Appl. Phys. Rev. 7, 011404 (2020); https://doi.org/10.1063/1.5129468 7, 011404

© 2020 Author(s).

Visually aided tactile enhancement systembased on ultrathin highly sensitive crack-based strain sensors Cite as: Appl. Phys. Rev. 7, 011404 (2020); https://doi.org/10.1063/1.5129468Submitted: 28 September 2019 . Accepted: 17 January 2020 . Published Online: 18 February 2020

Jing Li , Rongrong Bao , Juan Tao , Ming Dong, Yufei Zhang , Sheng Fu , Dengfeng Peng ,

and Caofeng Pan

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Visually aided tactile enhancement system basedon ultrathin highly sensitive crack-based strainsensors

Cite as: Appl. Phys. Rev. 7, 011404 (2020); doi: 10.1063/1.5129468Submitted: 28 September 2019 . Accepted: 17 January 2020 .Published Online: 18 February 2020 . Corrected 24 February 2020

Jing Li,1,2,3 Rongrong Bao,1,3,4 Juan Tao,1,3 Ming Dong,5 Yufei Zhang,1,3 Sheng Fu,1 Dengfeng Peng,2

and Caofeng Pan1,2,3,4,a)

AFFILIATIONS1CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and Sensor, Beijing Institute of Nanoenergyand Nanosystems, Chinese Academy of Sciences, Beijing 100083, People’s Republic of China

2College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, China3School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China4Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning, Guangxi 530004,People’s Republic of China

5Beijing Institute of Tracking and Telecommunications Technology, Beijing 100094, People’s Republic of China

a)Author to whom correspondence should be addressed: cfpan@binn.cas.cn

ABSTRACT

Attenuated tactile sensation may occur on people who have skin trauma or prolonged glove usage. Such decreased sensation may causepatients to become less responsive to minute skin deformations and consequently fail to regulate their limbs properly. To mitigate suchhealth conditions, an integrated tactile enhancement system that exceeds the human skin’s sensitivity is indispensable for patients to regainthe touch sensation of minute deformations. Here, we develop a visually aided tactile enhancement system for precise motion control bycombining ultrathin, highly sensitive, crack-based strain sensors and signal acquisition circuit with real-time display equipment. By optimiz-ing the thicknesses of the substrates and sensitive films of the strain sensors, our device has a detection limit as low as 0.01% and an ultrahighgauge factor of 44 013 at a strain of 0.88%, which exceeds the performance of previous devices with crack-based strain sensors within minutestrain range. The high sensitivity of the ultrathin crack-based strain sensor makes it possible for our visually aided tactile enhancement sys-tem to detect tiny deformations such as the slight brush of a feather, the fall of water droplets on fingers, and even the touch of invisiblewires. Our study demonstrates promising applications of integrated visually aided tactile enhancement systems in human-machine interac-tions and artificial intelligence.

Published under license by AIP Publishing. https://doi.org/10.1063/1.5129468

I. INTRODUCTION

Tactility is one of the vital foundations of human fine manipula-tion.1–3 Our precise motion control in daily life is based on the syner-gistic effect of visual sensation, tactile sensation, and nervous systems.However, people may undergo impaired tactile sensation after skindamage or wearing gloves, leaving them unable to properly commandtheir body and handle forces, especially when they undergo tiny defor-mations. For instance, surgeons are mostly insensitive to soft biologicaltissue and blood vessels while putting on gloves during surgery, andthe movements of astronauts are slightly clumsy after wearing space-suits. Electronic skin (e-skin) that with ultrasensitive response, simu-lating or surpassing human skin, and capable of converting external

tiny stimuli into electric signals, is an alternative for people with atten-uated touch sensation.4–10 However, for people with attenuated touchsensation to modulate their motions more precisely, apart from ultra-sensitive e-skin, using other senses (such as visual signals and auditorysignals) as tactile signal supplements is extremely necessary. Therefore,it is rather urgent to develop a tactile enhancement/substitution systembased on ultrasensitive e-skin. In one scenario, people with attenuatedtouch sensation can modulate their movements and forces throughpracticing looking at real-time display equipment that display real-time tactile deformations sensitively, while wearing ultrasensitivee-skin. However, this is a very difficult goal to achieve, because it needsnot only a complete signal extraction as well as a processing and

Appl. Phys. Rev. 7, 011404 (2020); doi: 10.1063/1.5129468 7, 011404-1

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display system to simulate human neural network but also highly flexi-ble and sensitive sensors.

