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A Spider-Web-Like Highly Expandable Sensor Network for Multifunctional Materials

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By Giulia Lanzara , * Nathan Salowitz , Zhiqiang Guo , and Fu-Kuo Chang

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The skin of living animals is an inspiration for the next generation

of materials, devices, and structures. Human skin is sensitive to pressure, strain, and temperature; [ 1 , 2 ] dolphins or bats drastically reduce drag by sensing fl uid fl ow [ 3–5 ] and adapting either their skin shape [ 6 , 7 ] or rigidity; [ 8 ] and snakes use their skin to detect vibrations. [ 9 ] The fundamental common factor in these examples is that the living tissue is integrated with a network of distributed nano- or microscale sensors and actuators. To mimic such sys-tems, novel materials and devices, such as paperlike displays, [ 10–12 ] biomedical electronics, [ 13 ] intelligent textiles, [ 14 ] artifi cial skin, [ 15–17 ] morphing materials, [ 18 ] robotics, wired or wireless sensor net-works, and structural health monitoring systems, [ 19 ] should be integrated with distributed networks of miniaturized sensors and advanced electronics that span large macroscopic areas. The core problem in producing such networks is the current inability to integrate millions of microscale devices in precise predefi ned locations on a large macroscopic scale and to do so at reasonable cost. Common technologies for large-area electronics are based on manually assembling numerous individual and relatively large components into networks that cover large areas, but this is inva-sive to the material and/or structure and prohibitively expensive. New technologies like fl uidic self-assembly of silicon dies, [ 20 , 21 ] stretchable silicon, [ 22 , 23 ] stretchable metal interconnects, [ 24–28 ] or highly stretchable two-dimensional silicon wired networks [ 29 ] show promise, but have signifi cant drawbacks including wiring, [ 20 , 21 ] integration into structures, [ 20 , 21 , 29 ] invasive nature, [ 22 , 24 , 25 , 27 , 28 ] low expandability, [ 22–28 ] lack of mechanical fl exibility, [ 29 ] handling dif-fi culties, [ 20 , 21 , 29 ] and fi nally low temperature resistance, [ 22 , 23 ] which limit the types of applications. An effective multiscale method is required to overcome these issues. An innovative technology [ 30 ] consists of fabricating highly expandable, fl exible, and conductive substrates by forming a polyimide layer as a network of micros-cale wires and nodes. Here, we generalize the major fi ndings and extend this multiscale and cost-effective concept, [ 30 ] which can be used to integrate thousands of wired or wireless sensing elements into macroscopic material noninvasively. The key concept relies on the fact that by properly removing unnecessary material (99.7%) from a microscale polyimide thin fi lm hosting a network of thou-sands of devices, the network can be greatly expanded at low strain levels to cover large macroscopic areas. The expansion is character-ized by area dilatations that are several orders of magnitude higher than those demonstrated to date in the literature. [ 15 , 22–28 ] The ratio

© 2010 WILEY-VCH Verlag GmAdv. Mater. 2010, 22, 4643–4648

[∗] Dr. G. Lanzara , N. Salowitz , Z. Guo , Prof. F.-K. Chang Department of Aeronautics and Astronautics Stanford University Stanford, CA, 94305 (USA) E-mail: glanzara@stanford.edu

DOI: 10.1002/adma.201000661

between stretched and unstretched areas (area dilatation) can be of the order of 10 2 up to 10 3 . The expanded network resembles a giant, ultralight and fl exible spider web that spans a large area and is barely visible to the naked eye. Very small and conductive (if necessary) microwires run long distances and connect an array of thousands of micronodes which house the sensing elements in a complex network. Due to the light weight and small size of the microwires and micronodes, the web is tremendously nonin-vasive once integrated into a wide range of materials of any size, 3D shape, and rigidity. The integration is proven here with several examples such as composite plate, folding paper, human hand, and fl exible polymer, which suggests the wide range of potential applications as well as the capability of the web to resist harsh manufacturing environments that may occur during the integra-tion processes (such as high temperature, high pressure, or fl uid fl ow). The functionality is demonstrated by fabricating temperature sensors on the web and metallized microwires that may also work to sense strain, extension, and sensor positioning (e.g., for smart textiles, artifi cial skin, and composites). The reliability of the web during the in-service life of a hosting material is also presented. Experimental, numerical and theoretical studies are reported in this contribution as a proof of the proposed multiscale approach and of its wide applicability.

