A button switch inspired duplex hydrogel sensor based on both triboelectric and piezoresistive effects for detecting dynamic and static pressure

Zhensheng Chen, Jiahao Yu, Xiaoxi Zhang, Haozhe Zeng, Yunjia Li,Jin Wu,and Kai Tao,a)

ABSTRACT The capability to sense complex pressure variations comprehensively is vital for wearable electronics and flexibl human-machine interfaces.In this paper,inspired by button switches,a duplex tactile sensor based on the combination of triboelectric and piezoresistive effects is designed and fabricated.Because of its excellent mechanical strength and electrical stability,a double-networked ionic hydrogel is used as both the conductive electrode and elastic current regulator.In addition,micro-pyramidal patterned polydimethylsiloxane(PDMS)acts as both the friction layer and the encapsulation elastomer,thereby boosting the triboelectric output performance significantly The duplex hydrogel sensor demonstrates comprehensive sensing ability in detecting the whole stimulation process including the dynamic and static pressures.The dynamic stress intensity(10-300 Pa),the action time,and the static variations(increase and decrease)of the pressure can be identifie precisely from the dual-channel signals.Combined with a signal processing module,an intelligent visible door lamp is achieved for monitoring the entire“contact-hold-release-separation”state of the external stimulation,which shows great application potential for future smart robot e-skin and flexibl electronics.©2022 Author(s).All article content,except where otherwise noted,is licensed under a Creative Commons Attribution(CC BY)license(http://creativecommons.org/licenses/by/4.0/).https://doi.org/10.1063/10.0010120

KEYWORDS Duplex hydrogel sensor,Triboelectric nanogenerator,Piezoresistive sensor,Dynamic and static sensing

I.INTRODUCTION

With the dramatic growth in intelligent robots and smart wearable electronics,smart sensors with multifunctional sensory abilities-especially for pressure-have attracted considerable attention.Conventional pressure sensors are now well developed and are used widely because of their excellent reliability and sensitivity for detecting complex pressure variations.1However,constrained by their complicated structures,rigidity,and essential power-supply requirement,traditional sensors are seriously limited in their applications,and there is an urgent need for self-powered ability and flexibility such as for intelligent robotic and electronic skins.2-5Therefore,triboelectric nanogenerators(TENGs),which have considerably flexibl substrates and excellent output performance,have attracted considerable attention because of their selfsustaining ability,simple structure,and easy integration.6-8With the rapid development of a new generation of tactile sensors with self-powered technology,9-12much effort has gone into optimizing their performances and sensitivities for the accurate detection of the contact-separation pressing process,13-15including novel hierarchical harvester structures,16ultra-stretchable and transparent electrical skins,17-19multifunctional sensor units,14and hybrids of different generator technologies.20Consequently,the dynamic pressures applied on these devices can be easily measured.However,TENGs still cannot detect the static variations of the external stimulation in detail,which is mainly because of their limited triboelectric working principles.21-24

Piezoresistive and piezoelectric effects have been identifie as the most promising means of recording static pressure variations,25and they are used widely in tactile sensors because of their excellent sensitivity and accuracy.26-29Various materials and structures based on these effects have been investigated in detail to increase the sensitivity and reliability of pressure sensors,such as advanced composites,30-32graphene mixed materials,33,34nano-particle elastomers,modifie hydrogel,and ultrathin membranes.35-37Nevertheless,some flaw remain,such as complex structures,high cost,rigidity,and slow response rate,and these place serious restraints on potential applications.38-41

Herein,a duplex hydrogel sensor(DHS)based on both triboelectric and piezoresistive effects is proposed and fabricated for monitoring external pressures in detail.With a newly designed double sandwiched structure and hierarchical fabrication method,micro-pyramidal patterned polydimethylsiloxane(PDMS),an ionic hydrogel membrane,a PDMS membrane,and a hydrogel elastomer are assembled.The upper triboelectric dynamic pressure sensor and the lower piezoresistive static compression transducer work together to achieve multidimensional pressure-sensing abilities.To increase the output performance of the triboelectric part,a micropyramidal array is implanted on the top surface of the PDMS layer to enlarge the contact area during the triboelectrificatio process.Moreover,because of its excellent mechanical and electrical compression properties,the lower PDMS-hydrogel-PDMS structure acts as the piezoresistive part for monitoring the static pressure variations.In addition,with a customized dual-channel signal processing system,an intelligent visible door lamp is fabricated and investigated;this has long-term monitoring ability for detecting pressure variations,thereby showing great potential for intelligent sensor systems and flexibl electronics.