Highly flexible sensors, even ultrathin sensors, enable the e-skincapability to monitor tiny deformations more precisely and reduce thepossibilities of detachment due to the much more conformable contactwith target substrates. In special applications, such as tactile assistanceand recording during surgical operation, the application of ultrathinsensors is essential to avoid affecting the touch of the doctors’ fingers.Dramatically reducing the thickness of e-skin is an efficient way toimprove its flexibility and can also reduce its weight.11–16 However,handling thinner sensors presents enormous difficulties in transferprocesses. Introducing an easy-to-control, cost-effective, water-solublesacrificial layer as part of the fabrication process of ultrathin sensorscan avoid strain damage and the destruction of harsh etchants.17,18

Piezoresistive, crack-based sensors that were inspired by the spi-der’s slit organ are remarkable for their ultrahigh sensitivity and simplefabrication process compared to other resistive sensors.19–23 To date, avariety of novel functional materials have been utilized to optimize theperformance of crack-based sensors.24–35 To build a tactile enhance-ment/substitution system that can exceed the human skin’s sensitivity,crack-based strain sensors, with dramatically promoted flexibility andenhanced sensitivity, are urgently needed.

Herein, we demonstrate a visually aided tactile enhancement sys-tem based on ultrathin, highly sensitive, crack-based strain sensors,signal acquisition circuits, and real-time display equipment that canprovide precise motion control. An ultrathin strain sensor is preparedthrough utilizing a water-soluble sacrificial layer, with the structure ofa cracked Ag film on an ultrathin polydimethylsiloxane (PDMS) sub-strate (14.8lm). The ultrathin crack-based strain sensor (UCSS) islow cost, simple in preparation, and possesses an excellent electrome-chanical performance with a sensitivity as high as 44 013 at a strain of0.88% and considerably low detection limit of 0.01%. With another

PDMS layer for encapsulating, the device steadily achieves over 13 000cycles. The response of UCSS is fast and stable with negligible hystere-sis. Applying UCSS directly on fingertips can clearly detect humanpulses with representative peaks and deformations of touching a petal.In particular, a visually aided tactile enhancement system (VATES) isbuilt based on gloves fitted with UCSSs, signal acquisition circuits, andreal-time display equipment. The VATES enable the ultrasensitivesensation to the tiny deformation of gloves, such as the slight brush offeather, the fall of water droplets, and even the touch of invisible wires,presents promising assistance for doctors, astronauts, and manyothers. As a proof-of-concept experiment, an ultrathin crack-basedstrain sensor array was prepared and applied for detecting the spatialstrain distribution.

II. PREPARATION AND PROPERTIES OF UCSS

Figure 1 schematically illustrates that a visually aided tactileenhancement system is built through combining ultrathin crack-basedstrain sensors [Fig. 1(a)], signal acquisition circuits [Fig. 1(b)], and areal-time display [Fig. 1(c)], mimicking the tactile sensation of humanskin and neural network. The VATES enables people with tactileimpairment to realize precise motion control. The photographs in Fig.1(a) show an ultrathin crack-based strain sensor transferred onto a fin-gertip. As a result of the ultrathin characteristic of the strain sensor,the fingerprint of the fingertip is clearly displayed in the enlarged pho-tograph. The fabrication process of UCSS is illustrated in Fig. 2(a).First, a sacrificial layer was prepared by spin-coating an aqueous poly-vinyl (PVA) solution on a glass substrate. Then, a PDMS layer wascoated on the sacrificial layer, followed by using de-ionized water toetch the sacrificial layer over about 380min. Afterward, the smoothPDMS layer without strain damage was obtained and transferred toa flexible polyethylene terephthalate (PET) film. A sensitive layerof 37 nm Ag film (see Fig. S1 in the supplementary material) was

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FIG. 1. The visually aided tactile enhancement system applied for precise motion control. The ultrathin highly sensitive crack-based sensor, signal acquisition circuit, and real-time display system are used to realize the induction of tiny deformation. Ultrathin sensors do not disturb the human skin’s tactile sensation and exceed the human skin’s sensi-tivity, and real-time display can be used as a supplement to assist human skin’s tactile sensation. (a) The photograph of ultrathin crack-based strain sensor transferred onto ahuman fingertip. (b) Schematic illustration and photograph of a signal acquisition circuit. (c) Schematic illustration of real-time display and analysis equipment.