A schematic of our approach is described in Figure 1 . A thin polyimide fi lm (step 1) which is heat-resistant up to 550 ° C is functionalized and patterned at the microscale with a high-density array of micronodes interconnected by highly extend-ible microwires (step 2). Unnecessary material is then removed from the fi lm (step 3) leaving a discrete polyimide-based mem-brane (substrate) of interconnected microscale elements. This step not only reduces the amount of material that is ultimately integrated into a hosting material, thereby negating impact, but is also necessary to allow the microwires extendibility at low strain levels together with nearly undeformed nodes. The func-tional substrate is then expanded from the micro- to the mac-roscale (step 4) achieving a fi nal area coverage several orders of magnitude higher than the original area. The expanded sub-strate is then bonded to or embedded into macroscopic mate-rials or structures (step 5) of any shape and rigidity.

The fi nal area coverage, the expansion level, nodal density, node size, internode distance, wire size, and material reduction level are all related and can be optimized for specifi c applica-tions. In this paper metallized substrates (down to 25 μ m in thickness) with a network of thousands of micronodes with diameters as low as 200 μ m and microwires with widths as low as 4 μ m that are capable of an area dilatation of 265 and a con-sequent 99.7% material reduction are demonstrated.

Several important issues were studied to investigate the feasibility of the proposed approach and can be summarized as: 1) realization of a noninvasive and highly expandable fl exible

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Figure 2 . The extendible microwire. a) Extendible microwire design and extendibility concept. In the metallized microwire case strain/elongation and node placements can be monitored by measuring the electrical resistance vari-ation during the extension process. b) Deformation modes of a 4 μ m wide, 50 μ m thick Kapton microwire while being extended in air under an optical microscope (by fi xing one of the two nodes and applying a load to the other). c) Optical microscope image of a microwire portion fully extended on a sub-strate (1600% L / L 0 ). d) F / A vs L / L 0 curve of a 16 μ m wide, 50 μ m thick micro-wire tested in a quasi-static load control mode. Inset: microscope image of the loop of a 4 μ m wide microwire which is undamaged and planar after extension. e) Maximum tensile strain (computational study) in the loops and in the linear segments during the extension process. f) Experimentally measured Δ R / R vs L / L 0 for a 65 μ m wide, 50 μ m thick polyimide microwire coated with a thin Au layer. g) ε max vs L / L 0 calculated from the experimental Δ R / R (assuming Au to have a Poisson ratio ν = 0.42) and the ε max vs L / L 0 template (numerical) used to detect L / L 0 with the measured ε max . The slight variation between the two trends is due to nonlinear local instabilities when w / t > 1 (here w / t are 1.3 and 0.08 in the experimental and numerical case, respectively).

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substrate that can easily be expanded to the macroscale at low strain levels in a controlled, uniform, and mechanically stable manner, allowing accurate placement of a network of micron-odes in predefi ned locations at the macroscale; 2) easy and non-invasive integration of the expanded substrate into materials or interstructures and its reliability during the in-service lifetime of a structure or material of any rigidity; and 3) functionalization to demonstrate potential for customization to specifi c applications.

