset is an enlarged view of the thermochromic color bars, and the upper right inset is an image of the fabricated device.

FIG. 13: photo-electric temperature sensing and control process. The process enables accurate local temperature control in the microfluidic device when combined with computer storage of the corrected control signals. The process is operated via a control box shown in the upper right of the figure.

FIG. 14: temperature control was performed in a microreaction involving sodium thiosulfate and hydrochloric acid. The set target temperature on the thermochromic color bar is shown on the left. The corresponding reactions are shown on the right. Here, the reaction product (sulfur) makes the cycle clearly visible.

FIG. 15: (a) the micro-heater is controlled by a square wave trigger pulse signal generated by the system. (b) The CdS output voltage (fitted by the blue line in the lower part of the graph) is juxtaposed with the predicted temperature change (red line). The upper part of the figure is the corresponding trigger voltage pulse train. The system delay time is 0.7 seconds.

Detailed Description

The present invention relates to the synthesis of elastic biocompatible functional microstructures, wherein the designed electrical function results from mixing a PDMS gel with conductive nano-to micron-sized particles, wherein the critical volume fraction of solid particles is selected to ensure good conductivity, reliable mechanical properties and ideal thermal properties. By using such composite materials, methods for building planar and three-dimensional microstructures by soft lithography have been developed. Applications such as electrodes, conductive strips, two-and three-dimensional microstructures for wire bonding, micro-heaters, micro-heater arrays, flexible thermochromic displays, and microfluidic devices have all been demonstrated, all of which demonstrate resilient flexibility and fall-resistant characteristics while maintaining functionality. The results obtained have a wide prospect for the use of such composites in microfabrication, in particular biochips.

The term "PDMS based conductive composite" as used in this patent refers to a composite chemical structure comprising at least one conductive particle component that imparts conductivity to part or all of the entire structure. The term "conductive particle component" refers to a conductive nano-sized or micro-sized particle component. In some embodiments, the particulate component is selected from silver powder or carbon black. Other conductive particle compositions known to those skilled in the art may also be used to make the PDMS based conductive composite.

The term "mechanically elastic and flexible" as used herein refers to the ability of a PDMS based conductive composite to bend when subjected to mild to moderate mechanical stress without substantial permanent deformation of the structure or compromising the electrical conductivity of the structure. Mild to moderate mechanical stress includes wrapping or applying a thin layered structure on a curved or irregularly shaped surface, or bending the structure with fingers to conform to a frame or scaffold.

The term "fall-through" as used herein refers to PDMS based conductive composites and structures made substantially of PDMS based conductive composites that resist cracking or breaking with their conductive properties when subjected to mechanical stress due to sudden impacts such as, for example, hitting a support or falling to a hard surface.

The term "thermochromic color stripe" as used herein refers to a device or composition containing in at least one localized area a thermochromic chemical composition that changes color in response to temperature. In general, this temperature-dependent color can be sensed optically. For example, color can be sensed by a photodetector such as the human eye, photographic film, CCD camera, and the like. The thermochromic color bar may comprise a single localized thermochromic chemical composition that changes color over a broad color spectrum with temperature. Alternatively, the thermochromic color bar may comprise two or more localized thermochromic chemical compositions, wherein each localized thermochromic chemical composition changes color in response to a narrow range of temperatures, and wherein a series of such localized compositions can be configured to sense a change in a wider range of temperatures.

Synthesis of PDMS-based conductive composites

The present invention relates to a composite material obtained by mixing conductive nano-to microparticles with a PDMS colloid, wherein the critical volume fraction of solid particles is selected to ensure good electrical conductivity, reliable mechanical properties and ideal thermal properties. In one embodiment, the conductive composite comprises conductive nano-to micron-sized particles selected from silver (Ag) particles or carbon black (C) particles, wherein the particles are mixed with PDMS to form conductive composites Ag + PDMS and C + PDMS suitable for microfabrication. The synthesis process involves mixing silver powder or carbon black and PDMS colloids at the designed concentrations. In one embodiment of the invention, the Ag/PDMS weight concentration ranges from about 83% Ag to about 90% silver. In a further embodiment, the Ag/PDMS weight concentration ranges from about 84% to 87%. In another embodiment, the carbon black (C)/PDMS weight concentration range is from about 10% C to about 30% C carbon. In a further embodiment, the C/PDMS weight concentration range is about 15% C to 27% C. In another embodiment, the silver or carbon black particles are about 1-2 microns (silver) and about 30 to 100 nanometers (carbon black) in size, respectively, as can be seen in the inset of fig. 1(a) and 1 (b). In a preferred embodiment, the carbon black particles have a diameter in the range of about 20 to 30 nanometers. Fig. 1 shows an SEM electron micrograph of a cross section of the cured composite material, in which the solid particles are in contact with each other and uniformly distributed in the PDMS. Silver particles and carbon black particles are easily mixed with PDMS gels, possibly due to their desirable wetting properties.