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Appl. Phys. Rev. 7, 011404 (2020); doi: 10.1063/1.5129468 7, 011404-2

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deposited on the PDMS substrate, and the entire device was bent to astrain of 10% to generate parallel cut-through channel cracks.34

The SEM image of PDMS before being stretched is shown in Fig.S2(a) in the supplementary material, indicating that there is no straindamage during the etching and transferring process. The optical pho-tographs in Fig. S2(b) in the supplementary material show the parallelcut-through channel cracks of UCSS after pre-stretching. Meanwhile,the scanning electron microscopy (SEM) images of the UCSS in Figs.

S2(c) and S2(d) in the supplementary material distinctly present mag-nifying parallel cracks in Ag film, and the amplifying SEM imageexhibits the overlap of adjacent cracks.

The electromechanical properties of ultrathin crack-based strainsensors were investigated. To study the effect of the thickness ofPDMS on the sensitivity of crack-based strain sensors, crack-basedstrain sensors with different thicknesses were fabricated by controllingthe rotation speed of PDMS substrates, as shown in Fig. S3 in the

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FIG. 2. Fabrication process and electromechanical properties of the ultrathin crack-based strain sensors. (a) Schematic illustration of the fabrication process of UCSS. (b) and(c) Relative resistance variation and GF of crack-based strain sensors with different thicknesses, and the insets show the relative resistance change and GF of the 14.8lmcrack-based strain sensor. (d) Five cyclic tests of UCSS were performed under different strains. (e) Response time of UCSS. The inset shows that the response time andrecovery time are 87 and 68 ms, respectively. (f) Relative resistance change of UCSS under a 0.70% strain with different frequencies. (g) Mechanical durability of UCSS undera strain of 0.78%, and the inset shows many cycles of the sensor response.

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supplementary material. Here, gauge factor (GF) characterization ofthe sensitivity is defined as DR=R0ð Þ=e, where e represents the externalstrain, DR=R0 represents the relative resistance change under externalstrain of e, and R0 represents the resistance of the sensor under noload. The variations in the relative resistance (DR=R0) and GF ofcrack-based strain sensors with different thicknesses to the appliedstrain are shown in Figs. 2(b) and 2(c). Obviously, the crack-basedstrain sensor with a thinner PDMS substrate clearly presents highersensitivity to the applied strain, while the crack-based strain sensorwith a thicker PDMS substrate exhibits a wider detection range. Inpractical use, according to Fig. 2b, the ultrathin sensors show advan-tage in detecting tiny deformations, while the thicker sensors can beused for monitoring higher strain. From the inset in Fig. 2(c), the GFof UCSS reaches 44 013 when the strain is 0.88%, which is an ultrahighvalue among crack-based strain sensors, practically in minute strainrange reported to date, as shown in Table I. Recently, Zhou et al.(2019) widened the sensing range and drastically improved the sensi-tivity of the crack-based strain sensor through an electrospinningfibrous thermoplastic polyurethane (TPU) mat and spray-coating car-bon nanotube (CNT) ink onto it.36 The crack-based CNT/TPU strainsensor only presents ultrahigh value of GF in large strain range during220%�300%, therefore it was not considered for comparison of GF ofcrack-based strain sensors under tiny strain. It is because of the ultra-thin substrate obtained through using a water-soluble sacrificial layerthat the as-fabricated UCSS exhibits ultrasensitivity and excellentflexibility.

Next, we studied the effect of the thicknesses of Ag films on thesensitivity and sensing range of ultrathin crack-based sensors. Fivekinds of devices with different Ag films are fabricated by controllingthe time of magnetron sputtering. As shown in Fig. S4(a) in the sup-plementary material, a device with thinner Ag films presents highersensitivity, while the sensing range is not affected. We carefully calcu-lated the crack density of every kind of device by counting the numberof cracks per 2.75mm in the devices. Clearly the density of cracks indevices with thinner Ag films is higher than that of devices withthicker Ag films [see Fig. S4(b) in the supplementary material], whichaccounts for the higher sensitivity of devices with thinner Ag films.37

The resistance of sensors under no load also varies with the thicknessesof Ag films, and the thinner Ag films possess higher resistance [seeFig.S4(c) in the supplementary material].

