WO2012050938A2

WO2012050938A2 - Wearable tactile keypad with stretchable artificial skin

Info

Publication number: WO2012050938A2
Application number: PCT/US2011/053803
Authority: WO
Inventors: Carmel S. Majidi; Robert J. Wood; Rebecca K. Kramer
Original assignee: President And Fellows Of Harvard College
Priority date: 2010-09-29
Filing date: 2011-09-29
Publication date: 2012-04-19
Also published as: WO2012050938A3

Abstract

A hyper-elastic, thin, transparent pressure sensitive keypad can be fabricated by embedding a silicone rubber film with conductive liquid-filled microchannels. Applying pressure to the surface of the elastomer deforms the cross-section of underlying microchannels and changes the electrical resistance across the affected channels. Intersecting conductive channels form a quasi-planar network within an elastomeric matrix that registers the location, intensity and duration of applied pressure. Pressing channel intersections of the keypad triggers one of twelve keys, allowing the user to input any combination of alphabetic or numeric letters. A 5% change in channel output voltage can be required to trigger a key. In some embodiments, approximately 100 kPa of pressure is necessary to produce a 5% change in voltage across a conductive microchannel that is 20 microns in height and 200 microns in width. The sensitivity of the keypad is adjusted by changing the channel geometry and the choice of elastomeric material.

Description

WEARABLE TACTILE KEYPAD WITH STRETCHABLE ARTIFICIAL SKIN

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims any and all benefits as provided by law, including the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 61/387,740 filed September 29, 2010, which is hereby incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] This invention was made with government support under grant no. DMR-

0820484 awarded by the National Science Foundation. The government has certain rights in the invention.

REFERENCE TO MICROFICHE APPENDIX

[0003] Not Applicable

BACKGROUND

Technical Field of the Invention

[0004] The present invention relates to flexible and wearable computing devices and components. Specifically, the invention relates to a tactile keypad that can be embodied in a flexible artificial skin material for use on arbitrary and dynamic surfaces, such as robots and the human body.

Description of the Prior Art

[0005] Recent developments in wearable computing [1], as well as flexible pressure sensors and circuits [2], [3], have brought the robotics community closer towards the realization of skin-like tactile sensing. Flexibility and stretchability can expand the scope of applications of sensors, particularly towards wearable sensing for which surfaces are arbitrary and dynamic. Pressure sensors and tactile interfaces for wearable electronics and soft robots that can be made elastically soft and remain functional when stretched to several times their natural length can provide a significant improvement over existing rigid sensors and interfaces.

[0006] Stretchable capacitive pressure sensors for tactile sensing and humanoid robots have been demonstrated with elastic insulator layered between conductive fabric [4], [5], [6] and a silicone rubber sheet embedded with thin gold film [7]. Such sensors also operate by continuously supplying electrostatic charge and measuring the electrostatic induction induced by surface pressure [8]. Other recent efforts include resistive sensors composed of elastomer embedded with conductive microparticle filler [9], [10], [11] and ionic liquid [12].

[0007] Various types of highly conductive stretchable materials have been developed for stretchable electronic applications. Many exploit structures such as waves and nets, as in the case of wavy thin metals [13], [14], [15], [16], [17], [18], graphene films [19], and single- walled carbon nanotubes [20] . Stretchable electronics consisting of elastomers embedded with channels of conductive liquid have also been investigated [21], [22], [23], [24]. Liquid conductors eliminate the need for rigid electronics and preserve the natural hyperelasticity of the embedded elastomer. Thus, this technology offers a vast range of applications for which large deformations and pressures might be sustained.

SUMMARY

[0008] The present invention is directed to wearable tactile sensors that can be embodied in a thin, flexible elastomeric sheet. The thin, flexible elastomeric sheet can be embedded with a conductive liquid in microchannels that can be used to construct various all compliant and pressure sensitive sensing devices. In accordance with one embodiment of the invention, sensors can be used in a pressure sensing keypad in the form of a thin, highly stretchable elastomer sheet. The sensing devices according to the present invention can be formed of thin, transparent, translucent or opaque, highly flexible materials that can conform to rigid as well as non-rigid and arbitrary surfaces and objects.

