Research Explorer | The University of Manchester · Web viewTo start, basic mechanical properties and electrical properties were investigated to evaluate the sensor’s limitation.

Stable Wearable Strain Sensors on Textiles by

Direct Laser Writing of Graphene

Wen Liu1,2, Yihe Huang2, Yudong Peng1,2, Monik Walczak3, Dongwang1, Qian Chen1, Zhu Liu1,2*

and Lin Li2

1 Department of Materials, Faculty of Science and Engineering, The University of Manchester,

Oxford Road, Manchester M13 9PL, UK

2 Laser Processing Research Centre, Department of Mechanical, Aerospace and Civil

Engineering, Faculty of Science and Engineering, The University of Manchester, Manchester,

M13 9PL, UK

3 Department of Chemistry, Faculty of Science and Engineering, The University of Manchester,

Oxford Road, Manchester M13 9PL, UK

*Corresponding author: [email protected]

Kewords: Laser-induced graphene, fabric strain sensor, wearable electronics, polyimide fabric,

laser fluence.

Abstract: Strain sensors for smart wearable textiles have recently attracted great attention and

interest due to their potential in the healthcare applications, specifically, functions to track

heartbeat, pulse signals and movements of limbs and joints. Traditional methods typically require

relatively complicated procedures including dip-coating in different solutions to prepare sensing

fibers before weaving into fabrics. In this study, we used an ultraviolet picosecond laser to

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directly induce graphene on the polyimide (PI) fabric to produce a strain sensor. The process,

which is mask-free, easy-to-fabricate and the graphene tracks are well-adhered to the substrate.

High-quality 3D-porous graphene was produced directly using appropriate laser parameters with

a sheet resistance as low as 20 /sq. The We demonstrated graphene strain sensors showed on

polyimide (PI) fabric with high sensitivity within a small strain range (strain below 4%)

(GFmax=27), good linearity, a low threshold value (strain=0.08%) and high stability (4%

resistance loss after 1000 cycles). Furthermore, reliable signals gathered from varies human

motions demonstrated the potential for health-care monitoring using for this laser-induced

graphene (LIG) fabric strain sensor.

1. Introduction

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Wearable electronics or devices with functions to transform external stimuli into readable

electrical signals are needed in recent years for healthcare 1-6. As one of the most critical

wearable electronics, flexible and portable strain sensors have shown great potential in detecting

human motion7, muscle movement8 and metabolic rate9-10. Conventionally, strain sensors

normally use nano metallic particles or semiconductor materials to detect and transmit force

signals11-12. These sensors, which are mainly used for rigid surface and designated environment,

have low sensitivity and low flexibility, failing to meet the demand for complex signal detection

in the human body13. To achieve high-reliability and multi-functionality in sensing performance,

various properties of strain sensors need to be optimised, such as sensitivity, stretchability,

durability, and mechanical strength, etc. Numerous innovations have been made to simplify the

procedure for of sensing material preparation or reduceing the cost in sensor fabrication14-15.

The basic structure of strain sensors includes active materials, substrates and electrodes16. For the

three components, aActive materials stand at the core of flexible devices as their electrical

properties and controlled structures determine the performance of these strain sensors17. To date,

nanomaterials such as metallic nanostructures18, carbon nanotubes19-20, graphene21-22, conductive

polymers23 and nano-composites24-25 are considered to be competitive alternatives to achieve

satisfying sensing responses. Among them, graphene with various designed structures has

demonstratesd promising potentials due to its excellent mechanical and electrical properties26-27.

However, traditional methods for the graphene preparation are complex and costly challenging,

typically involving an excessively complicated synthesis process. For example, chemical vapour

deposition which was is commonly utilised for large area graphene growth requires flammable

gas precursors like CH4 and metallic substrates such as copper and nickel mesh28. TAnd the

transfer of graphene to a another textile substrate is very difficult and patterning requires another

process step. Chemical or thermal reduction of graphene oxide is limited by complex solution

reaction to produce reduced graphene oxide29. These existing methods also confront the problem

that post-synthesis steps are inevitable to transfer the attained graphene onto substrates and their

curing. In most cases, the whole procedure has demands for a reliable interface attachment.