The working principle of a crack-based strain sensor, as shown inthe diagram in Fig. S5 in the supplementary material, can be ascribedto two mechanisms: the overlap effect and the tunnel effect.19,34 After

pre-stretching, cut-through channel cracks were generated and partlyclosed forming the overlap region [see Fig. S5(i) and S2(d) in the sup-plementary material]. Because the resistance of the overlap regions ismuch larger than that of the Ag strip, the resistance of the overlappingregion is a dominant factor when external strain is applied, and theresistance of the whole sensor varies. In minor strain variation regions[see Fig. S5(ii) in the supplementary material], the resistance of theoverlap region increases relatively slowly due to the decrease in thecontact area of the two cracked blocks. When the strain increases fur-ther, the two adjacent Ag strips separate from each other [see Fig.S5(iii) in the supplementary material], and the current is conducted bythe tunnel effect; the resistance increases quickly with increasing strain.The relationship between the resistance of crack-based strain sensorsand external strain can be described by the following formula:19

S ¼ 1=2 1� erf ln e=e0ð Þ=lð Þ½ �

in which the resistance is R ¼ 1=S, erf(x) is the error function, and e0and l are fitting parameters. The experimental data of devices withdifferent Ag films fit well with the formula, as shown in Fig. S4(a) inthe supplementary material. However, the response of devices with26nm Ag film to external strain is relatively unstable in practical tests,which may result from the fact that the resistance of a device with26nm Ag film under no load is too high. For crack-based strain sen-sors with different PDMS thicknesses, when a thinner sensor isstrained by a small stress, a large strain is concentrated near the stress,which causes the Ag strips to separate from each other much faster;the resistance of a sensor will rise markedly and then rapidly displaythis small strain as a result of the tunnel effect. However, when athicker sensor is subjected to the small stress, the stress is absorbed orevenly dispersed inside thicker substrates, which makes it difficult totransmit to the position of the silver film on the surface, which inducesthe overall resistance’s slight increase. Considering the balance of sen-sitivity and sensing range, crack-based strain sensors with 14.8lmPDMS substrates and 37nm Ag films are the best choice for VATES.

Next, Fig. S6(a) in the supplementary material shows the varia-tion in the relative resistance, as the strain sensor undergoes increasingstrain and corresponding decreasing strain. The resilience of the strainsensor obtained from Fig. S6(a) is shown in Fig. S6(b) in the supple-mentary material. The variation in the relative resistance underincreasing strain matches well with that under decreasing strain, alongwith negligible hysteresis, which ensures the accuracy of the strain sen-sor in practical applications. It is noted that the detection limit ofUCSS is only 0.01%, as shown in Fig. S6(c) in the supplementarymaterial, showing that UCSS has outstanding advantage in measuringsmall strains. Figure 2(d) presents a stable performance of UCSS underdifferent strain cycles. The response and relaxation times are 87 msand 68ms, respectively, as shown in Fig. 2(e). When a strain of 0.35%with different frequencies ranging from 0.10 to 1.19Hz was applied,the UCSS still operates steadily all the time [Fig. 2(f)]. The repeatabilityof UCSS guaranteeing the stability is studied and illustrated in Fig.2(g). The relative resistance was almost unchanged after 600 cycles ofmeasurement, presenting good stability in cyclic usage. Moreover, inorder to improve the durability of UCSS and avoid the oxidation of Agfilm,38 another PDMS layer was used to encapsulate UCSS. As shownin Fig. S7(a) in the supplementary material, the sensitivity of an encap-sulated UCSS decreased slightly compared to a UCSS without encap-sulation, but still maintained 23 455 under strain of 0.88%. Especially,

TABLE I. Comparison of gauge factors of crack-based strain sensors under tinystrain.