II.EXPERIMENTAL WORK

A.Structural design and materials of DHS

As shown schematically in Fig.1(a),the basic structure of the DHS comprises two overlapping sandwiched parts,i.e.,the upper triboelectric part and the lower piezoresistive part,each of which can be fabricated directly from PDMS and elastic ionic hydrogel(as the conductive electrode).To further increase the sensitivity of the triboelectric part,micro-pyramidal structures[25μm×25μm×17μm,as shown by Fig.1(b-i)obtained using a confocal laser scanning microscope]were fabricated on the surface of the PDMS layer via a micromachined silicon mold.Moreover,by means of a spin-coating fabrication process,the thickness of the whole triboelectric part is controlled to within 500μm[as shown by Fig.1(b-ii)obtained using a scanning electron microscope].Such methods have generally been confirme for enlarging the friction areas of contact interfaces,thereby increasing the sensitivities of triboelectric devices.Combined with the lower piezoresistive part,the DHS can act as a pressure sensor for both dynamic contact-separation pressures and static force variations.With a total device height of 30 mm and radii of the triboelectric and piezoresistive parts of 10 mm and 11 mm,respectively,the fabricated DHS is easily compressed by a finge[see Fig.1(e)]and then recovers its original state once released because of its excellent self-rebounding property[see Fig.1(f)].By analyzing the voltage signals and current variations from two channels,a novel mechanism for the comprehensive variations of the pressure is proposed based on the DHS.Once an external pressure is applied,the upper triboelectric part acts as a TENG,responding to the dynamic variation by generating voltage signals.During the compression process,the lower piezoresistive part deforms continuously because of the static pressure,which leads to resistance variations that regulate the current accordingly.Therefore,the complex and comprehensive details of the pressure variations can be detected accurately and immediately[see Figs.1(d)-1(f)].

FIG.1.(a)Structuraldesign of duplex hydrogelsensor(DHS)with triboelectric and piezoresistive effects.(b)Images of micro-pyramidalpatterned polydimethylsiloxane(PDMS)and sandwich-structured triboelectric part obtained using confocallaser scanning microscope[image(i)]and scanning electron microscope[image(ii)].(c)Photographs ofDHS in originaland compressed states.(d)–(f)Output signals of DHS under different pressure conditions.

FIG.2.Compression properties of hydrogel:(a)curves of compression stress versus strain ofhydrogelfor strains from zero to 60%;(b)cyclic test of hydrogelfor fiv cycles in compressing states from zero to 50%;(c)fiv cyclic tests ofhydrogelin compression states of10%,20%,30%,40%,and 50%.

In previous work,we designed and carefully investigated a double-networked ionic hydrogel with multifunctional capabilities.42-44The basic double-networked structure(polyacrylamide chains and carrageenan chains)of the ionic hydrogel gives it excellent strength,being able to withstand over 1100%stretchability and over 15 000 cycles of applied pressure without any obvious damage.Moreover,modifie by LiBr solution,this hydrogel has strong water retention and wide temperature tolerance,thereby making it the ideal material from which to fabricate the flexibl electrode and piezoresistive part,responding well to both dynamic and static pressures.The durability of the hydrogel electrode can be enhanced by PDMS encapsulation,which provides physical protection and prevents water evaporation.Herein,this hydrogel is also used for the electrode membrane and the variable resistance.As shown in Fig.2(a),the compressive recoverability of the hydrogel was investigated under loading and unloading pressures.It can be compressed to 60%strain and withstand a maximum stress of 230 kPa,and once the compression is removed,the hydrogel recovers its original state naturally with only a slight hysteresis loop,thereby demonstrating its excellent mechanical compression properties.The repeatability and stability of the hydrogel were further studied in cyclic compression tests.The dynamic strain-stress curves of the hydrogel in Fig.2(b)show its excellent stability under fiv cyclic compressions for 50%strain.Also,the firs cycle often involves the largest hysteresis loop,exhibiting the typical elastomerlike behavior of the hydrogel,and the hysteresis loops from the following cycles almost overlap,demonstrating the stable consistency in the repeatable deformations.Additionally,the mechanical stability of the hydrogel was studied through fiv compression cycles under strains of 10%to 50%[Fig.2(c)],and the stabilized trends of each repeating curve demonstrate the elastomer-like behavior of the hydrogel.