Characterization of PDMS-based conductive composites

The conductivities of two examples of composite materials are shown in fig. 2(a) as a function of the concentration of conductive particles. In the Ag + PDMS composite, the threshold concentration, which starts to have good conductivity, is about 83 wt% Ag. Beyond this threshold, the conductivity σ increases rapidly. The C + PDMS composite behaves similarly, but the threshold concentration value is much smaller (-10 wt% C) and the conductivity is also much smaller (in some cases, five orders of magnitude smaller for the conductivity of the Ag + PDMS composite). The latter is in fact advantageous, for example, for the manufacture of micro-heaters, but is not suitable for applications where good electrical conductivity is required. It should be noted that when the concentration of the solid conductive phase is too high, the composite is difficult to process since the mechanical properties are no longer similar to those of PDMS. Therefore, for PDMS based conductive composites, the optimal concentration is very critical.

The resistivity of the cured composite material varied with the temperature T, as shown in fig. 2 (b). In the temperature region 25 ℃ to 150 ℃, the resistivity of the C + PDMS increases with increasing temperature; whereas the resistivity of Ag + PDMS peaks at about 120 c and decreases above this temperature. Because these features have reliable reproducibility, this temperature variation of the resistivity enables the design and fabrication of thermal sensors by employing PDMS-based conductive composites and their unique features.

We investigated the mechanical reliability of PDMS based conductive composites under deformation processing. In one example, to measure the mechanical reliability of the two composites under deformation processing, two 25 × 2 × 1mm of C + PDMS (26 wt% carbon) and Ag + PDMS (86 wt% silver) were prepared³Bars were used for experiments on a drawing system (MTS, Alliance RT/S). Conductivity was monitored as a function of strain by stretching the sample and recovering at a constant rate of 1.5 mm/min. The results for both samples are shown in fig. 3(a) and 3 (b). Note that the conductivity of both samples increased monotonically with increasing strain. The reason for this strain-dependent change in conductivity of the sample can be attributed to the change in the conductive particles contacting each other, i.e., the nano-carbon black particles or silver microparticles have a better chance to contact each other when the sample is stretched, and vice versa. When the strain is released, the conductivity of the C + PDMS sample returns to the original value with only a small change. However, the relaxation behavior of Ag + PDMS is very slow compared to the C + PDMS sample. It was shown that Ag + PDMS would take more than 1 hour to return to the original state. By varying the frequency of the stretch-recovery cycle, of the sampleThe dynamic characteristics are also determined. This is done by mounting one end of the sample to a static platform and the other end to a mechanical vibrating arm. The peak-to-peak amplitude is 1mm at a vibration frequency of 50Hz, as shown in FIG. 3 (c). It should be noted that the waveform seen in fig. 3(c) remains resolvable even at 200Hz, which means that such a composite material can potentially be used as a pressure sensor to detect dynamic pressure changes in a micro-chamber or micro-duct, e.g. small pressure changes can be easily detected by using a thin PDMS membrane with embedded wires. The Ag + PDMS samples showed similar dynamic mechanical properties, as shown in fig. 3 (d).

Production of planar microstructures

Fig. 4(a) shows an example of a procedure for embedding a layer of conductive composite in a PDMS elastomer. A thick layer of photoresist, such as AZ4620, is patterned onto the glass substrate using standard photolithographic techniques. This is to form a mold to pattern the conductive composite. Various other photoresist and/or lithographic techniques known to those skilled in the art may also be employed. After baking, the mold is treated with a mold release agent, such as tridecafluoro-1, 2, 2, 2-tetrahydrooctyl-1-trichlorosilane. Various other release agents or techniques known to those skilled in the art may also be employed. The conductive composite is synthesized by mixing PDMS (e.g., Dow Corning 184) with carbon black powder (e.g., Vulcan XC72-R, Cabot, Inc USA) or silver flakes (e.g., 1.2-2.2 microns, unist business Corp. (shanghai)) at various concentrations to form a C + PDMS colloid or a Ag + PDMS colloid. Various other PDMS compositions and conductive particles known to those skilled in the art may also be used. The gel is then spread on the mold. The unwanted portions of the gel are preferably removed from the mold surface (e.g., with a doctor blade) to ensure that only a clear pattern remains in the mold. The gel is then cured to a solid, for example by baking. For example, after baking at 60 degrees celsius for 1 hour, the colloid solidifies into a solid. Subsequently, the photoresist is removed from the mold substrate. For example, the photoresist AZ4620 may be removed by the following procedure: the entire mold substrate is immersed in a solvent such as acetone, then immersed in ethanol, and then rinsed with deionized water. After baking, only the PDMS based conductive composite should remain on the substrate, as shown in step 3 of fig. 4 (a). The integration or embedding of the conductive micropattern into the PDMS bulk layer can be achieved by pouring a PDMS gel over the substrate (with the desired microstructures immersed in the PDMS). After spinning to ensure uniformity of the layer, the PDMS slab with embedded conductive microstructures can be easily lifted off the substrate (as shown in step 4 of fig. 4 (a)). The method enables the prepared microstructure and the PDMS to have excellent combination. After heating, for example annealing at 150 ℃ (see the last step 5 of fig. 4 (a)), the prepared samples were found to be free of debonding or cracking.