e (%) GF References References

2 2079.23 Nature, 2014 192 16 000 Advanced Materials, 2016 351 5000 Materials Horizons, 2016 341 5000 Nano Energy, 2017 262 2000 Nanoscale, 2018 281.5 5888.59 Nanoscale, 2018 270.88 44 013 Our work Our work

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the encapsulated UCSS works steadily after over 13 000 cycles understrain of 0.70% [see Fig. S7(b) in the supplementary material], which iscomparable to ultrasensitive strain sensors reported recently.39,40

The ultrathin crack-based strain sensor has been demonstratedwith outstanding sensitivity, fast response, and stable repeatability,then, the applications of UCSS in sensing minute strain are studied.The responses of UCSS to subtle stimuli are measured and shown inFig. 3. To demonstrate high sensitivity of UCSS, crack-based strainsensors with different PDMS substrate thicknesses are applied to mon-itor pulse by directly adhering them onto a fingertip.41,42 A cross-sectional SEM image of 76.3lm thick PDMS is shown in Fig. S8(a) inthe supplementary material. The photographs in Fig. S8(b) in the sup-plementary material show that ultrathin crack-based strain sensor fitsthe skin far better than a thicker one. Clearly, crack-based strain sen-sors with different thicknesses are all able to monitor the pulses on ahuman fingertip, and the relative resistance change of the pulse of afingertip measured by UCSS is much higher than that of the thickersensor, as shown in Fig. 3(a). The amplified curves of a single pulsedetected by crack-based strain sensors with different thicknesses areshown in Fig. 3(b); it is worth noting that the pulse curve detected byUCSS displays representative peaks of human pulse. The statistical rel-ative resistance change of pulse signals detected by crack-based strainsensors with different thicknesses are calculated and shown in Fig.3(c). The outstanding performance of UCSS in monitoring pulsesresults from their ultrahigh sensitivity to tiny deformations as well astheir superior conformable contact with skin, therefore the pulse of

the fingertip causes much greater deformation of UCSS than that ofthicker sensors. Touch sensation is a vital function for e-skin to per-ceive the surrounding environment.43–45 The response of UCSS to afingertip touching a petal is shown in Fig. 3(d). The UCSS can detectthe tiny deformation of a fingertip in the process of touching a petal,while simultaneously sensing the human pulse, as shown in Fig. 3(e).Ultrathin crack-based strain sensors can also be utilized as highly sen-sitive electronic whiskers for detecting tiny changes in environmentalpressure.46 Immobilizing one end of a 200lm thick PET substrate andtransferring a strain sensor to this end with the other end of the PETsuspended, the strain sensor is highly sensitive to pressure applied onthe suspended end of the PET. The relative resistance change of thestrain sensor varies sharply when a small leaf weight of 0.048 g isdropped to the suspended part of the PET, as shown in Fig. 3(f). Theultrahigh sensitivity endows our ultrathin crack-based strain sensorwith potential applications in health monitors, tactile sensors, and elec-tronic whiskers.

The ultrahigh sensitivity, extraordinary flexibility, and good sta-bility enable ultrathin crack-based strain sensors to detect various tinydeformations in practical applications. The UCSS is very thin, makingit difficult to handle; therefore, transferring UCSS onto a Very HighBond (VHB) tape facilitates fitting the sensor to arbitrary surfaces andimproves its reusability. Figure S9(a) in the supplementary materialshows a scheme of transferring UCSS onto VHB tape. Putting UCSSon a non-jointed area clearly allows repeated detection of correspond-ing deformation. Figure S9(b) in the supplementary material shows

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FIG. 3. Responses of the ultrathin crack-based strain sensors to minute stimuli. (a) Relative resistance change of the fingertip pulse signals detected by crack-based strainsensors with different thicknesses. (b) The amplified curves of a single pulse detected by crack-based strain sensors with different thicknesses. (c) The statistical data of rela-tive resistance change of the fingertip pulse signals derived from (a). (d) Response of UCSS placed on a fingertip to touching a petal, and the inset at the upper left is the corre-sponding photograph. (e) The enlarged view of the pulse signal of a fingertip while finger is touching the petal. (f) Response of a UCSS placed on the fixed end of a cantileverstructure while a lightweight leaf is dropped onto the free end, along with a corresponding schematic diagram and photograph in inset.