FIG.3.Fabrication method for DHS including processing of triboelectric and piezoresistive parts.

FIG.4.Working principle of DHS in states of(i)just contacted,(ii)static compression,(iii)released back to just contacted,(iv)separating,(v)separated,and(vi)approaching.

B.Fabrication and working principle of DHS

Figure 3 shows schematically the fabrication process for the DHS.First,an inverted micro-pyramidal array is etched into the surface of a silicon wafer using the anisotropic method.Then,by means of spin coating(800 rpm for 15 s),a PDMS pre-mixture is coated onto the surface of the silicon mold,and a micro-structured PDMS membrane is obtained after storage for 6 h at high temperature(60○C).Next,liquid hydrogel mixture is poured onto the PDMS layer and spin-coated(600 rpm for 20 s)to a thin film The prepared hydrogel-PDMS layers are then kept at 5○C for 1 h and then heated to 70○Cfor another hour to solidify the hydrogel electrode.Also,the hydrogel fil is cut into a circular shape with a radius of 8 mm and connected with silver wire.Finally,another PDMS fil is formed on the hydrogel surface,thereby encapsulating the hydrogel electrode.It is then easy to peel the triboelectric part of the DHS out of the wafer.

A similar method is used to fabricate the lower piezoresistive part.The lower PDMS layer is fabricated by means of spin-coating on a plain silicon substrate.A PDMS cylinder is formed by means of a simple die and used as the hydrogel container.Hydrogel mixture is poured into the PDMS mold and solidifie immediately.To process the elastic unit,PDMS is also used to encapsulate the hydrogel bulk.The external electrodes for both the triboelectric and piezoresistive parts are silver wires(diameter 50μm),which are connected with Dupont lines for testing.Upon assembling the two parts together with adhesive PDMS,the DHS is formed for complex and comprehensive force detection.

To monitor the applied compression force comprehensively,a novel force-detection mechanism is proposed,and its basic working principle is investigated.As shown in Fig.4,the upper triboelectric part with a simple sandwiched structure is connected to the ground and can be used as a TENG in single-electrode mode to test the contact-separation process.The lower piezoresistive part based on the conductive elastic hydrogel serves as the static pressure monitor via its deformation-induced resistance variations.In the initial state shown in Fig.4-i,when the micro-pyramidal patterned PDMS makes contact with the external material,because of their different triboelectric series,electrificatio occurs at their interface and generates the same number of charges with opposite polarities between the two contact surfaces.With continuing pressure,the DHS is compressed naturally as an elastomer,but limited by the operating principle,it is almost impossible for the upper triboelectric part to detect the deformation process.By contrast,the resistance of the lower hydrogel unit decreases accordingly and is converted into a current variation when a constant voltage is applied.Therefore,the detailed variations of the external force(including increasing,holding,and decreasing)can be recorded effectively(as shown in Figs.4-ii and 4-iii).Once the contacting object separates from the DHS,electrostatic induction means that positive charges flo from the ground to the upper hydrogel electrode(Fig.4-iv)and a current is generated in the external circuit.At the maximum distance between the two surfaces,electrostatic equilibrium is achieved and no current flow in the circuit(Fig.4-v).When the upper positively charged surface approaches the negatively charged PDMS,the induced positive charges in the hydrogel decrease,thus electrons flo from the ground to the hydrogel electrode,thereby producing a reversed current signal(Fig.4-iv).Consequently,the closer the external object and the PDMS are to each other,the fewer positive charges are induced in the hydrogel electrode.During the whole working cycle,the variations of the hydrogel resistance and output performance can be measured by a digital multimeter and an electrometer respectively,which we analyze to evaluate the comprehensive states of the external pressure.