Fig. 4(b) shows SEM photographs of samples with different patterns made with Ag + PDMS composite. In these samples, the pattern size can be tens of microns to hundreds of microns, showing that this method enables microfabrication of conductive devices of different sizes and with engineered variations in microscale detail.

Three-dimensional wiring

Three-dimensional connection of electrical signals is an important issue in integrated microchips, such as transmission of electrical signals between different layers, communication between the inner and outer components of a multilayer chip. Structures comprising the PDMS based conductive composites of the present invention can also be fabricated with integrated circuits and/or structures that allow electrical signal connections. For example, for the microstructure shown in fig. 5(a), the fabrication method can be described by the two mask method shown in fig. 5(b), where a thin layer (e.g., 8 microns thick) of photoresist is first patterned using a first mask. After development, the remaining photoresist structure is baked, for example at 150 ℃ for 30 minutes, to deactivate the photoresist during the next development. A thick layer of photoresist (e.g., 20 microns in this example) is then applied and patterned to create an "n" type cavity on the mold substrate. The Ag + PDMS or C + PDMS mixture was then poured into the chamber. After the two layer PR is dissolved, for example with acetone, for about 30 minutes and rinsed, for example with ethanol and deionized water, silane is evaporated onto the sample. The pure PDMS mixture is then poured onto the mold and the sample is placed under vacuum, e.g., for 20 minutes, to ensure that all chambers are filled with PDMS. After curing, the PDMS sheet with the conductive pattern can be peeled off the substrate, the pattern being shown in the last block of fig. 5 (b). The two halves shown in fig. 5(c) were aligned face-to-face under the microscope and bonded together using an oxygen plasma treatment. The resulting three-dimensional microstructure can be seen in the rightmost block of fig. 5 (c). With this structure, electrical signals can be transmitted independently in the X or Y direction without crosstalk. Fig. 5(d) is an example of an exemplary test method for the above-described test specimen. This experiment tested the function of the circuit connection with different electronic components shown in fig. 5 (d). One LED is connected to each line and the light emitted from these LEDs is controlled separately by the Labview program. Since the Ag + PDMS composite has elasticity and good flexibility, the inserted metal pins can be very tightly attached to the patches of the conductive composite, and thus, the electrical connection is very stable. Test results show that such three-dimensional microstructural wiring can be used to compactly connect electronic components located on different layers. Because the entire structure is resiliently flexible, all of the electrical functions of the structure are not affected by a drop, such as an accidental drop from a table or other accident.

Micro-heater fabrication and characterization

PDMS based conductive composites can also be used to fabricate micro-heaters. Fig. 6 shows an example using a PDMS based conductive composite, in which a schematic view and SEM photographs (inset) of the microheater are shown. The spiral pattern of micro-heaters was sealed and supported on a PDMS base and protruded upward from the surface. Since the composite material is electrically conductive, when the two outwardly extending wires are connected to positive and negative voltages, there is a current generation, and therefore a heat generation. In an example, conductive strips are made using various dies having a width of 25 to 100 microns. The height of all the heating bars in the example was 14.4 microns,but other heights may be used. As can be seen from the inset of FIG. 6, the width of the heater strips in the example was 25 microns (for SEM pictures, the microheaters were not encapsulated with a PDMS layer) and the dimensions of the microheaters in this example were about 200X 200 μm². In addition, since the composite material and the base material (PDMS) are rubber-like with good flexibility, the test results showed that the micro-heater could be operated even when the entire chip was slightly bent.