The microwires were identifi ed as the key element to allow a stable and uniform network expansion to areas several orders of magnitude larger than the original area and to position the micronodes in predefi ned locations. In the case of wired applica-tions, or wherever metallized microwires can be used, the micro-wires were identifi ed as an additional tool that could sense the microwire extension level, measure micronode locations at the macroscale, and monitor the electrical resistance of the micro-wire during the in-service life of a structure. These functions are potentially useful for strain-gauge-type measurements or for calibration purposes. The basic microwire design is made from a thin polyimide fi lm monolithically connecting two micronodes (see Experimental Section for the fabrication process [ 30 ] ). The fi lm is formed as a series of curved regions (loops) and linear segments that run along the microwire length that are folded into the shape shown in Figure 2 a . When a load is applied to the nodes, the microwire unfolds (“extends”) in the direction of the applied load with the 2D deformation modes shown in Figure 2 b (see also Supporting Information) and with the trend shown in Figure 2 d, which is representative of the force per unit area ( F / A ) vs the stretching ratio ( L / L 0 where L is the increasing nodal sepa-ration during the extension process and L 0 the initial internodal distance). The response is linear up to 750% extension (threshold point) and non-linear for larger values (due to stress/strain con-centrations in the curved regions). The microwire extension is permitted by the curved regions opening in-plane and the simultaneous in-plane rotation and alignment of the linear seg-ments in the direction of the applied load (up to 1600% linear extension). Simultaneously, the micronodes move with a stable translational motion. Microwires were extended using the same procedure as described for the network expansion (see Experi-mental Section). Experimental and theoretical studies have shown that the ratio of the microwire width ( w ) over thickness ( t ) has to

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be kept lower than 1 to avoid nonlinear local instabilities which would give rise to unstable 3D deformation modes. The micro-wire length increase is mostly related to an “unfolding” effect instead of a “material stretching” leading to low tensile strain levels localized in the inner part of the loops (computational model in Figure 2 e and described in the Experimental Section). The inner part of the curved regions is subjected to local strain

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concentrations (inset) up to three orders of magnitude higher than in the linear segments. The loop trend becomes non-linear for extensions greater than the threshold point. Both maximum strain ( ε max ) curves increase at the same rate above 1400% L/L 0 (dominant “stretching” condition in the material once the loops are signifi cantly opened). The maximum strain along the micro-wire (Figure 2 g) was calculated according to a nonlinear theory [ 31 ] by experimentally measuring the electrical resistance variation (Figure 2 f) of a metal-coated extendible polyimide microwire (Al or Au) and was compared with that resulting from the compu-tational model. It was found that strain levels are below 5.3% with linear extensions of up to 1400% and that no damage or microcracks were observed once fully extended (up to 1600% in Figure 2 c). Node placement at the macroscale is controlled by the microwire extension level (Figure 2 g) and can be detected by monitoring ε max of the microwire while it is being extended, giving the ε max vs L / L 0 template for a specifi c microwire design (from the computational model).

© 2010 WILEY-VCH Verlag GmAdv. Mater. 2010, 22, 4643–4648

Figure 3 . Expandable network and expansion results. a) Image of a fabricatework on its metal side. The network is made of 5041 micronodes (400 μ m nected by 8 μ m wide, 25 μ m thick extendible microwires coated with a thin view of the network described in a) prior to being expanded. The network loomembrane patterned in high density at the microscale. c) A fabricated 256 nopolyimide side is easily held by hand without damaging the network. The netized by 16 μ m wide, 50 μ m thick microwires. d) 3D deformation mode of a neside) captured under a scanning electron microscope. The image shows the fl bility of the network to adapt to 3D complex confi gurations. e) Expanded 504in contrast to a hand to show the physical nature of the membrane once it is expanded 5041 node network (fi nal area coverage of 1 m 2 ).

An expandable sensor network can be realized by designing an array of micronodes connected to extendible microwires. Networks containing up to 5041 micronodes were successfully fabricated and tested ( Figure 3 a and 3 b). The networks can be easily held by hand as in Figure 3 a and 3 c because of both the good mechanical properties of the forming material and the microwire shape, which can partially extend and bend to adapt to new 3D stable confi gurations during handling (Figure 3 d). The peripheral nodes of the network were connected to an external frame through sacrifi cial microbeams. The external frame facilitates the handling process and avoids undesired pre-expansion of the network. The sensor network was then easily expanded, in air or on a supporting substrate, fi rst in one direction and then in the other direction (see Experimental and Supplementary Video). The network expansion is achieved by a simultaneous stable and uniform extension of each indi-vidual microwire interconnecting the micronodes, which reach a planar and predefi ned confi guration and resembles a