III.RESULTS AND DISCUSSION

The pyramidal patterned PDMS surface boosts the output performance of the triboelectric part significantly As shown in Fig.5(a),the sensitivity of the triboelectric part(K1=23 mV Pa-1for the fla one andK2=46 mV Pa-1for the pyramidal-structured one)is doubled in the pressure range of 0-1000 Pa,which we attribute mainly to the larger friction area between the two triboelectric layers.The sensitivityKunder pressurePis calculated as

FIG.5.Performance tests oftriboelectric and piezoresistive parts:(a)outputperformance oftriboelectric partwith pyramidalpatterned surface(blue,triangles)and fla surface(green,squares);(b)real-time response of piezoresistive hydrogelpartunder different pressures atinputvoltage from 1 to 6 V;(c)sensitivity of piezoresistive part,with high sensitivity of58.1%kPa-1 below 1.55 kPa and 15.3%kPa-1 at higher pressures(1.55–7 kPa).

whereΔVis the peak voltage change of the triboelectric part under applied pressure,andΔPis the applied pressure variation.To investigate the piezoresistive effect of the elastic hydrogel,we use the silver electrodes with various current signals to detect the dynamic change of the resistance.As shown in Fig.5(b),the slope of the current variation increases accordingly,which indicates the effective sensing ability of the piezoresistive hydrogel part.When the voltage source is decreased to 1 V,a clear difference under pressure is still obvious,and 1-V power sources are ubiquitous(e.g.,AA batteries)and so are convenient for daily applications.Similarly,the sensitivity of the piezoresistive part is also investigated to evaluate the device performance.This sensitivity is calculated as

whereΔIis the change of the current under applied pressure,I0is the initial current with no compression,andΔPis the pressure variation.As shown in Fig.5(c),the current is linearly related to the applied pressure in two regions.With increasing applied pressure,the height of the piezoresistive part decreases correspondingly,which further reduces the total resistance and increases the current.The piezoresistive part has a relatively high sensitivity of 58.1%kPa-1in the low-pressure range(0-1.55 kPa)and 15.3%kPa-1in the high-pressure range(1.55-6.5 kPa),which we attribute mainly to the large deformation rate of the hydrogel initially and the decreased variation rate at higher pressure,respectively.

By encapsulating both the triboelectric and piezoresistive parts,the output signals of the DHS under different stimulations(contact and separation)are further acquired and processed through the dual-channel testing system(Fig.6).With increasing applied pressure from 10 Pa to 300 Pa,the amplitude of the triboelectric voltage grows correspondingly with a short time lag(～50 ms),which shows excellent ability for detecting the dynamic contact-separation pressure.Meanwhile,the piezoresistive part is also sensitive to the pressure strength,with the current variation ratio not only responding to the compression strength but also maintaining static continuity according to the applied pressure.However,because of elastic damage and the dissipation of pressure energy,a slight hysteresis was found in the load-release cycles.Consequently,the piezoresistive part exhibits stable sensitivity to compression at the expense of a lower response rate,which is suitable for detecting static pressure.Therefore,the triboelectric and piezoresistive pressure sensors can work together effectively to measure multidimensional pressure information including strength,duration,and long-term static variations.Compared with a traditional tactile sensor based on tribo-only or piezo-only mode,the DHS provides a comprehensive method with double-detection dimensions for pressure sensing.

The DHS not only monitors contact and separation times rapidly and accurately but also responds accurately to the pressing strength,thereby providing considerable potential for self-powered button switches.A conventional switch is merely either on or off,which constrains the flexibilit and functionality of electronic devices.In the present study,we designed and fabricated an intelligent button switch based on the DHS as a visible door lamp for monitoring finge motions and dynamic pressure variations.As shown schematically in Fig.7(a),this visible notificatio system connected to a simple lamp acts as pressure-sensitive intelligent button switch to control the brightness.While the tester’s finge remains separate from the DHS,the lamp remains off[Fig.7(a-i)].When the finge touches the DHS,the lamp receives power because of the customized logic strategy[Fig.7(a-ii)],and then the brightness changes dynamically according to the pressure intensity(increasing or decreasing)[Figs.7(a-iii)and 7(a-iv)].

FIG.6.Sensing abilities of DHS to detect dynamic and static pressures under differentstimulation strengths.

