Advancing the pressure sensing performance of conductive CNT/PDMS composite film by constructing a hierarchical-structured surface

Ye Zho,Toyu Shen,Minyue Zhng,Rui Yin,b,Ynjun Zheng,＊,Hu Liu,＊＊,Hongling Sun,Chunti Liu,＊＊＊,Chngyu Shen

a Key Laboratory of Materials Processing and Mold(Zhengzhou University),Ministry of Education,National Engineering Research Center for Advanced Polymer Processing Technology,Zhengzhou University,Zhengzhou,Henan,450002,China

b China Astronaut Research and Training Center,Beijing,100094,China

Keywords: Flexible pressure sensor Hierarchical structure Polydimethylsiloxane Carbon nanotubes Electronic skin

ABSTRACT Flexible pressure sensors have attracted wide attention due to their applications to electronic skin,health monitoring,and human-machine interaction.However,the tradeoff between their high sensitivity and wide response range remains a challenge.Inspired by human skin,we select commercial silicon carbide sandpaper as a template to fabricate carbon nanotube (CNT)/polydimethylsiloxane (PDMS) composite film with a hierarchical structured surface (h-CNT/PDMS) through solution blending and blade coating and then assemble the h-CNT/PDMS composite film with interdigitated electrodes and polyurethane (PU) scotch tape to obtain an h-CNT/PDMS-based flexible pressure sensor.Based on in-situ optical images and finite element analysis,the significant compressive contact effect between the hierarchical structured surface of h-CNT/PDMS and the interdigitated electrode leads to enhanced pressure sensitivity and a wider response range (0.1661 kPa-1,0.4574 kPa-1 and 0.0989 kPa-1 in the pressure range of 0–18 kPa,18–133 kPa and 133–300 kPa)compared with planar CNT/PDMS composite film (0.0066 kPa-1 in the pressure range of 0–240 kPa).The prepared pressure sensor displays rapid response/recovery time,excellent stability,durability,and stable response to different loading modes (bending and torsion).In addition,our pressure sensor can be utilized to accurately monitor and discriminate various stimuli ranging from human motions to pressure magnitude and spatial distribution.This study supplies important guidance for the fabrication of flexible pressure sensors with superior sensing performance in next-generation wearable electronic devices.

1.Introduction

Wearable electronic devices have drawn increasing attention due to their excellent flexibility and promising applications to electronic skin(Eskin) [1,2],health monitoring [3,4],and human-machine interaction[5–7].Flexible strain/pressure sensors play a key role in wearable electronics,which are typically categorized on the basis of various working mechanisms[8],including piezoresistivity[9–12],capacitance[13–15],piezoelectricity [16,17],and triboelectricity [18].In particular,piezoresistive pressure sensors,which convert physical pressure into resistance/current signals,have been widely developed due to their low cost,simple signal readout process,and reliable sensing properties.However,piezoresistive pressure sensors do not typically have high sensitivity and wide response range simultaneously,which would make them difficult to satisfy many different requirements of human motion and health monitoring.

Nowadays,flexible conductive polymer composites (CPCs) typically composed of elastomers and conductive fillers are extensively utilized as the core components of wearable pressure sensors owing to their costeffective fabrication,diversified material selection,and high sensing performance.Elastomers,such as polydimethylsiloxane(PDMS)[19–21],thermoplastic polyurethane (TPU) [22,23],and Eco-flex [24],could endow CPCs with good flexibility and stretchability.Various advanced nanomaterials,such as carbon black (CB) [25,26],carbon nanotubes(CNT)[27–30],reduced graphene oxide(rGO)[31–34],metal nanowires[35,36],and nanoparticles [37,38]have been used as conductive fillers due to their outstanding electrical and mechanical properties.However,CPCs-based wearable pressure sensors generally exhibit low sensitivity due to the subtle change of the conductive network structure of CPCs under compression,which could limit their applications in full-range human motion monitoring.

To solve this problem,scientists have introduced a variety of micro/nanostructures,such as micropyramids [39–41],nanowires [42,43],hemispheres,and microdomes [44–47]in order to enhance the sensitivity of CPCs-based piezoresistive pressure sensors arising from their significant compressive contact effect.For example,Zhu et al.[48]fabricated rGO/PDMS-based pressure sensors with micropyramids using photolithography and silicon etching,and these exhibited a sensitivity of 5.53 kPa-1at 0–100 Pa and 0.01 kPa-1at 100–1400 Pa.Bae et al.[49]fabricated a bioinspired hierarchical graphene/PDMS pressure sensor by photolithography,selective wet etching,and chemical vapor deposition(CVD),and their sensor showed a sensitivity of 8.5 kPa-1in the pressure range of 0–12 kPa.Nevertheless,the limited compressive deformation of these designed micro/nanostructures usually endows the pressure sensor with a narrower albeit highly sensitive response range,and their complicated processing techniques also restrict their development in practical application.