The technique of laser reducing has been widely employed to turn graphene oxide into

functionalized graphene30. Interestingly, researchers in the Rice University accidentally found

that using a CO2 laser, one can directly induce a polyimide(PI) tape into 3D-porous graphene

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without any precursor or support chemical31. Tour’s group32 successfully disclosed identified the

essential conditions in the chemical structure and related mechanism to form laser induced

graphene (LIG) from a wide range of carbon resources including commercial polymers, wood,

paper, food with a by CO2 laser irraidation. By testing a large variety of materials, they found

that only the materials containing poly- or heterocyclic structures such as the imide group

structure can be converted to the LIG with appropriate laser fluences through high temperature

conversion. Some natural materials with predominantly cellulose content can also be converted

to LIG after pretreatment to form amorphous carbon before laser scribing. Han and his

colleagues33 produced LIG composites by impregnating metal salt during laser scribing and the

LIG composites performed as well as electrocatalysis electrodes for the oxygen reduction

reaction. Huang and co-workers from Tthe University of Manchester34 developed a

Polybenzimidazole (PBI) ink to produce heteroatom(N and S)-doped LIG using a UV laser

beam, which greatly improved surface hydrophilicity and electrical properties of prepared

graphene. Further study in this field has shown that outstanding electrical properties of this LIG

make it possible for applications like energy storage, electrochemical catalyst as well as

sensors35-37.

Beside active materials, substrates are also of vital importance in strain sensors. To endow a

strain sensor with flexibility, some human-friendly supporting polymers like

polydimethylsiloxane(PDMS) and ecoflex have been well-studied and combined with active

materials to build up strain sensor systems38. For the usage of the commercial fabric matrix,

however, few studies have reported similar work, or strain sensors based on fabric are mostly

fabricated by dip-coating which takes a significant amount of time to coat sensing particles on

fibres and hard to achieve large area coating39. Muhammad and collegues developed a

mercerized conductive cotton fabrics based on PEDOT:PSS/graphene through spraying coating

which is free from complicate synthesis processes and shows good strain-resistance responsed40.

However, the adhesion strength of the active materials limits the stability of the fabric based

sensor in long-term working.

In this paper, we report the development of a new method of laser direct writing to fabricate PI

fabric based strain sensors through a pulsed ultraviolet picosecond laser irradiation. Considering

both the cost and the complexity of synthesis, we chose the PI fabric rather than natural fibers

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like cotton which requires pretreatment to form amorphous carbon before laser fabrication. Many

studies on the generation of graphene on PI tapes and their application on different devices have

been reported,. However, but no one report has have been found on directly producinged

graphene on an ultra-thin PI fabric for precise sensing applications without using methods like

coating or transferring from other raw materials. Our work has It was found that the quality of

LIG was is heavily related to the laser fluence which means LIG only can be converted with an

appropriate laser energy range. DigitallyProgramm-designed graphene patterns grown from the

PI fabric surface followed the woven direction of the fibre, which demonstratesd excellent

sensitivity (GFmax=27) in assembled strain sensors. Experimental results also showed that good

linearity, low minimum strain response limit (strain=0.08%) and good mechanical durability for

over 1000 cycles were can be achieved by using appropriate laser energy. In addition to these,

real-time detection for finger, wrist bending and muscle tension prove the ability of the PI fabric

strain sensors to monitor human-body activities.

2. Materials and experimental Section Procedure.

Formation of porous laser-induced graphene: The laser-induced graphene with different scanning

traces was synthesized in ambient condition. An ultraviolet pulsed laser was focused on the

polyimide fabric with a focused beam spot size of 40 m, a pulse frequency of 3372.8 kHz, and a

constant scanning speed of 100 mms-1 controlled by with a galvo multi-axis scanner. The laser

beam scanning spaceing is was set as 0.02 mm to make the overlapping scanning area just cover

the beam spot. In this study, the laser fluence used to fabricate the LIG patterns rangeds from

15.8-25.2 mJ/cm2. The polyimide fabric for laser carbonizing in this research is was aramid

polyimide fiber woven fibric which was purchased from Alibaba.com. The specification of this

polyimide fabric is shown in Table S1. The laser device in this experiment was an is Edgewave

400Ww picosecond laser (PX400-3-LW) with a maximum power of 400Ww, a pulse length of

10 ps, a the wavelength of 355 nm and a the repetition rate ranging from 490 kHz to 19.9 MHz.

The diagrams of the laser model are shown in S1.