To verify the heating capacity of the microheaters in these examples, thermal images and local temperatures were detected using an Infrared (IR) camera (FLIR System trademark, model Prism DS). An infrared camera is placed directly above the micro-heaters to record thermal characteristics when the micro-heaters are applied with different voltages. By employing this infrared sensing technique, not only is an accurate temperature reading obtained, but also the entire thermal distribution pattern is obtained. By focusing the IR camera onto the central spiral range of the microheater, the relationship between temperature and applied voltage was determined. Fig. 7 shows the results of testing a heater having a 75 micron wide strip, from which it can be seen that the temperature monotonically increases from ambient temperature with increasing applied voltage. This relationship can be well fitted by an exponential curve. When the applied voltage was 2.5V, the maximum temperature reached about 250 ℃. Two actual infrared images of the microheater taken at different voltages are shown in the inset. In the left figure, the heating profile is rectangular with wide thermal temperature, and in the right figure, localized heating profile (bright spot area-400X 400 μm)²) Where the microheater temperature rises to 250 ℃. These heat distribution pictures show that the heated area is much larger than the micro-heater size and that the low temperature zone extends further beyond the heater than the high temperature zone, which is caused by heat conduction. The area of the high temperature zone is relatively small, indicating that the micro-heater can be used for sample annealing or local reactions, such as those performed on biochips and microchemicals.

Fabrication and characterization of flexible thermochromic displays

Another example of using the PDMS based conductive composite of the present invention is for a flexible display device. A flexible display device manufactured using the PDMS based conductive composite material of the present invention may have further advantages of light weight, improved portability, and/or improved durability. [11, 12] many flexible display devices are based on a combination of liquid crystals and polymeric structures. For example, displays with high flexibility can be made using liquid crystals encapsulated in a single pixel in an elastomeric substrate [13] or in a field-induced polymer structure [14 ]. To drive these displays, wires/conductive patterns are indispensable for transmitting control signals. More recently, ultra-low power organic circuits have been implemented. [15] It is reported that conductive traces can be fabricated using electro-and-optical lithography [16, 17] and also direct ink jet printing techniques of conductive compositions [18, 19 ].

Some embodiments of the present invention provide for the design and manufacture of thermally activated displays using films made of thermochromic composites and embedded conductive wiring patterns. Thermochromic powders are materials whose optical properties, such as color, can be adjusted by changing the temperature in a reversibly repeatable manner. The preparation of the material is mainly combined with the reversible thermochromic effect to carry out research [20-22 ]. Due to the accurate, fast and stable nature [23] this material can be widely applied across the range of applications from smart windows, colour filters and temperature sensors [24, 25 ]. Polydimethylsiloxane (PDMS) plays an important role in our thermal display, mainly due to its ideal challenging properties for thermochromic nanoparticles and silver powders. Therefore, thermochromic or conductive polymer colloids can be easily prepared [26 ].

The display of the present invention is based on the use of two materials: (a) thermochromic polymers and (b) conductive particles + PDMS conductive composites described in this specification. Various thermochromic polymers known to those skilled in the art can be used to make the display. In one embodiment, a microencapsulated thermochromic powder (e.g., 3-7 microns in diameter, LIjinkeji co. ltd) may be employed that is, for example, dark green at ambient temperature and white above, for example, 60 degrees celsius. When this powder is mixed with PDMS, e.g. PDMS 2025(dowcon 184) at a particle concentration of e.g. 20% (weight ratio) and thoroughly ground, a dark green liquid-like composite is formed. To prepare the conductive composite, for example, micron-sized silver powder (1.2-2.2 microns) is employed and mixed with PDMS at a silver weight concentration of, for example, 86.3% (by weight). After vigorous stirring, the composite material formed a soft gel mixture. With soft lithography, the conductive composite provides advantages such as easy patterning of the micro conductive wiring and easy integration of the circuit. When the thermochromic composite material is spun at 400rpm over the designed pattern for 18 seconds and cured after a short bake, a thermochromic display, for example, having a thickness of 150 microns is formed. The thermochromic conductive composite exhibits polymer properties with excellent flexibility due to the PDMS matrix. The ease of shaping the conductive pattern provides a great advantage in the design of the display device of the present invention.