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d expandable net-in diameter) con-Al layer. b) Closer ks like a discrete de network on its work is character-twork (polyimide

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spider-web upon full expansion (Figure 3 e and 3 f). Expanded networks were integrated into and onto various materials and objects to prove the enormous potential of the pro-posed design for a variety of applications. The network immediately adapted to the shape of a hand ( Figure 4 a ) and of a paper (Figure 4 b) and was capable of following the 3D deformation modes of these objects. The net-work was successfully integrated into a fl exible polymer (poly(dimethylsiloxane) (PDMS)), withstanding phase changes and fl uid fl ows during the PDMS curing process (Figure 4 c). The network was not affected by bending or torsion loading conditions (Supplementary Video) of the integrated PDMS, allowing for its use in applications that require large deformations. This was also quantitatively proven by testing conductive microwires mounted on a thin Kapton tape. In the latter case, the integrated Kapton tape was folded under pure bending conditions around bars with decreasing radius as shown in the sche-matic in Figure 4 d. Simultaneously, the elec-trical resistance of the extended microwire was monitored by bringing two probes into contact with contact pads connected to the extended microwire. The test results, together with a microscopic inspection of the samples after testing, showed that the microwires were undamaged under extreme bending conditions (fully folded around a needle with a 350 μ m radius). The R / R 0 vs α curves in Figure 4 d represent the response of an integrated Kapton tape while it is being folded around the bars where α is the con-tact angle that defi nes a half portion of the sample in contact with the bar and R 0 is the initial electrical resistance ( α = 0). The larger R / R 0 variation in the needle case was due to the higher strain values (12%)

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Figure 4 . Integration of the expanded conductive network into materials and functionalization. a) Expanded functional network mounted on a hand. b) Expanded functional network (Al-coated) mounted on a rolled paper. c) Expanded functional network (Al-coated) integrated in a fl exible material. An accurate microscopic analysis showed that the microwires and micronodes were undamaged and kept their regular predefi ned pattern after bending or torsion. d) R / R 0 vs α of an extended Au-coated polyimide microwire bonded to a Kapton tape and tested under pure bending conditions by folding it on bars ( r 1 = 4 mm, r 2 = 2.4 mm and r 3 = 350 μ m) with the test procedure depicted in the inset. The resulting tensile strain values once the sample was fully folded were: ε 1 = 1.2%, ε 2 = 2% and ε 3 = 12%. e) Expanded sensor network (Al-coated) surface mounted on a composite plate using the procedure described in the Experimental Section. f) Δ R / R % vs time of a Au-coated extended microwire bonded to a composite plate under tensile fatigue loading conditions. The microwire is subjected to the same strain levels as the supporting composite plate and works as a strain gauge with a calculated 0.9 gauge factor. The plot represents the microwire response after 100 000 cycles, 250 000 cycles, and 400 000 cycles. g) Microscope image of an expandable sensor network integrated with Pt-RTDs built on the nodes and connected by Au-coated polyimide microwires. h) Results of the RTD temperature sensors built on an expandable substrate. The RTD (896 Ω ) was tested through the extended Au-coated microwires (195.1 Ω ), after the network expansion.

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that were reached once the sample was fully folded ( α = 0.5 π ).

An expanded network was also surface-bonded on an alter-nating 0 ° /90 ° pre-impregnated (prepreg) carbon fi ber composite (Figure 4 e) during the plate manufacturing. The network was not affected after being exposed to the harsh manufacturing environ-ment (see Experimental Section) and its reliability under fatigue loading conditions was investigated by testing a functional (extended) microwire bonded to a composite plate and subjected to half a million loading cycles (see Experimental Section). The microwire was undamaged and fully working after testing, and was found to function very well with resistance variation as a “strain gauge”, as shown in Figure 4 f.