[0009] In accordance with some embodiments of the invention, an input device in the form of a functional keypad can be fabricated with flexible sensing elements. In accordance with some embodiments of the invention, the keypad can be composed of a flexible sheet material (e.g., polydimethylsiloxane - PDMS) and embedded with conductive liquid microchannels, filled with a conductive liquid (e.g., non-toxic Eutectic Gallium-Indium - eGaln) and can range from approximately 700 μπι to 5 mm in total thickness. The keypads can be rapidly fabricated via a maskless photolithography technique.

[0010] In accordance with some embodiments of the invention, serpentine patterned microchannels extending along a path can be overlaid perpendicularly, such that the locations of the channel intersections behave as 'buttons.' The path can be a linear path, a non-linear path or an arbitrary path. The serpentine-like pattern of the microchannels allows the highly compliant and stretchable sensors to sense pressure over a larger area. Applying pressure to the surface of the elastomeric sheet deforms at least a portion of the cross-section of the nearby microchannels and changes the electrical resistance of the conductive liquid in the microchannel of one or more channels. The relative change in the electrical resistance of all of the channels within the network can be used to determine the location and intensity of the applied pressure. Enhanced sensitivity of the keypad can be achieved through reduction of the microchannel aspect ratio and increased density of the channel network. The elastomeric sheet according to the invention can be integrated with wearable electronics, human- computer interfaces or robotic systems for soft and stretchable sensing functionality.

[0011] In accordance with some embodiments of the present invention, a transparent, all-compliant, pressure sensing keypad with embedded conductive liquid-filled

microchannels can be formed by bonding together layers of an elastomeric material. The elastomer keypad can include any number of keys, for example twelve keys, such as a telephone keypad. The keypad can be is approximately 700 μπι in total thickness. The embedded rectangular microchannels can be 20 μπι in height and 200 μπι in width, yielding an aspect ratio of 0.1. Applying pressure to the channels changes the electrical resistance of the area of the channels where the pressure is applied. By arranging the channels in a crossing or intersecting configuration, pressure applied in the area of two or more channels allows the location, intensity and duration of the pressure to be determined. In accordance with one embodiment of the invention, perpendicularly overlaid serpentine patterned channels allow the channel intersections to act as keys on a keypad or keyboard. According to the location and duration of pressure, a signal representative of an alphanumeric character can be output from the keypad based device, decoded by a computer system and displayed on a computer monitor in real-time. In accordance with one embodiment of the present invention, the output voltage through a channel must increase by at least a predefined threshold (e.g., 5%) for a key press signal to be valid, which can correspond to a pressure of approximately 100 kPa or more. Sensitivity of the pressure sensing keypad can be predetermined as a function of the channel aspect ratio (lower aspect ratio provided higher sensitivity) and the selection of substrate material properties (softer materials provide higher sensitivity). [0012] These and other capabilities of the invention, along with the invention itself, will be more fully understood after a review of the following figures, detailed description, and claims.

BRIEF DESCRIPTION OF THE FIGURES

[0013] FIG. 1 is a diagram of an elastomer keypad according to an embodiment of the invention.

[0014] FIG. 2A is a diagram of an elastomer keypad system according to an embodiment of the invention.

[0015] FIG. 2B is a diagram of an output signal produced by an elastomer keypad system according to an embodiment of the invention shown in Fig. 2A.

[0016] FIG. 3 is a diagram showing a process for fabricating an elastomer keypad according to an embodiment of the invention.

[0017] FIG. 4 is a diagram of key of an elastomer keypad according to an

embodiment of the invention.

[0018] FIG. 5 is a diagram of key of an elastomer keypad according to an alternative embodiment of the invention.

[0019] FIG. 6 is a diagram of a flexible elastomer keypad positioned on a person's wrist according to an embodiment of the invention.

[0020] FIG. 7 is a diagram of a flexible elastomer keypad positioned in the palm of a person's hand according to an embodiment of the invention.

[0021] FIG. 8A is a diagram of a flexible elastomer keypad according to an embodiment of the invention in an unstressed configuration.

[0022] FIG. 8B is a diagram of a flexible elastomer keypad according to an embodiment of the invention in a stressed configuration.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0023] The present invention is directed to wearable, tactile sensors that can be embodied in a thin, flexible elastomeric sheet. The flexible elastomeric sheet can include microchannels carrying a conductive liquid that can be used to construct various all compliant and pressure sensitive sensing devices. In accordance with one embodiment of the invention, sensors can be used in a pressure sensing keypad in the form of a thin, highly stretchable elastomer sheet. The sensing devices according to the present invention can be formed of thin, transparent, translucent or opaque, highly flexible materials that can conform to rigid as well as non-rigid and arbitrary surfaces. The sensing devices can operate in a stretched or flexed configuration.