Fig. 8 is a three-dimensional picture showing an example structure of a display unit. It is a single layer thermochromic sheet in which a pattern of conductive lines in the shape of a logo is embedded. When a voltage is applied to the two protruding electrodes, the resulting current will cause a localised heating of the thermochromic layer directly above the wire [27 ]]. Once the local temperature is raised to, for example, 60 degrees celsius or above, the color of the thermochromic layer changes immediately, for example, from dark green to white, thereby displaying a white visible image of the indicia. Since the mean thermal diffusivity of the thermochromic composite material is very small, for example, about 2.4X 10^-3cm²s^-1Well controlled localized temperatures will keep the mark edges clear and not blurred by thermal conduction. To ensure accurate and simple control, the conductive pattern of the present example is designed as a series circuit to ensure the same current through the entire path. The local conductance of the pattern can be pre-designed by varying the width of the conductive lines: the lines used to generate heat generally require higher resistance and are therefore designed to be, for example, 100 microns wide; for telexThe other lines of the conductor are wider (e.g., 300 microns) to reduce resistance. The top right inset of fig. 8 is a top view of a 22 mm wide square example display. The thermochromic material completely covers the conductive pattern and shows a uniform dark green coloration. The conductive traces are visible in the bottom view, which is shown in the lower right inset.

An important feature of the display performance is the response time to the applied voltage. Experiments were performed in test samples with a resistance of 80 ohms at ambient temperature, e.g. 20.4 degrees celsius. A charge coupled device camera is used to record the image changes when the thin thermochromic film is subjected to a step function DC voltage. The images are arranged in chronological order, with the most complete and accurate image identified for this example display being recorded and displayed in inset (b) of fig. 9. This image is then defined as the one hundred percent display (clearest). For these experiments, the degree of visualization of the image was determined by using commercially available software (photoshop). The time dependence of the image display at different voltages is plotted in fig. 9. In both cases, the corresponding images are also displayed as shown in the insets (a) and (c). They are clearly inferior to those shown in the inset (b). The left picture of fig. 9 shows the speed of image display after voltage application in this example. It can be seen that the response time decreases significantly as the voltage increases from 6 volts to 14 volts. At a fixed voltage, the display increases with time. When the voltage is higher than 8 volts, the film can obtain a clear image in about 2 seconds. As shown in the right inset (c) of fig. 9, an increase in duration or an increase in applied voltage results in a display that may overheat resulting in a blurred image.

To overcome the problem of overheating, for example, periodic square wave pulse trains with a fixed duty cycle are employed. This can avoid overheating, maintain a desired clear pattern, and also reduce power consumption. To optimize the square wave pulse duration T and voltage V, we performed a series of experiments with the pulse period T fixed at 20 ms. The table in fig. 10 provides the optimum values of the voltages (for the sake of clarity) at different T/T ratios (from 5% to 50%)To the best image). It is clear that the optimum voltage value increases monotonically as T/T decreases. The energy loss as a function of T/T can also be calculated. For example, when the resistivity of the silver-PDMS conductive composite increases by 70% from 22 degrees Celsius to 60 degrees Celsius [28 ]]The resistance of the conductive pattern is 136 ohms in the display mode. The energy consumption W is thus, for example, determined by W ═ V (V)²X (T/T))/R, plotted with a solid line in fig. 10, where the V values used are for the best image. The solid squares are measured values. Very good consistency is seen. These results show that the energy can be reduced to a minimum of 0.13w at T/T of about 40%. When the T/T value exceeds this optimum point, the energy consumption increases rapidly. The lowest power consumption is therefore the result of the competition between pulse duration and applied voltage. While a decrease in T/T is beneficial for reducing energy, the consistency of the optimal V value increases by V²The relationship of (c) offsets this reduction. Based on the above results, in one embodiment, the application of a periodic square wave pulse sequence not only addresses the overheating problem but also reduces energy consumption.

The mechanical properties of the PDMS based thermochromic material and the conductive composite material give the thermochromic display a high degree of flexibility. The thickness of the film is, for example, about 150 microns, so that the film can be bent, folded and twisted at will while maintaining normal display function. Fig. 11(a) is an example of the display wrapped around a post. Once the voltage is applied, the logo image is displayed quickly as shown in fig. 11 (b). With such mechanical flexibility, the thin film thermochromic display of the present invention can be easily adapted to various application environments.

Based on the ease of manufacturing and the simple hierarchical structure, thermochromic displays can have the advantage of reducing the cost of the display unit. The heat pulse control scheme can also provide lower energy consumption, and the light weight and mechanical flexibility can provide additional portability, convenience, and durability. With a matrix-like hot pixel, for example, a programmable image can be generated by digital control.

Fabrication and characterization of microfluidic reaction systems

Another example of the use of the PDMS based conductive composites of the present invention is in microfluidic reaction systems. Flexible display devices fabricated using the PDMS based conductive composites of the present invention may provide further advantages of lighter weight, improved portability, and/or improved durability.