Assembly of LIG fabric strain sensor: After designed LIG patterns were formed on the PI fabric,

the sensors were assembled using copper tape (thickness of 0.1 mm, 99.9% purity) as the

electrode. The sSilver paste was utilised to connect the copper tape and the LIG patterns,

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reducing the connection resistance. A Ppolyimide tape was used to encapsulate the connection

area.

Characterization of LIG: The surface morphology were investigated using by a Zeiss ultra 55

field-emission scanning electron microscopy. A Talos F200A transmission electron microscopy

was employed to study the microstructure and morphology of stacked LIG layers. The

crystallization and defects of LIG were characterized with by a Renishaw RM System 1000

Raman spectrometer with Cobalt Fandange 514 nm 50 mW laser. The distribution of LIG on the

PI fabric was also recorded using the Raman spectrometer in the form of Raman mapping in a

50m x 50m square. A powder X-ray diffractometer(prote AXRD) and an ultra-high vacuum

X-ray photoelectron spectroscopy(Thermo Scientific) were used to understand the chemical

composition of LIG.

Electromechanical characterization of the LIG fabric strain sensor: The sheet resistance was

tested using thought a 4-probe method which utilised a 6220 current resource (Keithley

Instruments, Cleveland, OH, USA). The eExternal strain was applied on the sensor with by an

Instron 3344L3927 2kN static testing machine. The diagrams of electromechanical test devices

are shown in figure S2. The electrical signals were recorded in real time with by a digital

multimeter (Keysight Technologies 34460A) during the stretching-releasing process. The

sensitivity can be described using the gauge factor which can be estimated through41:

GF=∆ RR ε

(1)

where ε is the strain applied to the sensors; R is the initial resistance at ε = 0, and the ∆ R is the

resistance change induced by the variation of strain.

3. Results and dDiscussion

3.1 Synthesis of LIG on PI fabric

Figure 1a-c show the fabrication process of LIG on PI fabric which is scalable, mask-free and

program-controlled. Firstly, a piece of commercial PI fabric was cut into the geometry of 50 mm

length and 10 mm width. The thickness of the PI fabric was is 0.2 mm. Afterwards, the a multi-

axis Edgewave 400 W picosecond laser machining system was employed to pattern the

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digicallyprogram-designed LIG traces on the PI fabric. In this step, the laser wavelength was 355

nm to introduce photo-chemical reactions and photothermal effects in PI fabric. The designed

lasing pattern is a rectangle of 30 mm length and 10 mm width. Referring to the previous

research, the primary mechanism of LIG conversion can be explained in microscale. Figure 2

demonstrates the chemical nature of the conversion of PI fabric into porous laser-induced

graphene with by a picosecond UV laser. It can be seen that tThe molecular structure of this PI

fabric is mainly composed of a ring-like structure based on the carbon skeleton, with numerous

C-O, C-N bondings attached. The 355 nm laser wavelength can generate photons with the photon

energy of 3.49 eVv, which leads to the photochemical process to break the C-O, C-N bondings

without requiring an extremely high temperature34. The A photothermal process also partially

controls the process of the carbon bonds breaking and reorganizing. The localised high

temperature caused by lattice vibration makes original bonding break and carbon atoms will

subsequently rearrange to form Sp2 hybridization42. Figure 1d shows a photograph of the carbon

pattern on PI fabric after laser scribing. To assembly strain sensors, copper tapes were used as

conduction electrodes, and silver paste was filled between LIG and electrodes for connection

which is illustrated as figure 1e and 1f.

Figure 1.(a-c) A schematic of LIG fabrication on PI fabric using the ultraviolet picosecond laser;

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(d) PI fabric with laser scribed carbon traces; (e) An assembled LIG fabric sensor with copper

electrodes; (f) A serpent-shape LIG fabric sensor with copper electrodes.

Figure 2. A schematic diagram shows the conversion of PI fabric into porous laser-induced

graphene by picosecond UV laser from a chemistry perspective.