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the response of UCSS to a palm making a fist and relaxing while thesensor is adhered to the arm. Figure S9(c) in the supplementary mate-rial shows that UCSS responds to the increasing strain applied to thearea adjacent to the sensor on the forearm. The change in the resis-tance of UCSS increases with increasing force. The UCSS placed onthe corners of the mouth responds accurately and repeatedly to thesmiling, as shown in Fig. S9(d) in the supplementary material.Similarly, putting an ultrathin strain sensor on the corners of the eyeand the ridge of the brow allows it to monitor the blinking of the eye

as well as frowning of the brow, respectively [see Figs. S9(e) and S9(f)in the supplementary material], which provides clear informationabout a person’s countenance to identify the mood or quality of sleep.A UCSS attached to a stainless steel ruler can precisely respond to theflexion movements and vibrations of the ruler, while one end is immo-bilized and external pressure is applied and then released on the otherside of the ruler [see Figs. S9(g) and S9(h) in the supplementary mate-rial]. The UCSS was transferred to a subject’s throat to monitor thedeformation of the throat tissues when words are spoken, as shown in

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FIG. 4. Design of visually aided tactile enhancement system for precise motion control. (a) Schematic illustration of design of multichannel signal acquisition circuit and wirelessconnection to a computer with homemade software to accomplish image processing. (b) Photograph of glove with every finger fitted with UCSS. (c) Photograph of the entireVATES for detecting the touch of a soft feather. (d) The response of UCSSs to deformation of finger slightly touched by a soft feather, and the inset shows corresponding pho-tograph and real-time display. (e) Schematic illustration of VATES applied for detecting the touch of invisible tungsten wire, and visually getting tactile information. (f) and (g)show the photograph of grabbing a soft ball with different poses and corresponding VATES’s responses to deformation of UCSSs, the inset in (g) is the real-time display of theVATES’s responses. (h) Well-encapsulated VATES applied underwater for monitoring the deformation of hand caused by water dropping. The inset shows the schematic dia-gram and photograph of feeling the water dropping.

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the inset of Fig. S9(i) in the supplementary material. Figure S9(i) in thesupplementary material shows that the variation in the relative resis-tance is in accord with the speaking of the words, exhibiting potentialapplications in voice recognition systems. These results demonstratethe wide applications of our presented ultrathin strain sensor in e-skinand human-machine interfaces.

III. DESIGN AND APLICATIONS OF VATES

On the basis of high sensitivity of ultrathin crack-based strainsensor to minute strain, a homemade visually aided tactile enhance-ment system was designed combining signal acquisition circuit andreal-time display equipment. The multichannel signal acquisitioncircuit was designed and connected wirelessly to a computer based ona PIC16F1526 single chip microcomputer with homemade software toaccomplish image processing. As shown in Fig. 4(a), the multichannelsignal acquisition system can collect data of 30 resistive sensors at thesame time and 20 sets of data per second. The collected data are outputto a computer in real time by wireless serial port. The homemade soft-ware can read, display, and store the system data. There are two waysto display the selected data: hot spot and digital. Five UCSSs weretransferred onto the five fingers of a glove reversely and encapsulatedwith glue, ensuring the stability in practical use [Fig. 4(b)]. When wewear a glove, it is hard to feel the slight touch of a soft feather.However, with the help of highly sensitive VATES and vision as a tac-tile supplement, the mild touch of a soft feather on fingers can be real-time displayed and “felt” in a way [Fig. 4(c)]. We slightly touched thefingers in turn using a soft feather, and the deformation of everystressed finger can be real-time displayed without cross talk, as shownin Fig. 4(d) and Movie S1 in the supplementary material. Sometimesfor a surgeon, feeling minute deformations of soft tissue, even invisible

tissue or tiny blood vessels, is extremely challenging. As a referablesolution, with the help of VATES, we gently touched an invisible15lm tungsten wire and distinguished the deformations exactlythrough looking at the real-time display [Fig. 4(e)]. It is noted that themagnitude of strain can be distinguished through the color depth ofthe hot spot, as shown in Movie S2 in the supplementary material. Toverify the real-time multichannel signal acquisition characteristic andthe application for precise motion control, we put on gloves fitted withUCSSs and grabbed a soft ball with different poses [Fig. 4(f)]. Thestress on every finger can be real-time displayed, and differentiatedindividually and collectively, as shown in Fig. 4(g). With tight encapsu-lation of UCSSs and every node of the circuit by using glue, theVATES can, in particular, be applied underwater. The VATESresponds to deformations of glove caused by water dripping withoutshort circuiting, presenting specific applications in underwater e-skin[see Fig. 4(h) and Movie S3 in the supplementary material]. Theseresults verified the significant practical application value of VATES forassisting people who have a dull touch.