The term "microfluidic chip" is used interchangeably with "microfluidic reaction system" in the present invention and refers to a device that conveniently supports the separation and/or analysis of chemical and/or biological samples on a scale of as little as a few nanoliters or even less. Generally, these chips are formed of several microchannels that may be connected to various reservoirs containing fluid materials. The fluidic material may be driven or displaced in these microchannels throughout the chip by electromotive forces, pumps, and/or other driving mechanisms. These microfluidic devices may use Micro-electrical-systems (mems) elements: such as chemical sensors, biosensors, microvalves, micropumps, microheaters, micro-pressure transducers, micro-flow sensors, micro-electrophoresis columns for DNA, RNA and/or protein analysis, micro-heat exchangers, microchip chemical laboratories, and the like. These microfluidic devices can conveniently mix, separate, and/or analyze fluid samples in an integrated system formed on a single chip. The term "biochip" as used herein refers to a "microfluidic chip" which is primarily used to separate and/or analyze biological samples.

Temperature is a fundamental environmental parameter that can affect many material properties. There are various types of temperature sensors, such as an optical fiber sensor [29] that measures high temperature, an organic thin film transistor sensor [30], and the like. Recent interest in chemical and biological functions in microfluidic chips has focused on temperature control in these systems, since thermal detection and control are important in microreaction and bioprocesses, such as experiments relating to DNA sequencing and cell biology applications [32 ]. In microchips, platinum films are commonly used as temperature sensors [33 ]. Thermal microscopic scanning using fluorescent particles as sensors has also been reported [34 ]. In another approach, an infrared camera is also often used to obtain not only the surface temperature distribution from the image [35], but also to construct a feedback system [36] for temperature control. For these purposes, low cost infrared sensors [37] have been developed.

Polydimethylsiloxane (PDMS) is considered as the main substrate material for microchip fabrication due to its simplicity of fabrication, biocompatibility, and other advantages [38 ]. However, due to its weak bonding property with a metal material, it is difficult to implant a micro temperature sensor in a PDMS chip in a soft lithography fabrication method. In addition, since this material shields the signal from the IR camera, it is difficult to sense the local temperature inside the microchip in a non-contact manner. To address the above issues, the design and preparation of thermochromic micro-color bars is presented in the present invention that provide a localized temperature indicator inside a microfluidic chip that can be optically sensed. Together with the embedded PDMS/silver particle-based microheaters and optical sensors of the present invention [39], another embodiment demonstrates that the local thermal properties of microfluidic chips can be easily monitored and controlled by a feedback electronics system.

To demonstrate the functionality of the method of the present invention, a microfluidic chip for well-known chemical reaction experiments, such as that shown in FIG. 12, was designed. The top right inset is a top view of an image of an exemplary microfluidic chip, 32 mm long and 10 mm wide. The underlying color bars are composed of, for example, six different bars, each made of a specific mixture of pure PDMS and thermochromic particles (e.g., 3-7 micron diameter, Lijinkeji co., Ltd.) [39 ]. The color transition temperatures of each of the 6 bars are arranged in order and in a temperature range (e.g., 30 degrees celsius to 60 degrees celsius). For example, when the temperature exceeds a certain value, the corresponding color bar changes from its starting color to a different color, such as white. Each color bar may be associated with a circle (for optical sensing, see below), an arrow and a number indicating its color transition temperature, as shown in fig. 12. The contrast variation of the color bars is very sensitive to variations in temperature, which can be calibrated by a thermal stage precisely controlled by a thermocouple temperature control system. Micro-heaters (e.g., synthesized using a silver-PDMS composite) having an initial predetermined resistance of, for example, 69 ohms are also embedded in the layer to generate heat in pre-designated areas. Examples of the method of manufacturing the microheater are described above, and we can also see our previous work [40 ]. Microfluidic channels, for example 200 microns wide and 100 microns deep, are located in the upper microchip layer. These channels may have three functional regions: heating zones, temperature detection zones and/or reaction loops (reaction loops). In one embodiment, the heating zone has two symmetrical zigzag channels for heating the chemical solution when two different chemicals (blue A and red B) are injected into the chip. As the two heated fluids flow through the temperature sensing zone, the solution temperature causes the color bar (which is in contact with the microfluidic channel) to change color (see bottom left inset in fig. 12), and its temperature becomes apparent in the process. After flowing through the temperature detection zone, the two chemical solutions are mixed in, for example, a reaction loop, thereby initiating a chemical reaction at the desired temperature.