FIG.7.DHS used as intelligent visible door lamp to further monitor pressing motions and dynamic variations of strength:(a)schematics ofintelligentvisible door lamp while(i)separated,(ii)touching,(iii)pressing,and(iv)releasing;(b)dual-channelsignalprocessing system for triboelectric and piezoresistive parts in DHS;(c)changes in brightness of intelligent door lamp under different pressures;(d)operating outputs of DHS with applied dynamic and static pressures(the insets show the considerable response speed ofthe DHS).

As shown in Fig.7(b),the whole system has three main parts:(i)the DHS sensor used to detect the external stimulation(upper triboelectric part)and regulate the current in the circuit(lower piezoresistive part);(ii)the signal acquisition and processing system comprising a signal acquisition module,threshold value comparison module,signal amplifie module,and driving control module,and(iii)the terminals of the controlled electrical device.The triboelectric part is connected to the signal acquisition card(USB-6289;National Instruments Corporation),and the output channel is scanned sequentially by real-time customized data processing procedures based on the LabVIEWsoftware.Then,a pre-set threshold value comparison function processes the voltage signals into a binary signal(0/1)that is processed by an STM32 amplifie module and then converted to the ON/OFF status of the power circuit,which then drives the controlled electrical device directly.When the circuit turns on,the lower piezoresistive part acts as the current regulation module to control the output power directly.According to the above output characteristics,we introduced an array of light-emitting diodes(LEDs)into this system to fabricate an intelligent door lamp as a visible manifestation of the system’s application value.When a person just touches the DHS,the LED array is illuminated[Fig.7(c-i)]and then brightens with increasing pressure[Fig.7(c-ii)].The whole working process and the output signals of the DHS were recorded by a digital multimeter,as shown in Fig.7(d).The tester compressed the DHS three times with incremental pressure,followed by a slow release and then full separation from the DHS.The signals generated in the dual-channel system indicate the rapid contact-separation response of the upper triboelectric part,which demonstrated faster response(55 ms for pressing and 43 ms for releasing)compared with the lower piezoresistive part(60 ms and 200 ms,respectively).The lower hydrogel part responded effectively when force was applied,this was because the pressure and deformation occurred almost simultaneously.When the pressure was released,the recovery speed of the hydrogel usually lagged behind the variation speed because of the elasticity limitation,which led to the lower response speed of the lower piezoresistive part when the force was released.During the static variations,the piezoresistive part recorded the compression in detail[see the three distinct peaks in the piezoresistive channel in Fig.7(d)].With the support of the sensing and controlling system,similar current-sensitive devices could be introduced to enlarge the application scope of the DHS,such as doorbells and shape-memory alloys.Therefore,the DHS offers promise for intelligent button switches and the control of electronic devices.

IV.CONCLUSIONS

In summary,triboelectric and piezoresistive parts based on ionic elastic hydrogel and PDMS have been integrated together in a duplex sensor to measure the complex pressing process.Because of the stable compressive recoverability and stable piezoresistive property of the hydrogel,in addition with the pyramidal-patterned triboelectric part,the DHS can be used to detect detailed pressure information.Excellent sensing ability(46 mV Pa-1)and rapid response(<50 ms)of the fabricated triboelectric part have been realized.Also,the current regulator based on piezoresistive hydrogel bulk achieved the high sensing ability of 58.1%kPa-1in the lowpressure range(0.01-1.55 kPa)and 15.3%kPa-1in the high-pressure range(1.55-6.5 kPa).Therefore,multidimensional pressure information including stress intensity,application period,and force variations can be assessed in detail according to the dual-channel signals.Combined with a customized signal acquisition and processing system,the DHS was used in an intelligent visible door lamp that could not only detect different pressure states but also control the output power directly,thereby demonstrating the considerable potential of the DHS for intelligent sensor systems and flexibl electronics.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China(Grant Nos.51705429 and 61801525)and the Fundamental Research Funds for the Central Universities,Guangdong Natural Science Funds(Grant No.2018A030313400).

AUTHOR DECLARATIONS

Conflict of Interest

The authors have no conflict to disclose.

DATA AVAILABILITY

The data that support the finding of this study are available from the corresponding authors upon reasonable request.