Aiming to enhance the pressure sensing performance of conductive CNT/PDMS-composite-film-based pressure sensors,we fabricated a piezoresistive wearable pressure sensor based on hierarchical structured CNT/PDMS composite film (h-CNT/PDMS) through blending and blade coating techniques using commercial silicon carbide sandpaper as the template.In order to compare the sensing performance between hierarchical and planar structured pressure sensors,we then conducted pressure sensing tests and in-situ morphology characterization under uniaxial compression.We then applied finite element analysis (FEA) in order to study these test results.In addition,we implemented a series of sensing tests to explore the response/recovery time,stability,and durability of a h-CNT/PDMS-based pressure sensor.Finally,we utilized our pressure sensor to detect different human motions and external stimuli to evaluate its application potential as E-skin.

2.Experimental materials and methods

2.1.Materials

We purchased PDMS(Sylgard 184 Silicone Elastomer),including base polymer(Kit A)and curing agent(Kit B),from Dow Corning Corp.,USA and bought Multi-walled CNT (TNM1,inner diameter: 2–5 nm,outer diameter:5–15 nm,length:10–30 μm)from Chengdu Organic Chemicals Co.Ltd.Xylene and ethanol were obtained from the Tianjin Fuyu Fine Chemical Co.,Ltd,and commercially available silicon carbide sandpapers(#60) were purchased from a local market.Polyimide (PI) based interdigitated electrodes were bought from Guangzhou Yuxin Sensor Technology Co.,Ltd,and polyurethane (PU) scotch tape was bought from Shanghai Hons Medical Technology Co.,Ltd.

2.2.Fabrication of h-CNT/PDMS film

To fabricate our film,we added a certain amount of CNT into 80 mL of xylene and treated it by ultrasound for 30 min and magnetic stirring for 1 h to obtain a homogeneous CNT dispersion,which we then mixed with diluted PDMS base polymer(15 g PDMS base polymer in 15 mL xylene)under magnetic stirring for 6 h.After evaporating most of the solvent of the above mixture in a petri dish,we added curing agent (1.5 g) and mixed it homogeneously to obtain the final CNT/PDMS mixture with a CNT loading of～ 2 wt%.Subsequently,the CNT/PDMS mixture was blade-coated onto the sandpaper template and then degassed in a vacuum desiccator for 15 min to remove the final traces of solvent.Finally,we obtained h-CNT/PDMS film after curing the mixture at 80°C for 3 h and peeling it off from the sandpaper template.For comparison,we also fabricated the planar CNT/PDMS composite film(p-CNT/PDMS)using a glass template under the same procedure.

2.3.Fabrication of the flexible pressure sensor and E-skin

We assembled our flexible pressure sensor by sandwiching the prepared h-CNT/PDMS film between PI-based interdigitated electrodes and PU scotch tape.The flexible conductive tape was then connected with the interdigitated electrodes for our subsequent sensing performance tests.In accordance with our previous work [50–52],we placed flexible E-skin with 4×4 pressure sensor arrays onto a polycarbonate(PC)film with the aid of double-sided adhesive tape.

2.4.Characterization

To analyze the microstructure morphology of our sample material,we used a field-emission scanning electron microscope(SEM,Zeiss MERLIN Compact,Germany) and ultra-depth three-dimensional microscope(UDTDM,Olympus,DSX 510).We recorded the Raman spectrum using a Raman spectrometer(Renishaw inVia,Renishaw Company,UK)with the excitation wavelength set at 633 nm.The pressure sensing performance of our sensors was assessed on a homemade test system,which simultaneously connected a universal test machine(UTM2203,Shenzhen Suns Technology Stock Co.,Ltd.,China) and an electrochemical workstation(CHI660E,Shanghai Chenhua Instruments Limited (SCHI),China) to a computer for in-situ current collection during the compressive process.We also recorded the current-voltage curves of the pressure sensors using the CHI660E electrochemical workstation.