In another embodiment, a temperature detection and feedback control system for micro-heaters is designed and constructed for precise control of local temperature in a microfluidic chip, as exemplified with reference to the flow chart in fig. 13. A color detector is placed in close proximity to the chip to monitor the color bar area. In one embodiment, a microscope connected to a Charge Coupled Device (CCD) camera is placed directly above the chip to monitor the color bar area. As the color bars change their contrast at different temperatures, their color images are detected and displayed by a color detector, such as a CCD camera, and displayed on a monitor. In one embodiment, a photoconductive cell sensor (e.g., (CdS) (NORP 12, Silonex Inc)) will convert the detected image contrast (calibrated by the standard temperature control system mentioned above) into a digital electrical signal that is input to a feedback system, such as that shown in fig. 13. Those skilled in the art will also recognize that various other photoconductive cell sensors may be used with the feedback system described herein. Thus, for example, if the microfluidic temperature is set at 35 degrees celsius, the sensor (e.g., CdS sensor) will be configured to focus on the circular region of the associated color bar, indicating the sensor sensing region. For example, CdS sensors are sensitive to image contrast; for example, CdS conductivity is high when the sensing region is bright; when the area becomes dark, the conductivity decreases. Accordingly, the sensor detects the color brightness from the sensing area to determine the on/off state of the micro-heater. This is achieved by operating an amplifier that amplifies the signal from the sensor and sends it to a functional comparator (e.g., the red path in fig. 13). The functional comparator determines the output state of the power supply. If a signal is received that is representative of a dark color in the sensing region (temperature below the set temperature), the comparator will generate a trigger signal to turn on the power to the micro-heater, thereby increasing the temperature. When the temperature of the sensing region reaches a set temperature value, the corresponding color bar will become, for example, white and the comparator will cut off the voltage output from the driver. In this way, the feedback system regulates the temperature of the microfluid.

In the case where the desired set temperature should be maintained for a long time, the analog control signal may be converted to digital form and stored in random access memory. The signal selector is then disconnected from the feedback loop and instead receives a control signal from the CPU after the inverse digital-to-analog conversion. In this way, the opto-electric feedback control loop would be used for initial calibration purposes only, with subsequent temperature control independent of the microscope and CCD camera.

Chemical reaction experiments were performed to test the thermochromic color bars and the associated temperature control aspects of the system for functionality. Liquid solutions of 3 mol per liter sodium thiosulfate and 6 mol per liter hydrochloric acid, respectively, were injected into the microchannel by an injection pump at a rate of 0.02 ml/m. When the two chemical solutions are mixed together, a reaction occurs and sulfur (yellow) becomes visible. Thus, on the right side of fig. 14, the visible region of the reaction loop indicates chemical solution that has not reacted, whereas the clearly visible region indicates the presence of sulfur. The intensity of the reaction was observed to increase with increasing reagent temperature, with more sulfur becoming visible in the loop channel. When the CdS sensor was placed on a 30 degree celsius color bar, the reaction did not proceed substantially, and sulfur particles were formed only in the last two loops, as can be seen to the right in fig. 14 (a). But when the temperature was set to 45 degrees celsius, the reaction accelerated and the sulfur became visible after the first loop. A similar situation is observed when the temperature is set to 60 degrees celsius, whereby the reaction proceeds very fast, large sulphur particles being visible almost immediately after mixing. The left image shows that as soon as a given color bar reaches a set temperature, even very small changes in contrast are immediately detected by the sensor and a corresponding output signal is generated to the control system to accurately maintain the heater state. The different reaction results demonstrate the ability of our control system to adjust the temperature in the microreaction to the desired range.

To quantitatively verify the control of temperature, an oscilloscope was used to record the synchronization signal to the micro-heater and the voltage output from the CdS sensor. Fig. 15(a) shows a square wave train driving the micro-heater when the temperature is set at 40, 45 and 60 degrees celsius. It can be seen that at a fixed pulse amplitude, a higher set temperature of the microheater requires a longer pulse duration, and a slightly increased duty cycle. In fig. 15(b), the CdS voltage output (below) (after fitting through the dark blue line) at a set temperature of 45 degrees celsius is compared to the corresponding trigger pulse (above). As the temperature of the color bar increases and the contrast ratio becomes lighter, the sensor resistance decreases, thereby decreasing the voltage output. Therefore, by inverting the output voltage of the CdS sensor shown by the blue line, a temperature variation trend (indicated by the red line) was obtained. It can be seen that once the desired temperature of 45 degrees celsius at point a is reached, the trigger pulse (above) turns off, but the temperature will still continue to rise to peak B and then fall back again to 45 degrees celsius at point C. When the trigger pulse to the heater is turned on at the next pulse, the heater will have a delay to heat the fluid; thus, the temperature drops to point D and then rises again to point E. It can be seen that the voltage from the CdS sensor is a very small value, with a response time measured as-0.7 seconds. Thus, the temperature may remain stable with only small fluctuations due to the response time of the system.