The types of network functionalities may be selected depending on the specifi c application. Resistance temperature detectors (RTD), which can be used for a wide variety of appli-cations, were chosen as an example to prove the possibility of incorporating additional functionalities onto an expandable substrate. Figure 4 g shows the functional, expandable sub-strate with gold-coated extendible microwires and platinum RTDs fabricated in the nodes. Once expanded, the sensor network was bonded onto a silicon wafer and tested on a hot plate (temperature range from 24 ° C to 120 ° C). The sensor response in each node was detected through the extended microwires and solely across the RTD. The RTDs were able to detect temperature gradients in the entire testing range with a fi rst-order dependence of resistance on temperature (Figure 4 h).

In conclusion, the major results of the fi rst multiscale method that allows a non-invasive and precise integration of thousands of nano- or microscaled electronic devices and materials into large macroscopic materials were summarized. The designed polyimide-based web can reach expansion levels (26500%) and temperature ranges (550 ° C) several orders of magnitude larger than those demonstrated in the literature, its fl exibility allows the integration of sensor networks into materials of any 3D shape and rigidity, and it withstands even harsh manufac-turing environments (high pressure, high temperature, fl uid fl ow). As functional examples, temperature sensors and strain gauges were successfully built into the web. The functional-ized web was undamaged after it was tested under a wide range of loading conditions (i.e., after integration in fl exible or rigid materials). These results represent a revolutionary approach that has great potential for the extension of nano- or micros-cale devices to the macroscale. This is a major step toward the realization of the next generation of multifunctional materials resembling living systems.

Experimental Section Fabrication : The Kapton HN fi lm, selected as polyimide material, was

cleaned by sonifi cation and cured at 80 ° C. A 100 mm diameter silicon wafer was coated with a 7 μ m-thick photoresist layer. The wafer was used during fabrication to rigidly support the Kapton fi lm, which was bonded to it on a hot plate under high pressure. An LOL2000 lift-off layer (Shipley Microposit) and a 1 μ m-thick photoresist layer (Shipley 3612) were spin-coated on the sample, exposed to UV light with a Cr-patterned quartz mask, and developed. A thin Ti/Pt layer was deposited by e-beam evaporation and the photoresist was lifted off leaving the Pt-RTDs only

© 2010 WILEY-VCH Verlag GmAdv. Mater. 2010, 22, 4643–4648

on the substrate. The same steps were used to functionalize the Al or Au microwires and to pattern an Al/Cr sacrifi cial mask (2000/3000 Å), which was shaped as a network of micronodes connected by folded microwires. The sacrifi cial mask was used to etch the Kapton fi lm in an oxygen plasma. [ 30 ] The resulting etched membrane was then released from the silicon wafer and the metal mask was removed by wet or dry etching.

Computational Study : A (2D shell elements) non-linear fi nite element numerical model (ABAQUS) was implemented to study the mechanical response of a 4 μ m-wide and 50 μ m-thick microwire in air (no frictional forces) with an explicit dynamic analysis. 3D models were implemented to study the response of specifi c portions of the microwire (loops, linear segments and the interaction loops/linear segments) with varying geometrical dimensions (width and thickness).

Network Expansion : The set of microbeams on two opposite sides was cut just prior the expansion process to separate the network from the external frame. The opposite left frame portions were then bonded to a fi xed and to a movable support keeping the network free-standing in air to avoid frictional forces (or alternatively on a supporting substrate). By moving the movable support by hand, the network was expanded in the direction of the applied load. The network was then picked up (breaking the remaining microbeams), reversed, and extended in the other direction with the same procedure described above.

Integration into a Composite : The expanded network was overlapped on the prepregs and held in place by the sticking force of the prepregs at room temperature. A 0.52 MPa pressure was applied to the sample while heating at 177 ° C to cure the integrated composite.

Fatigue Testing : The composite was tested in an MTS System using cyclic tensile loading conditions characterized by a minimum stress of 100 MPa and an R value of 0.15 at 9 Hz. The electrical resistance of the microwire was simultaneously and continuously monitored during the 500 000 loading cycles.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors appreciate the support of this work by NASA and the Air Force Offi ce of Scientifi c Research (AFOSR). The program monitors are William Winfree (NASA) and Les Lee (AFOSR). This article was amended September 9, 2010 to correct the display of Figures 2 and 4.

Received: February 22, 2010 Revised: April 27, 2010

Published online: September 7, 2010

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