3.Results and discussion

Fig.1a shows the fabrication process of the h-CNT/PDMS-based flexible pressure sensor according to the details listed in Section 2.As shown in Fig.1b,the outstanding physical characteristics of PDMS are well maintained after mixing with CNT and constructing the designed hierarchical structure,endowing the obtained h-CNT/PDMS with excellent overall flexibility,making the assembled pressure sensor (Fig.1c)readily applicable to flexible electronic devices.In addition,as depicted in Fig.1d and Video S1,a blue light-emitting diode(LED)connected with the pressure sensor becomes brighter with increasing pressure and then turns to the original brightness once the pressure is released,showing the typical piezoresistive effect.Hence,our prepared h-CNT/PDMS-based flexible pressure sensor may have great potential to be utilized in the field of tactile sensing.

Fig.1.(a)The schematic diagram of the fabrication process of our h-CNT/PDMS-based flexible pressure sensor.(b) Photographs of h-CNT/PDMS with good cutting ability,bendability,stretchability,and twistability.(c) Photographs of the assembled pressure sensor and its flexible characteristics.(d) Light intensity variation of LEDs connected with the pressure sensor under different pressures.

We display the SEM top view and lateral view images of h-CNT/PDMS with different magnifications in Fig.2a–c.Evidently,numerous hierarchical protrusion structures with different heights appear on the surface,and the surface hierarchical structure displays a random height distribution within the range of 0–0.6 mm based on the 3D reconstruction image of h-CNT/PDMS (Fig.2e and f),indicating the successful construction of the designed hierarchical structured surface.In addition,we also see that CNTs are uniformly distributed in the PDMS matrix(Fig.2d),ensuring the construction of a stable conductive network,which is critical for good pressure sensing performance.

Fig.2.(a–b) SEM top view and (c–d) SEM lateral view images of h-CNT/PDMS with different magnifications.(e) The 3D reconstruction image of the h-CNT/PDMS and its corresponding height distribution.(g) Raman spectra of the PDMS,CNT,and h-CNT/PDMS.

Furthermore,as shown in Fig.2g,the Raman spectra of pure PDMS reveals three peaks at around 492,618,and 710 cm-1,which are ascribed to Si–O–Si symmetric stretching vibration and Si–C symmetric stretching,respectively.In addition,peaks at around 1262,1413,2907,and 2967 cm-1correspond to the symmetric bending,asymmetric bending,symmetric stretching,and asymmetric stretching vibration modes of CH3,respectively.The Raman spectrum of the CNT shows three characteristic peaks at around 1346,1590,and 2687 cm-1,corresponding to the D-band,G-band,and 2D-band of CNT,respectively[53,54].Finally,for the h-CNT/PDMS,all the typical peaks of CNT and pure PDMS can be completely observed,but the peak intensity of PDMS exhibits a weaker value,indicating a good combination between CNT and PDMS.

Fig.3a and b show the relative current variation under pressure(ΔI/I0=(I– I0)/I0,where I and I0correspond to the current with and without pressure,respectively) of h-CNT/PDMS-and p-CNT/PDMS-based pressure sensors as a function of external pressure(P).The sensitivity(S) of the pressure sensor can be calculated by S=(ΔI/I0)/P[55,56].As shown in Fig.3a,the h-CNT/PDMS-based pressure sensor displays more obvious relative current variation within the total pressure range compared with that of p-CNT/PDMS-based pressure sensor,even at a low strain range(Fig.3b).In other words,the h-CNT/PDMS-based pressure sensor displays higher sensitivity and wider sensing range than that of p-CNT/PDMS (0.0066 kPa-1in the range of 0–240 kPa).The corresponding sensitivities are calculated to be 0.1661 kPa-1,0.4574 kPa-1and 0.0989 kPa-1in the pressure range of 0–18 kPa,18–133 kPa,and 133–300 kPa,respectively.

Fig.3.(a) Relative current variation of h-CNT/PDMS-and p-CNT/PDMS-based pressure sensors as a function of external pressure.(b) The zoomed-in plot of the dotted box in (a).(c) Optical images showing the evolution of the contact area between h-CNT/PDMS film and the test surface glass under different pressures.(Top:color dotted lines showing contact site variation under different pressures;Bottom:schematic diagram of the working mechanism under different pressures).(d)FEA modeling of the contact state of (Top) h-CNT/PDMS and (Bottom) p-CNT/PDMS films under different pressures.(e) Contact area versus applied pressure of p-CNT/PDMS and h-CNT/PDMS films based on the simulation results.

As for our sandwich-structured pressure sensor,the sensing mechanism is mainly related to the change of contact area between the conductive composite film and the interdigitated electrodes and variation of conductive network inside the composite film upon external pressure.For the p-CNT/PDMS-based pressure sensor,the planar surface is almost entirely in contact with the interdigitated electrodes at the initial state,and the pressure-induced contact area variation is subtle.In addition,the conductive network variation inside this composite film is also limited,resulting in relatively low sensitivity.However,the gradual and remarkable compressive deformation of our special hierarchical structure upon increasing pressure can lead to a significant change in contact area between the h-CNTs/PDMS film and interdigitated electrodes,leading to higher sensitivity under a wider sensing range.