The ability to sense spatial strain is indispensable for e-skin andhuman-machine interaction. To implement this function, we designedand produced a multipixel ultrathin crack-based strain sensor arraywith masks and an insulation layer of PDMS, as shown in Fig. 5(a);and the detailed production process of the ultrathin crack-based strainsensor array is shown in Fig. S10 in the supplementary material. Theactive layer of the Ag film is also approximately 37 nm thick, the sameas that of UCSS. In order to ensure good conductivity of the array’sinterconnections, insensitive Ag layers with the thickness of 200nmare selected as the top and bottom electrodes [see Figs. S11 and S12 inthe supplementary material]. The optical photograph of as-fabricatedultrathin crack-based strain sensor array transferred on opisthenar is

Ag

(a) (b) (c)

(d) (e)

PDMS

0.1

5.0

i Ag top electrode

Ag bottom electrode

ΔR/R0

0.0

80.0

ΔR/R0

PDMS insulating layeriiiii

iii ii i

FIG. 5. Design and application of an ultrathin crack-based strain sensor array for spatial strain distribution mapping. (a) Schematic diagram of the structure of a multipixel ultra-thin crack-based strain sensor array. (b) Photograph of the ultrathin crack-based strain sensor array on the back of a hand and corresponding enlarged view. (c) Photograph ofpressed ultrathin crack-based strain sensor array on human skin. (d) Photograph of a person gripping a wash bottle and squeezing it with the thumb with an ultrathin crack-based strain sensor array transferred on the bottle, and the spatial strain distribution of the wash bottle being squeezed. (e) Photograph of a soft feather touching an ultrathincrack-based strain sensor array transferred on a glove, and the corresponding spatial strain distribution of the hand.

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Appl. Phys. Rev. 7, 011404 (2020); doi: 10.1063/1.5129468 7, 011404-7

Published under license by AIP Publishing

in Fig. 5(b), together with the enlarged photograph in the inset thatreveals the highly flexible characteristics of the strain sensor array. Thestrain sensor array is intact and maintains conformal contact with skineven when compressed, as shown in Fig. 5(c). To prove the spatialstrain sensing ability, we transferred a crack-based strain sensor arrayonto a wash bottle and squeezed the wash bottle with the thumb [Fig.5(d)]. The derived spatial strain distribution is detected by multichan-nel signal acquisition circuit and plotted, as shown in Fig. 5(d), show-ing the precise spatial strain distribution of the deformed wash bottle.A crack-based strain sensor array is transferred onto a glove in perfectcondition and fulfills close contact. Even on the rugged surface of opis-thenar, the strain sensor array is capable of mapping the strain distri-bution of the deformation of opisthenar lightly brushed with feathers,and corresponding data acquired through multichannel signal acquisi-tion circuit is shown in Fig. 5(e). These experiments demonstrate thepromising applications of a crack-based strain sensor array in tactileassistance for people who frequently wear gloves.

IV. CONCLUSION

In summary, based on ultrasensitive and ultrathin strain sen-sors, signal acquisition circuits, and real-time display equipment, wedemonstrated a visually aided tactile enhancement system offeringhelp for people with an impaired sense of touch. By optimizing thethicknesses of PDMS substrates and Ag films, the ultrathin, highlysensitive, low-cost crack-based strain sensor with a low detectionlimit, fast response, and simple manufacturing process was preparedusing a water-soluble sacrificial layer. The detection limit of thestrain sensor is as low as 0.01%, and the GF reaches 44 013 at a strainof 0.88%, which is an ultrahigh value among crack-based strain sen-sors. Naturally, the ultrathin crack-based strain sensor is capable ofdetecting various subtle deformations, such as a fingertip pulse, thetouching of a petal, and the falling of a leaf. On the basis of UCSSand real-time display equipment, we designed a visually aided tactileenhancement system for helping people in the case of tactile impair-ment. The VATES is capable of assisting people who wearing glovesto feel tiny deformations, even the touch of invisible wires. Finally,we verified the capability of the multipixel strain sensor array tosense the spatial pressure distribution. Our ultrathin crack-basedstrain sensor and tactile enhancement system are promising in thefields of health monitoring, humanoid robots, human-machineinteractions, and artificial intelligence.