3.2 Characterization of LIG grown on the PI fabric

The surface morphologies of LIG grown on PI fabric with different laser fluence were

investigated using scanning electron microscopy. In lower magnified graphs (Figure 3), it can be

seen that the degree of surface damage gradually increased with the increase of laser fluence. In

figure 3a, b, the laser ablation left only a superficial texture on the surface while maintaining the

integrity of the fabric's fibre structure. When increasing the laser fluence from 18.8 mJ/cm2 to

23.8 mJ/cm2, the porous structure started to appear and a slight variation of pore size followed

with the gradience of laser fluence leading to the formation of 3D-LIG which is showns in figure

3(c-f). The formation of the porous structure may be due to the breakage of oxygen and nitrogen

bondings in the organic fabric during laser scribing43. These nitrogen and oxygen atoms after

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bond breaking were converted into corresponding gas oxides and escape from the substrates44. In

higher-resolution images, both surface texture and small particles embedded in larger carbon

flakes can be clearly seen. It is worth noting that the higher energy injectioning resulted in

carbon particles growing on the edge of fibre frame which replaced the network-like texture on

the surface (figure 4(a-f)). This was possibly achieved by thermal decomposition of the fibre

when laser energy was delivered to the substrate in a short time45. The interface between the PI

fabric and the LIG can be seen in figure S3, which proves the strong adhesion of the LIG

particles. Figure S10 shows investigated the sheet resistance of LIG fabric strain sensors after 10

cycles in the 10% detergent/deionized water solution, which further comfirms the roburst

interface between the LIG and the PI fabric.

Figure 3. SEM images of laser-scribed surface details with different laser fluence on PI fabric:

(a)16.8 mJ/cm2; (b)17.4 mJ/cm2; (c)18.8 mJ/cm2; (d) 20.6 mJ/cm2; (e) 22.2 mJ/cm2; (f) 23.8

mJ/cm2. (x1000 magnification).

To qualitatively confirm the chemical composition of LIG, advanced characterization methods

including X-ray diffraction, Raman and TEM were used. Figure 5a shows the Raman spectra of

the PI fabric and the LIG obtained at 20.5 mJ/cm2 laser fluence. It can be clearly seen that three

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peaks appear after the laser synthesis, which replaced a series of organic peaks in untreated PI

fabric. In the Raman spectra of graphene, the D-peak located in 1350 cm -1 indicates the

proportion of defects, and the G-peak situated in 1550 cm-1 demonstrates the existence of carbon

bonding stretching both in ring-structure and chain-structure46. At the wavelength around 2700

cm-1, the 2D-peak is related to the degree of crystallinity of graphene46. Therefore, the effect of

laser fluence on the quality of LIG was explored through comparing the relative intensity of D-

peak and 2D-peak with G-peak (I(D)/I(G) and I(2D)/I(G)) which are illustrated in figure 5c and

5d. As the laser fluence increased from 15.8 mJ/cm2 to 25.2 mJ/cm2, the intensity of 2D-peak

follows the trend of declining first and then rising, reaching the peak value at 20.5 mJ/cm 2. On

the contrary, the intensity of D-peak dropped to the lowest point at 20.5 mJ/cm2 and had a

significant growth subsequently. These results demonstrate that the best-qualified LIG was

obtained with the lowest proportion of defects and the highest carbon crystallization at 20.5

mJ/cm2 laser fluence. In figure 5b, the successful formation of LIG can be proved by the

emergence of two peaks respectively located at 2=26˚and 43˚. According to the Bragg equation,

the interplane space can be calculated as 0.35 nm from a reflection peak at 2=26˚(indexed to

(002) plane), indicating a high degree of graphene crystallization47. The in-plane structure was

associated with (100) reflection plane which is located at 2=43˚47. The XRD spectrum with

different laser fluence is shown in Figure S4.

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Figure 4. SEM images of laser-scribed surface details with different laser fluence on PI fabric:

(a)16.8mJ/cm2; (b)17.4mJ/cm2; (c)18.8mJ/cm2; (d) 20.6mJ/cm2; (e) 22.2mJ/cm2; (f) 23.8mJ/cm2.

(20kK magnification).

Figure 5a and 5b show the Raman spectrum mapping of LIG grown on the PI fabric in a 50

µm×50 µm square. The colour intensity distribution corresponds to the values of I(D)/I(G) and

I(2D)/I(G) which can be used to describe the uniformity of LIG in this area. Due to the rough

surface of the PI fabric, only one single fibre can be well focused on to obtain a the distribution

trend of laser-formed graphene. In figure 6a, the values of I(D)/I(G) are within the range from

around 0.4-0.7, following the woven pattern of the fabric. This phenomenon may result from the

various height of the fibre in the woven pattern which influences the absorption of laser energy.

