Abstract
Sensor designs found in nature are optimal due to their evolution over millions of years, making them well-suited for sensing applications. However, replicating these complex, three-dimensional (3D), biomimetic designs in artificial and flexible sensors using conventional techniques such as lithography is challenging. In this paper, we introduce a new processing paradigm for the simplified fabrication of flexible sensors featuring complex and bioinspired structures. The proposed fabrication workflow entailed 3D-printing a metallic mold with complex and intricate 3D features such as a micropillar and a microchannel, casting polydimethylsiloxane (PDMS) inside the mold to obtain the desired structure, and drop-casting piezoresistive graphene nanoplatelets into the predesigned microchannel to form a flexible strain gauge. The graphene-on-PDMS strain gauge showed a high gauge factor of 37 as measured via cyclical tension-compression tests. The processing workflow was used to fabricate a flow sensor inspired by hair-like ‘cilia’ sensors found in nature, which comprised a cilia-inspired pillar and a cantilever with a microchannel that housed the graphene strain gauge. The sensor showed good sensitivity against both tactile and water flow stimuli, with detection thresholds as low as 12 µm in the former and 58 mm/s in the latter, demonstrating the feasibility of our method in developing flexible flow sensors.
Highlights
Biological sensors found in living beings ranging from bacteria to plants to mammals display sensing capabilities that are unrivalled by any comparable man-made technologies, both in sensitivity and versatility, owing to millions of years of optimization through evolution and natural selection
The PDMS sensor consisted of a cilium-inspired pillar and a cantilever with microchannels that housed a graphene nanoplatelet strain gauge
The bioinspired sensor was subjected to both tactile and flow stimuli, and displayed good sensitivity in all cases, showing a detection threshold of 12 μm for an oscillating tactile stimulus and 58 mm/s for an oscillatory flow stimulus in water
Summary
Biological sensors found in living beings ranging from bacteria to plants to mammals display sensing capabilities that are unrivalled by any comparable man-made technologies, both in sensitivity and versatility, owing to millions of years of optimization through evolution and natural selection. The creative micromechanical designs of various biological sensors such as acoustic sensors in the inner ear, olfactory sensors in sharks, neuromast flow sensors in fishes, wake sensing whiskers in seals, tactile sensors in human finger tips, thermal sensors in beetles, and so on, exhibit impressive sensitivity and high efficiency in filtering biologically-relevant signals in noisy ambient conditions [1,2,3]. Ultrasensitive hair-like ‘cilia’ structures are ubiquitous in nature and perform flow sensing in numerous animal species, examples of which are shown in Figure 1a (blind cavefish or Astyanax mexicanus fasciatus) and Figure 1b (tiger wandering spider or Cupiennius salei). The sensing principle of the cilia in all these species is similar, namely, that the drag force induced by the flow is translated to mechanical bending of the high-aspect ratio cilia, which in turn elicits an electrical impulse across the Nanomaterials 2019, 9, x FOR PEER REVIEW. T[1h6e].dTimheendsiimonesnsoifomnsosotfbmioolostgibciaollocigliiacarlacniglieafrroanmge2–f1r5o0m0 μ2–m15in00heμimghtinanhdei0g.h2t–5a0n0dμ0m.2i–n50d0iaμmmeteinr, talddRohceiewuahtsmesieecaaavetsriltecole0odhnr.w0,ei1rmlits5inhmpghmuriascetvsissl−esaia(i1lvaal)-oisecbnwhsyslieoipencnwvilgrsoeeidstdacievsilislymiiet0aynmp.-0si(rnio1aems5rsspssihciimtrvikogeeidhbnsseg−ae1s,)sfneafs0nobbi.rstyr0ioi7ivcnr7caistsltyVoteatsd/o(nemalcusyebss,hei−nmti1hgg)fiehamaMbnaairndEcsikcaMt0aithno.tS0ergm7edt,7seyhcfuVohaosrn/lnidmdnoilngvfosesu−gtl1Mnaoy)cncaE[tic1nitMeoy7d,n]Sd.attehhRltitreeteeeycscshaetohiannoforaonlcltadlhooltiemegvmrryesyailtlohs[alc1ai(ninv7atd]yees. sfuennscotirosnfaoluitnydoifnlafitsehraelsl[i1n8e,1s9e]n.sors found in fishes [18,19]
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