SUPPLEMENTARY MATERIAL

See the supplementary material for details on the morphologycharacterization and working principle of device, and additional dataand video on performance and application of device, and so on.

AUTHOR’S CONTRIBUTION

J.L., R.B., and J.T. contributed equally to this work.

ACKNOWLEDGMENTS

The authors thank the support of a national key R&D projectfrom the Minister of Science and Technology, China (Grant No.2016YFA0202703), National Natural Science Foundation of China(Grant Nos. 51622205, 61675027, 51432005, 61505010, and51502018), Beijing City Committee of science and technology(Grant Nos. Z171100002017019 and Z181100004418004), Natural

Science Foundation of Beijing Municipality (Grant Nos. 4181004,4182080, 4184110, 2184131, and Z180011), and the University ofChinese Academy of Sciences.

APPENDIX: EXPERIMENTAL

1. Fabrication of ultrathin crack-based strain sensor

First, PVA water solution (2wt. % concentration) was coatedon a glass substrate followed by being cured at 100 �C for 10min.Then, PDMS with different thicknesses were obtained by mixingthe elastomer and cross-linker (Sylgard 184, Dow Corning) with aweight ratio of 10:1 and spin-coating the mixture on the PVA layer,followed with being dried at 120 �C for 10min. The PDMS layerwas transferred to a PET film after etching the PVA sacrificial layerby using de-ionized water. A layer of Ag microparticles was depos-ited onto the PDMS substrate by magnetic sputtering with a powerof 60W. The pre-stretch process was manipulated on the 3D micro-manipulation stages at a strain of 10% to form parallel cracks forstrain sensing. Later on, Cu wires were used to connect the twoends of the sensor with silver paste.

2. Fabrication of multipixel ultrathin crack-basedstrain sensor array

First, PVA sacrificial layer and PDMS substrate were spin-coated on a glass substrate. After etching the PVA sacrificial layerand transferring the PDMS to a PET film, a layer of 37 nm Ag witha designed pattern was deposited on the PDMS substrate, combin-ing the shadow mask and magnetron sputtering. Subsequently, thesample was pre-stretched under 10% strain and released to generatecracks controlled by 3D micromanipulation stages. Then, two layersof 200 nm Ag film were deposited on the sample as bottom and topelectrodes, respectively, and separated by drop-casting an insulatinglayer of PDMS array to avoid leakage.

3. Characterization and measurements

The morphologies and cross-sectional SEM images of thesamples are measured by the optical microscope (Zeiss ObserverZ1) and the field-emission scanning electron microscopy(SU8020 Hitachi). The thicknesses of Ag films are measured by astep profiler (BRUKER, DektakXT). The electromechanical prop-erties of the sensor are recorded with the LCR meter (AgilentE4980A) under a sampling rate of 10 kHz, and the StanfordResearch System (SR 570 low noise current amplifier equippedwith DS 345 synthesized function generator) under a samplingrate of 10 kHz.

4. Design of signal acquisition circuit

The multichannel signal real-time acquisition/display system isbased on PIC16F1526 single chip microcomputer. The signal acqui-sition system can collect data of 30 resistive sensors at the sametime and 20 sets of data per second. The collected data are outputto PC in real time by wireless serial port. The customized programin computer can read, display, and store the system data. Three dif-ferent ranges of 0–3000 X, 0–6000 X, and 0–12 000 X can be testedby selecting the test sensitivity of the program. There are two ways

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Appl. Phys. Rev. 7, 011404 (2020); doi: 10.1063/1.5129468 7, 011404-8

Published under license by AIP Publishing

for displaying data: hot spot and digital. The measurement principleof real-time resistive sensor data acquisition circuit is given in detailin the supplementary material.

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