To corroborate the sensing mechanism contemplated above,we apply in-situ observation to explore the evolution of the contact area between the h-CNT/PDMS and interdigitated electrode layer with increasing pressure.We note that a transparent glass sheet was utilized to replace the interdigitated electrodes for ease of observation.As shown in Fig.3c,the contact area variation between the h-CNTs/PDMS film and the glass under different pressures is outlined by differently colored dotted lines.Here,we observe only few contact sites (e.g.,red,orange,and yellow dashed lines) between the film and the glass without external pressure(Fig.3c1).When pressure is applied,compressive deformation of the initial contact sites occurs,causing a larger contact area.In addition,some new contact sites (e.g.,blue and green dashed lines) also appear,contributing to the increased current of the pressure sensor (Fig.3c2).With further increasing the pressure,larger compressive deformation and more new contact sites (e.g.,purple,pink,and dark blue dashed lines)lead to a higher sensitivity (Fig.3c3) [57].After this stage,the contact area gradually becomes saturated,and the level of sensitivity exhibits a decreasing trend.

To further elucidate the effect of the hierarchical structure on the pressure sensing performance,we conduct FEA analysis to simulate the microstructure surface deformation process of the h-CNT/PDMS and p-CNT/PDMS films under different external pressures [58].As shown in Fig.3d &Fig.S1,when we applied external pressure on the h-CNT/PDMS-based pressure sensors,stress can be concentrated on the initial contact sites of the hierarchical structure,and (the displayed)sliding caused by the compressive deformation of microstructure leads to increased contact area.As the pressure increases,the stress increases and more microstructures become squeezed,the contact area between the film and interdigitated electrode layer increases sharply until saturation(Video S2).

However,the surface of p-CNT/PDMS film always remains in complete contact with the interdigitated electrode layer during the whole process,and the contact area displays a slight change according to the sliding result(Video S3).Based on the FEA modeling,Fig.3e shows the simulated contact area of p-CNT/PDMS and h-CNT/PDMS film under applied pressure,which is consistent with the trend of relative current variation with pressure shown in Fig.3a.In addition,we note that the hierarchical structure exhibits persistent variation from the simulation results over the whole pressure range,indicating the wider sensing range of the h-CNT/PDMS-based pressure sensor.Hence,we achieve a high sensitivity over a wide sensing range for our h-CNTs/PDMS-based pressure sensor owing to persistent and significant contact area variation.

Next,we discuss the pressure sensing performance of the h-CNT/PDMS-based pressure sensor under different external conditions.As depicted in Fig.4a,the current-voltage(I–V)curves of the h-CNT/PDMSbased pressure sensors under various pressures present typical ohmic linear behavior,which is critical for a stable pressure response [22,31].Moreover,the increased slope of the I–V curve with increasing pressure indicates a higher current,which coincides with the results in Fig.3a.Fig.4b and c shows the pressure sensing behavior of the sensor with cyclic loading/unloading cycles under different pressures range from 5 to 190 kPa with a compression rate of 1.5 mm/min.Higher relative current variation peak intensity is obtained for a greater pressure as expected based on the sensing mechanism discussed above,but the h-CNT/PDMS-based pressure sensor also exhibits excellent stability and repeatability under different pressures.

Fig.4.(a) Current-voltage curves of the pressure sensor subjected to various pressures.(b,c) Relative current variation of the pressure sensor subjected to loading/unloading cycles at various applied pressures.(d)Relative current variation of the pressure sensor at different frequencies.(e)Response time and recovery time of the pressure sensor under the pressure of 125 kPa at a compression rate of 200 mm/min.(f)The reliability test of the pressure sensor under 50 kPa for 1500 cycles.The sensing behavior of the pressure sensor upon(g)bending,(h)torsion,and(i)water(the pressure and compression rate were 25 kPa and 1.5 mm/min,respectively).

We also considered the compression rate in order to evaluate the pressure sensing performance of our h-CNT/PDMS-based pressure sensor.As depicted in Fig.4d,the relative current variation peak intensity of the sensor stays constant at different compression rates(1.5,2.5,and 5.5 mm/min) under a pressure of 35 kPa.This typical compression-rate-independent sensing performance indicates that the sensor can efficiently avoid the interference of compression with the output signal.Fig.4e shows that the response and relaxation time of the sensor was 320 and 100 ms,respectively,indicating promise for real-time human motion monitoring.