Based on the detailed description of the present disclosure and with reference to embodiments thereof, it will be apparent that modifications and variations are possible, including additions of elements or rearrangements or combinations of one or more elements, without departing from the scope of the invention as defined in the appended claims. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

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Claims (23)

1. A micro-heater comprising a planar structure, a three-dimensional structure, or a combination thereof, the planar structure, the three-dimensional structure, or the combination thereof comprising at least one PDMS based conductive composite, wherein the structure provides electrical conductivity and has mechanical elasticity and flexibility;

wherein the at least one PDMS-based conductive composite is the following: (a) ag + PDMS gel composite material; (b) carbon black + PDMS gel composite; or (c) a combination thereof,

wherein when the conductive composite is an Ag + PDMS gel composite, the Ag/PDMS weight concentration ranges from 83% Ag to 90% Ag, and the average size of the Ag particles ranges from 1.0 micron to 2.2 microns; and

when the conductive composite is a carbon black + PDMS gel composite, the carbon black/PDMS weight concentration ranges from 10% carbon black to 30% carbon black, and the average size of the carbon black particles ranges from 30 nanometers to 100 nanometers.

2. The microheater according to claim 1, wherein the at least one PDMS based conductive composite is a Ag + PDMS gel composite.

3. The microheater according to claim 2, wherein the at least one PDMS based conductive composite is a Ag + PDMS gel composite, wherein the Ag/PDMS weight concentration range is 84% Ag-87% Ag.

4. The microheater according to claim 1, wherein the at least one PDMS based conductive composite is a carbon black + PDMS gel composite.

5. The microheater according to claim 1, wherein the at least one PDMS based conductive composite is a carbon black + PDMS gel composite, wherein the carbon black/PDMS weight concentration range is 15% carbon black-27% carbon black.

6. The microheater according to claim 1, wherein said structure is a rod array, a multilayer wire co-junction or a bridge, contains said electrical conductivity and is mechanically resilient and flexible.

7. The microheater according to claim 1, wherein the structure comprises at least one conductive wiring structure having a minimum dimension of 10 microns.

8. The microheater according to claim 1, wherein the structure is drop resistant.

9. The microheater of claim 1, comprising a heater strip having a width or length of at least 25 microns.

10. The microheater according to claim 9, wherein the maximum local temperature generated by the heating strip can be from ambient temperature to 250 degrees celsius.

11. The microheater according to claim 1, wherein (a) the monolithic structure has mechanical elasticity and flexibility while maintaining a local heating function; (b) the whole structure is anti-falling; or (c) a combination of (a) and (b).

12. A micro-device comprising the micro-heater of any of claims 1-11.

13. A thermal array comprising a microheater according to any of claims 1 to 11.

14. The thermal array of claim 13, wherein the microheaters comprise heating strips having a width or length of at least 25 microns, and the thermal array further comprises a temperature sensing mechanism coupled to the feedback control.

15. The thermal array according to claim 14, wherein said temperature sensing mechanism comprises thermochromic color bars whose colors can be optically sensed.

16. The thermal array according to claim 15, wherein said temperature sensing mechanism comprises at least one thermochromic micro-color bar whose color can be optically sensed, and wherein color detection from said at least one thermochromic micro-color bar is optically monitored, subsequent conduction through said heating bar is controlled by an electro-optical feedback system that stops heating when a desired thermochromic micro-color bar is activated by a desired threshold temperature.

17. A thermally activated display comprising a microheater according to any of claims 1 to 11.

18. A thermally activated display according to claim 17 wherein said structure comprises (a) a thermochromic composite and (b) an Ag + PDMS gel composite; wherein the structure is thermochromic, electrically conductive and flexible.

19. A thermally activated display according to claim 17 wherein said structure comprises (a) a thermochromic composite in contact with (b) a Ag + PDMS gel composite layer.

20. A thermally activated display according to claim 18 wherein the structure is embedded with a pattern of conductors corresponding to a pre-designed pattern of the display.

21. A thermally activated display according to claim 17 wherein said structure is embedded with a plurality of individual conductor patterns, said conductor patterns being positioned in a matrix-like array of individual pixels; wherein each pixel can independently display the same or different color as an adjacent pixel based on the degree of heating supplied to each individual pixel by the wire.

22. A thermally activated display according to claim 20 wherein said structure comprises (a) a thermochromic composite in contact with (b) a Ag + PDMS gel composite layer; wherein the wire pattern is embedded in the Ag + PDMS gel composite layer.

23. A heat-activated display according to claim 22 comprising a layer of Ag + PDMS gel composite having a Ag/PDMS weight concentration of 84% Ag-88% Ag and microencapsulated thermochromic powder as said thermochromic composite layer.