[0024] Figure 1 shows a functional keypad 100 according to an embodiment of the invention. In this embodiment, the keypad 100 can include 12 keys 110 formed by embedding a matrix of conductive liquid filled microchannels in a flexible sheet material 104. In accordance with one embodiment of the invention, the flexible sheet can include a patterned arrangement of conductive liquid filled microchannels 112. Each channel 112, labeled Ch. 0 through 6 can be formed in the flexible sheet 104 such that pressure applied to the microchannel causes a change in the resistance of the conductive liquid contained therein. Each of the channels can extend along a predefined path. The path can be a linear path (a shown in Fig. 1), a non-linear path or an arbitrary path. In some embodiments, a single channel can be used as a single sensor to measure or report pressure over a specific portion of the sheet. In other embodiments, such as those shown in Fig. 1, the channels can be arranged in a patterned array wherein at least some of the channels intersect such that two channels can report a change in resistance in response to pressure applied to the area of intersection. In this way, positional information can be determined by monitoring the resistance of each of the channels 112.

[0025] As shown in Fig. 1, the channels can be arranged in a pattern or array of 3 vertical channels, Ch.O, Ch. 1, Ch. 2, each extending along a linear path of the spaced apart rows and 4 horizontal channels, Ch. 3, Ch.4, Ch. 5, Ch.6, each extending along a linear path of the spaced apart columns, that intersect at 12 key locations 110 in the flexible sheet material 104. In this example, the 12 key locations can be labeled similar to a so called telephone keypad. For example, pressure applied to the location labeled "MNO" will result in the increase in the resistance of channels Ch. 1 and Ch. 4 and similarly contact pressure of the "MNO" key can detected by simultaneous increase in the resistance of channels Ch. 1 and Ch. 4. The length of time the key is pressed or number times the key is pressed in rapid succession can be used to select the M, N or O character. In some embodiments, capital and small letters can be selected in this fashion.

[0026] In accordance with the various embodiments, the flexible sheet 104 can be formed from any flexible material, including for example, silicone materials, rubber materials, (e.g., EcoFlex0030 and EcoFlex0050, SmoothOn, Easton, Pa; PDMS, Dow Corning, Midland, MI; P-20 and GI-1120, Innovative Polymers, Saint Johns, MI; Tap Platinum Silicone System, Tap Plastics, CA) and soft polyurethane materials (e.g., Dragon Skin, SmoothOn, Easton, PA; IE-35A, Innovative Polymers, Saint Johns, MI). In accordance with the various embodiments of the invention, the thickness of the flexible material can be determined by the application of the sensing device, where thicker layers can be used for more demanding physical applications (large stretching and deformation forces) and thinner layers can be used for more sensitive and less physically demanding applications. The microchannels can be formed by molding or etching the microchannel patterns into layers of the flexible sheet material 104. The layers can be bonded together to form the flexible sensing device, where the microchannels can be formed adjacent the intersection of two layers of material. In accordance with one embodiment, the flexible sheet material can be approximately 700 μπι in total thickness. In other embodiments, the sheet material can range from approximately less than 50 μπι to more than 5 mm in total thickness. In this embodiment, three layers can be used to provide two layers of microchannels, one layer for the pattern or array of 3 vertical channels, Ch.O, Ch. 1, Ch. 2 that form the columns and another layer for the pattern or array of 4 horizontal channels, Ch. 3, Ch.4, Ch. 5, Ch. 6 that form the rows. In addition, labels, legends, symbols or other indicia can be painted or screened onto the flexible material, such as shown in Fig. 1 to provide a user with an indication of where to press and what input the pressed location might generate.

[0027] Each of the microchannels can be filled with a conductive liquid, for example, a non-toxic Eutectic Gallium- Indium (eGaln, BASF) based material, a Galinstan based materil, an ionic liquid, or any other conductive liquid. The dimensions of the microchannels determine the dimensions of the conductive liquid and the electrical characteristics of the channel at rest and when pressure is applied. Depending on the application and the force sensing requirements, the microchannel can be in the range from less than 200 μπι to more than 1000 μπι wide and in the range from less than 10 μπι to more than 25 μπι high. The sensitivity of the sensors in the keypad can be enhanced by reducing the microchannel aspect ratio (height to width). In some embodiments of the invention, the microchannels can be straight or linear microchannels. In some embodiments, a user interface can be formed by mesh of linear microchannels extending along intersecting rows and columns, where the spacing between the rows and columns can be selected to define the positional sensitivity of the sensing device. In other embodiments of the invention, the microchannels can be formed in patterns, such as the serpentine pattern shown in Figs. 1 - 8. Other patterns, such as circular or spiral patterns can also be formed in the elastomer substrate. The serpentine pattern provides that several portions of the microchannel are close together and allows that the microchannel can become compressed in more than one position along its length, to increase the sensitivity of the sensor in a specific location.