We also conducted a test with 1500 loading/unloading cycles under a pressure of 50 kPa with a compression rate of 3.5 mm/min in order to study the fatigue resistance of the pressure sensor.Fig.4f shows the response pattern keeps almost unchanged even after 1500 loading/unloading cycles.In addition,there we also see no obvious change in relative current variation peak intensity (inset in Fig.4f),implying excellent long-term durability of the sensor,which can be attributed to the stable mechanical and electrical properties of the h-CNTs/PDMS composite film.Furthermore,when we also subjected the h-CNTs/PDMSbased pressure sensor to cyclic bending and torsion deformation(Fig.4g and h),we observed stable and reproducible response patterns.Finally,due to the protection of the sandwich structure,the sensor also possesses good waterproof properties,making its application possible in the presence of water as well(Fig.4i).Hence,our h-CNTs/PDMS-based pressure sensor showed outstanding sensing performance and great versatility useful in wearable devices.

Following its excellent pressure sensing performance,we tested our h-CNT/PDMS-based pressure sensor in monitoring various human activities.As shown in Fig.5a,relative current variation increases correspondingly when a finger touches the pressure sensor,and then it recovers to the initial value immediately after removing the finger.Crucially,the sensor shows obvious differences between the relative current variation peak intensities under different touch intensities.Similarly,different sensing signals are also accurately detected when the pressure sensor was attached to the index finger and bent to different angles (Fig.5b).

Fig.5.Response pattern of the pressure sensor towards various human motions,including(a)finger touch,(b)finger bending,and(c)elbow bending(d)Photograph of a 20 g weigh and a 100 g weight laying onto the E-skin and(e,f)the corresponding pressure sensing mapping based on the current variation.(g)Photographs of the E-skin attached onto a clenched hand and (h,i) the corresponding pressure sensing mapping based on the current variation.(j) Schematic diagram of the wireless sensing system design.(k) Signal of the wireless sensing system for monitoring pressures with different frequencies.

Moreover,stable and repeatable sensing signals are clearly observed when bending a human elbow to a fixed angle(Fig.5c).Additionally,we fabricated an E-skin consisting of a 4×4 sensor array to monitor spatial pressure distribution.Owing to the outstanding flexible character of our h-CNTs/PDMS-based sensor,the assembled E-skin can be bent arbitrarily and attached onto the human body easily(Fig.S2).As shown in Fig.5d–f,when two balance weights with different masses (100 g and 20 g) are placed on the E-skin,the spatial pressure location and corresponding pressure can be well identified from the current variation in pressure sensing mapping.Similarly,the pressure levels of different areas of human skin resulted from bending the elbow can also be precisely identified(Fig.5g–i).

Finally,as shown in Fig.5j,we also built a wireless sensing system,consisting of the pressure sensor,microprocessor,and mobile phone for data recording [56,59]to further test the practical application of our h-CNTs/PDMS-based pressure sensor.As a result,stable signals can be successfully received by the mobile phone when the finger presses on the pressure sensor with different frequencies(Fig.5k&Video S4).All these results indicate good potential for our sensor's application to detecting human motion and spatial pressure distribution.

4.Conclusions

In conclusion,the hierarchical structured h-CNT/PDMS film was fabricated using solution blending and blade-coating and then sandwiched between interdigitated electrodes and PU scotch tape to obtain the h-CNT/PDMS based piezoresistive pressure sensor,showing greater pressure sensitivity across a wider range when compared with the p-CNT/PDMS film.It is the successful construction of a hierarchical structure that endows the h-CNT/PDMS-based pressure sensor with this enhanced sensitivity under a wide response range (specifically 0.1661 kPa-1,0.4574 kPa-1and 0.0989 kPa-1in the pressure range of 0–18 kPa,18–133 kPa,and 133–300 kPa).The evolution of our hierarchical structure under pressure is visualized by in-situ observation and FEA simulation to explore the precise pressure sensing mechanism.From this we see that our prepared pressure sensor presents fast response/relaxation time (320/100 ms),good stability,and excellent durability (1500 cycles).Additionally,our pressure sensor is also robust to bending and torsion deformation and even the presence of water.Finally,our pressure sensor exhibits good potential for applications in the detection of different human motions and spatial pressure distribution.

Declaration of competing interest

The authors declare no competing financial interests.

Acknowledgements

This research was financially supported by the National Natural Science Foundation of China (NO: 51803191,12072325,52103100),the National Key R&D Program of China(2019YFA0706802),the 111 project(D18023),and the Key Scientific and Technological Project of Henan Province(202102210038).

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.nanoms.2021.10.002.