[0028] Figure 2A shows a tactile keypad system 200 according to an embodiment of the invention. The tactile keypad system 200 can include a keypad 100 connected to a voltage source 202 and a data acquisition device DAQ 210. The DAQ 210 can be connected to computing device 220 or other device that can receive keypad based input. In this embodiment, one end of each channel of the keypad 100 can be connected to a ground signal 108, 208 and the other end of each channel can be connected to the DAQ 210 and the voltage source 202 through a resistor 204. In this configuration, each channel can be part of a voltage divider and the application of pressure at a specific location causes an increase in resistance in the correspond channel increasing the voltage read by the DAQ 210 for that channel. In response to each key press, the DAQ 210 can output one or more numbers or symbols indicating the channels that responded or one or more numbers or symbols indicating the key that was pressed. In addition, the DAQ 210 can also output an indication of the level of pressure applied and the duration in time of the key press. Alternatively, the DAQ 210 can output one or more signals representative of the voltage or resistance of each channel, wherein a key press is indicated by a change in the output signal for one or more channels. The computing device 220 can include a central processing unit (CPU) and associated memory. The memory can store one or more computer programs or software including, for example, a basic input/output system (BIOS), an operating system and application programs. The software can be programmed to receive signals from the DAQ 210 and interpret or decode those signals to represent the different keys or symbols input by pressing areas of the keypad.

[0029] In accordance with one embodiment of the present invention, a keypad, as shown in Figure 1, can be fabricated with perpendicular serpentine channels for which the feature height is 20 μπι and the width of the channels is 200 μπι, yielding an aspect ratio of AR = 0.1. The serpentine pattern can be used to increase the effective width of the sensing channels and allow a change in local pressure to be detected over a larger area. As discussed by Park, et al. [24], pressure sensitivity of conductive liquid microchannels embedded in a silicon elastomer matrix can be determined by the elastic modulus of the matrix and the aspect ratio of the channels. In accordance with the invention, the lower the aspect ratio of a rectangular microchannel, the greater the sensitivity of the pressure sensor. The accordance with some embodiments of the invention, a channel height of 20 μπι can be used to increase repeatability of the fabrication process, and a channel width of 200 μπι can be used to increase transparency and overall aesthetics of the device.

[0030] In accordance with one embodiment of the invention, the ends of the eGaln filled channels can be wired to a DAQ 210 breadboard (e.g., Measurement Computing USB- 1208LS) for producing keypad system 200 output. As shown in Figure 2A, the circuit can be composed of voltage dividers, where the applied voltage is 2 V and the initial voltage read by the DAQ 210 can be in the range of 0.9-1.1 V (depending on the channel). In accordance with this embodiment of the invention, each of the sensors formed by the conductive channels, or sensors, can be connected in series with a 10 Ohm resistor, and it can be presumed that each of the sensors start out with a resistance of approximately 10 Ohm.

[0031] Pressing each sensor increases the output voltage, V_out , across that channel.

In order to eliminate noise in the voltage signal, a threshold (e.g., 2%, 3%, 4% 5%, 10%, 15%,) can be set, such that when the output voltage of the channel increases by more than the threshold, the key is interpreted to have been pressed. The threshold can be selected to avoid noise or false signals such as changes due to changes in temperature. For example, for an initial voltage readout of 1 V, the key gets triggered when the voltage increases to 1.05 V or greater. According to this example and the governing equation of a voltage divider,

R out

out y initial (1)

R i,nitial + R out

where V_out is the output voltage of the sensor in the voltage divider, Vmitiai is the initial voltage of the sensor in the voltage divider, Rinitiai is the initial resistance of the channel and Rout is the increased resistance of the channel after the key is pressed. In accordance with one embodiment of the invention, a 5% increase in voltage corresponds to approximately a 10% increase in resistance. A computer program or software code can be developed (for example, using the Data Acquisition Toolbox in MATLAB R2010a, The Math Works) to receive the voltage signals and determine whether a key is pressed. In accordance with this embodiment of the invention, the twelve-key keypad 100 can perform similarly to a mobile phone keypad. The functionality of the keys 110 of the keypad 100 is shown in Figure 1. By putting pressure on a key 110, two intersecting channels each output a change in resistance, thus registering that the key 110 was pressed and displaying the corresponding letter on a computer screen. The keys 110 can be used to produce multiple letters by either holding down a key or pressing the key in succession (<2 seconds) one could toggle through the available letter options. In addition, by waiting for a period greater than two seconds, a new letter could be inscribed.

[0032] In accordance with the invention, changes in electrical resistance and output voltage of the embedded, liquid-filled conductive microchannels can be sensed and measured, resulting in real-time display of an alphabetic message on a computer 220 monitor. In Figure 2B, the base voltage of each channel was normalized after data collection, such that only a relative change in voltage is relevant to the plot. The keypad system 200 was used to type the phrase 'HELLO WORLD.' In this embodiment, the change in voltage, and hence applied pressure, lasts over varying durations and denotes the toggling of letters specific to the key being pressed. For example, to achieve the letter 'L', which is the third letter for a key as seen in Figure 1, pressure can be maintained on the key for approximately three seconds. Alternatively, to obtain the letter 'D', which is the first letter for a key, the pressure duration can be less than one second.

[0033] In accordance with the invention, using a scale (ScoutPRO 6000g, OHAUS), the force necessary to increase Vout by 5% - 10% and trigger a key can be measured. In one embodiment, the force was measured to be about 10 N of force. In addition, a typical fingerprint area can be estimated to be ~1 cm , and thus the total pressure applied can be estimated to be approximately 100 kPa. In contrast, normal keyboard typing pressure can be estimated to be on the order of 5 - 10 kPa [25]. Trigger pressure, or the pressure required to change the output voltage of a channel by 5%, can be reduced by decreasing the aspect ratio of the channels or implementing softer materials. For example, EcoFlex silicon rubber (0030, SmoothOn) has an elastic modulus of -125 kPa (as opposed to 1 to 2 MPa for PDMS) can be used to reduce the trigger pressure. Alternatively, the threshold value can be decreased based on the signal/noise ratio of the sensors and the resolution of the analog/digital converter used in the DAQ 210.

[0034] Figure 5 shows a microchannel of a key of a keypad with a height of 10 μπι and a channel width of 1000 μηι, yielding an aspect ratio of 0.01. Park, et al. [24], derived that for a channel embedded near the surface of the elastomer, o (ΐ _ 2(ΐ - _ν ² )-¾

Ρ'

where v is Poisson' s ratio, p is the applied pressure, p ' is a characteristic pressure defined by p ' = Eh/w, E is Young's modulus, h/w is the aspect ratio of the channel, and AR/Ro is the percent change in electrical resistance of the microchannel. By reducing the aspect ratio by an order of magnitude (i.e. from 0.1 to 0.01) the relative change in electrical resistance can be increased by nearly 300%. Thus, a lower aspect ratio can be used to increase the sensitivity of the channels' pressure sensing capabilities.

[0035] The mechanical limits of stretchability for both functionality and failure of the keypad can be tested in pure tensile loading using an Instron Materials Testing System (model 5544A) in tensile extension mode. The functionality of the keypad can be considered to be maintained as long as all of the channels remained conductive. By this standard, the keypad was found to fail mechanically before failing in functionality. In some embodiments of the invention, the sensors can became slightly more (or less) responsive to pressure under the stretched condition. In some embodiments of the invention, the keypad device can be stretched to greater than 350% before mechanical failure.

[0036] Figure 3 shows a process for fabricating a sensing device, such as the keypad

100 of Figs. 1 and 2A, according to some embodiments of the invention. The keypad of Figs. 1 and 2A can be formed by combining 3 layers of PDMS material. Two of the layers can be patterned to form microchannels that make up the signal channels. The pattern of

microchannels can be formed in one layer and then covered by an unpatterned smooth layer or the unpatterned smooth back surface of another layer. In this way the layers can be built up to form the sensing device. The patterns can be formed by molding or etching the layers of flexible sheet material. In one embodiment, the layers of flexible sheet material can be formed by spin-coating the flexible sheet material onto a silicon positive relief.

[0037] In accordance with some embodiments of the invention, the tactile keypad devices according to the invention can be fabricated by using a photolithography process, as shown in Figure 3. The choice of photo-resist and spin-rate can be used to determine the subsequent depth of the patterned features formed on the silicon wafer positive reliefs. In accordance with one embodiment, the photoresist (e.g., SU-8 2050, MicroChem, Newton, Ma) can be spun onto a clean wafer at 500 rpm for 10 seconds (spread), followed by 4000 rpm for 30 seconds (spin). The wafer can then be placed on a hot plate at 65 degrees C for 3 minute and 95 degrees C for 6 minutes. The coated wafer can then be patterned via a maskless, direct-write laser exposure utilizing a diode-pumped solid-state (DPSS) 355nm laser micromachining system. This method of exposure can be used to form channels as small as 25 μπι in width and to produce arrays of channels having edges separated by little as 50 μπι, in order to produce sensors with densely packed fine features. After exposure, the wafer can be post-baked for 1 minute at 65 degrees C and 6 minutes at 95 degrees C and finally the photoresist can be developed in SU-8 developer for 5 minutes.

[0038] In accordance with one embodiment of the invention, one or more silicon wafers patterned with photoresist can be used to mold three PDMS layers that can be used to form the keypad device 100. A hydrophobic monolayer can be applied by vapor deposition to inhibit adhesion between the silicon molds and subsequently cured PDMS. For example, the wafers can be placed in an evacuated chamber (-20 mTorr) with an open vessel containing a few drops of Trichloro(lH,lH,2H,2H-perfluorooctyl)silane (Aldrich) for >3 hours. Next, the PDMS (Sylgard 184; Dow Corning, Midland, MI) can be spin-coated in liquid form (10:1 mass ratio of elastomer base to curing agent) onto the silicon mold to produce a thin elastomer film of tunable thickness.

[0039] In accordance with some embodiments of the invention, the keypad device

100 shown in Figures 1 and 2A can form of two patterned PDMS layers with a thickness of approximately 250 μπι (e.g., approx. spin-coating speed of 300 rpm) and a third and bottom layer of unpatterned PDMS having a thickness of approximately 200 μπι (e.g., by spin- casting PDMS on a blank silanized wafer at 400 rpm). Each of the PDMS layers can be cross-linked in the molds by oven-curing at ~ 60 degrees C for 30-40 minutes. The layers can be manually removed from the molds and bonded together via oxygen plasma surface treatment (e.g., conducted at 65 watts for 30 seconds). The patterned layers can be bonded together first, overlaying the channel patterns with the desired alignment.

[0040] In accordance with one embodiment of the invention, to facilitate subsequent filling of the channels within a thin device using conventional tubing and syringe dispensing, small blocks of PDMS can be adhered to the device inlet and outlet locations on top of the cured device. Adhesion can be achieved by heating the cured PDMS on a hot plate at 100 degrees C, applying a small amount of uncured PDMS onto the inlet and outlet locations, and then pressing the PDMS blocks into the uncured droplets. These blocks can be allowed to fully cure on the hotplate for approximately 30 minutes. In the final step, small holes can be formed in the adhered inlet and outlet blocks connecting the small holes to the ends of the microchannels.

[0041] After the PDMS blocks are adhered to the top, the device can be bonded to the unpatterned layer. The sealed microchannels can be filled with the eGaln conductive liquid and the filling blocks can be manually cut off. This step is largely enabled by the high surface energy of eGaln, which allows a channel to be filled and remain in tact even if the channel inlet and outlet are exposed (assuming no external pressure is applied). The eGaln- filled channels can be wired (inserting wires into the holes at the ends of the channels) and re- sealed with a final coating of PDMS. The final device thickness can be in the range from approximately 500 μπι to more than several mm.

[0042] In an alternative embodiment, the device can be formed without the PDMS blocks and syringes with needles can be used to inject the eGaln conductive liquid into the microchannels while sucking the air out at the same time.

[0043] Figure 3 shows a keypad fabrication process according to one embodiment of the invention. Initially, silicon wafer based master positive reliefs can be prepare using a maskless, direct-write laser exposure photolithography process. In element (a), a thin film of PDMS (250 mm) is spin-coated (300 rpm) onto a patterned silicon mold, thermally cross- linked, and manually peeled from the silicon master. In element (b), a thin film of PDMS is spin-coated onto a second silicon master and thermally cured. In elements (c-d), the existing PDMS layers are thoroughly bonded together via oxygen plasma treatment. In element (e), small blocks of PDMS are adhered to the channel pattern inlet and outlet locations on top of the cured PDMS layers. In elements (f-g), the existing structures are manually cut and peeled from the silicon wafer, and holes are stamped through the PDMS blocks at the channel inlets and outlets. The channels are then bonded to a final, unpatterned PDMS film by means of oxygen plasma treatment. The unpatterned film can be 200 mm in thickness and can be achieved by spin-coating PDMS onto a blank, unpatterned wafer at 400 rpm. In element (h), conventional microfluidic tubing can be connected at the channel inlet locations, and a syringe can be used to fill the microchannels with a conductive liquid, e.g., eGaln. In element (i), the PDMS blocks can be cut and removed from the keypad device, and conductive wires can be inserted into the channel ends. The channels can be sealed and wires can be held in place with a final coating of PDMS. In element (j), the final device can be peeled from the silicon wafer and used.

[0044] Figure 4 shows a diagram of a single key of the keypad 100 according to an embodiment of the present invention. In this embodiment, the microchannels of the keypad 100 can be 20 μπι high by 200 μπι wide (aspect ratio AR = 0.10) and formed in a serpentine pattern where the loops of microchannel can be spaced apart by up to 2 mm or more. Figure 5 shows a diagram of a single key of the keypad according to an alternate embodiment of the present invention. In this embodiment, the microchannels of the keypad can be 10 μπι high by 1000 μπι wide (aspect ratio AR = 0.01) and formed in a serpentine pattern where the loops of microchannel can be spaced apart by less than 1 mm. Depending on the desired size of the keys, response and sensitivity, and the application of the keypad, the spacing between the loops of microchannel can be less than 1 mm or greater than 3 mm.

[0045] Figure 6 shows a diagram of a keypad 100 with wires extending from the ends of the channels according an embodiment of the invention. In this embodiment, the flexible substrate material 104 enables the keypad 100 to conform to a person's forearm and be operated by the application of pressure from a finger or a stylus. Figure 7 shows a diagram of a keypad 100 with wires extending from the ends of the channels according an embodiment of the invention. In this embodiment, the flexible substrate material 104 enables the keypad 100 to conform to the palm of a person's hand and be operated by the application of pressure from a finger or a stylus.

[0046] Figures 8 A and 8B show a diagram of an elastomeric keypad 100 with wires extending from the ends of the channels according an embodiment of the invention. Figure 8 A shows the elastomeric keypad 100 in its relaxed configuration prior to the application of any deforming stress. Figure 8B shows the elastomeric keypad 100 in a deformed

configuration under tensile stress.

[0047] The stretchable sensors and fabrication technology according to the various embodiments of the present invention can be applied to create other types of functional electronics and sensing. A pressure sensitive keypad is only one of many applications for this all-compliant sensing technology. In other embodiments of the invention, the sensors according to the present invention can be used to integrate hyper-elastic pressure sensors with soft robotics, artificial skin, soft orthotics and integrated circuitry. Using the laser photolithography process, which can produce microchannel features in the range of 25 μηι resolution, the sensing elements and electrical connections of the sensors according to the present invention can be further miniaturized.

[0048] Other embodiments are within the scope and spirit of the invention. For example, due to the nature of software, functions described above can be implemented using software, hardware, firmware, hardwiring, or combinations of any of these. Features implementing functions may also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations.

[0049] Further, while the description above refers to the invention, the description may include more than one invention.

Claims

What is claimed is:

1. An elastomer keypad comprising:

an elastomer sheet material having two or more channels;

each channel including at least one microchannel extending along a path and the at least one microchannel including a conducting liquid, the conducting liquid having a rest resistance in a rest state; and

wherein an applied pressure to one surface of the elastomer sheet material causes the resistance of the conducting liquid in at least one channel to change.

2. An elastomer keypad according to claim 1 wherein at least one microchannel is formed in a pattern.

3. An elastomer keypad according to claim 2 wherein the pattern is a serpentine pattern.

4. An elastomer keypad according to claim 2 wherein the pattern is a circular pattern.

5. An elastomer keypad according to claim 1 wherein at least one channel extends along a linear path.

6. An elastomer keypad according to claim 1 wherein at least one channel extends along a non-linear path.

7. An elastomer keypad according to claim 1 wherein the elastomer sheet material include a first channel extending along a first path and a second channel extending along a second path; and

the first path intersects the second path.

8. An elastomer keypad according to claim 1 wherein the elastomer sheet material include a first channel extending along a first path and a second channel extending along a second path; and

the first path is perpendicular to the second path.

9. An elastomer keypad according to claim 1 wherein the conducting liquid includes a eutectic gallium indium material.

10. An elastomer keypad according to claim 1 wherein the elastomer sheet material includes one or more of silicone, rubber, and PDMS materials.

11. An elastomer keypad system comprising:

an elastomer keypad including an elastomer material having two or more channels, each channel including at least one microchannel filled with a conducting liquid having a rest resistance in a rest state and each channel extending along a path;

a voltage source;

a data acquisition device connected to at least one of the channels and adapted to receive a keypad signal when an applied pressure to an area of the elastomer sheet material causes the resistance of the conducting liquid in the at least one channel to change and produce a key pressed signal as a function of the keypad signal.

12. An elastomer keypad system according to claim 11 wherein the at least one channel is connected through a resistor to the voltage source and forms part of a voltage divider; and wherein an output of the voltage divider is connected to the data acquisition device such that when the pressure is applied to an area of the elastomer sheet material, the output of the voltage divider produces a change in voltage and the change in voltage is measured by the data acquisition device and the data acquisition device produces a key pressed signal as a function of the change in voltage.

13. An elastomer keypad system according to claim 12 wherein the key pressed signal includes an indication of a change in voltage an output of the at least one channel.

14. An elastomer keypad system according to claim 11 wherein the data acquisition device includes a connection configured to connect the data acquisition device to a computing device.

15. An elastomer keypad system according to claim 11 wherein the data acquisition device registers a key being pressed when the change in resistance of the conducting liquid is greater than a threshold value.

16. An elastomer keypad system according to claim Γ wherein the elastomer keypad includes a first channel extending along a first path and a second channel extending along a second path; and

at least one key formed by an intersection of the first channel and second channel.

17. An elastomer keypad system according to claim 16 wherein a simultaneous change in resistance of the first channel and the second channel indicates a key press at the key corresponding to the intersection of the first channel and the second channel.

18. An elastomer keypad system according to claim 11 wherein the voltage source produces a voltage in the range from 1 to 10 volts;

19. An elastomer keypad system according to claim 11 wherein the voltage source produces a voltage of approximately 2 volts.

20. An elastomer keypad system according to claim 11 wherein the conducting liquid includes a eutectic gallium indium material.

21. An elastomer keypad system according to claim 11 wherein the elastomer material includes one or more of silicone, rubber, and PDMS materials.

22. A method of fabricating an elastomer keypad comprising:

forming at least one patterned mold having at least one positive relief representing a microchannel; applying a first thin film of elastomer material on a first patterned mold; peeling the first thin film of elastomer material from the first patterned mold, the positive relief forming at least one microchannel in the first thin film;

applying a base thin film of elastomer material on a first unpatterned mold;

peeling the base thin film of elastomer material from the first unpatterned mold; bonding the base thin film to the first thin film; and

filling the at least one microchannel with a conductive liquid.

23. A method of fabricating an elastomer keypad according to claim 22 comprising:

applying a second thin film of elastomer material on a second patterned mold;

peeling the second thin film of elastomer material from the second patterned mold, the positive relief forming at least one microchannel in the second thin film; and

bonding the second thin film to the first thin film.

24. A method of fabricating an elastomer keypad according to claim 22 wherein the first thin film is applied to the first patterned mold by spin coating the elastomer material on to the first patterned mold.

24. A method of fabricating an elastomer keypad according to claim 22 wherein the second thin film is applied to the second patterned mold by spin coating the elastomer material on to the second patterned mold.

26. A method of fabricating an elastomer keypad according to claim 22 further comprising thermally cross-linking the elastomer material of the first thin film prior to peeling first thin film from the first patterned mold.

27. A method of fabricating an elastomer keypad according to claim 22 further comprising thermally cross-linking the elastomer material of the second thin film prior to peeling second thin film from the second patterned mold.

28. A method of fabricating an elastomer keypad according to claim 22 wherein filling the at least one microchannel with conductive liquid includes injecting a conductive liquid into the at least one microchannel.

29. A method of fabricating an elastomer keypad according to claim 22 wherein the conducting liquid includes a eutectic gallium indium material.

30. A method of fabricating an elastomer keypad according to claim 22 wherein the elastomer material includes one or more of silicone, rubber, and PDMS materials.