The sSimilar phenomenon also exists in figure 6b, representing the distribution of the degree of

graphitization. The TEM characterisation further discloses internal morphology and

microstructure of laser-formed graphene(figure 6c and 6d). In the lower magnified image, ultra-

thin graphene layers were interlaced to form a transparent 3D-porous graphene network. This can

be explained by the fact that the instantaneous laser energy rearranged the carbon atoms into a

wrinkled graphene structure which extended in random directions48. In the higher-resolution

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photograph, ripple-like graphene partially grows upwards. Therefore, the number of layers

(below five layers) can be seen from the edge of graphene flakes with an interface spacing of

0.35 nm.

Figure 5. (a) Raman spectra of PI fabric and LIG obtained at 20.5mJ/cm2 ; (b) XRD spectra of

PI fabric and LIG obtained at 20.5mJ/cm2; (c)Raman spectra of LIG synthesized with different

laser fluence, respectively at 15.8, 17.3, 18.9, 20.5, 22.1, 23.6, 25.2mJ/cm2; (d) Raman peak

intensity ratio of LIG with increasing laser fluence.

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Figure 6. Raman mapping of (a) d/g peak intensity ratio (d) 2d/g peak intensity ratio; (c,d) TEM

image of LIG obtained at 20.5mJ/cm2.

The synthesizeds of LIG based on the PI fabric was studied with by X-ray photoelectron

spectroscopy (XPS). Figure 7a compares the chemical composition of PI fabric and LIG,

showing the significant suppression of O and N elements after laser scribing. The peaks of Si2s

and Si2p also disappeared in the XPS survey of LIG, which may be related to the surface dust

removed by laser cleaning. This comparison might suggest that the decreased O and N elements

emitted from the surface of the PI fabric, in the form of gas oxidation, causing the porous

structure of LIG which can be observed in the SEM images. Rice University has investigated the

mechanism of the conversion of PI films to LIG using a long wavelength CO2 laser42. They

suggested that high localised temperature at least of 2500 ˚C was is required to break the C-O

and C-N bonds. These oxygen and nitrogen atoms would will then rearrange and form gases. In

our work, using a UV laser (355 nm wavelength), photon energy of 3.59 ev is enough to achieve

the breakage of these bonds through a the photochemical process without a high temperature34.

For the left remaining carbon atoms, a the graphitization process can be triggered through

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thermal dynamic effects. They also found that specific structure features including aromatic and

imide repeat units were are essential to generalize this laser-induced graphitization process49.

High resolution of C1s XPS spectraums of PI fabric and LIG, as illustrated in figure 7b,

indicate that C-O-C, C-N and C=O bonds are significantly decreased by the laser ablation. This

means that the Sp2 carbons dominate the composition of LIG50. Figure 7c shows the high

resolution of O1s XPS spectraums which consists of C-O and C=O bonds. It can be seen that

majority of C-O bonds were are converted into C=O bonds which have higher binding energy

during the laser process. The reduceion of C-N bonds also can also be observed in figure 7d.

Figure 7. XPS characterization of PI fabric and LIG. (a) XPS surveys of the PI fabric and LIG;

(b) High resolution C1s XPS spectrum of the PI fabric and LIG; (c) High resolution O1s XPS

spectrum of the PI fabric and LIG; (d) High resolution N1s XPS spectrum of the PI fabric and

LIG.

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3.3 Performance analyses of LIG fabric strain sensor.

The electromechanical performance of the LIG fabric sensor was comprehensively studied by

continuously recording the resistance change while the sensor was stretched. To start, basic

mechanical properties and electrical properties were investigated to evaluate the sensor’s

limitation. In figure 8a and 8b, a tension-recovery cycle of the PI fabric in parallel and diagonal

directions are shown. The inserted illustrations show the difference in the woven direction of the

PI fabric. It is clear that in the parallel direction, the PI fabric cannot recover after stretching

even under subtle strains. However, the fabric exhibited good recovery in the diagonal direction

within a certain range which means that it is possible to assembly sensors in this direction. The

anisotropy of the stretchability of the fabric was due to the different mutual tension and friction

in different directions after weaving51. In general, the weaving method endows the non-elastic PI

fabric with stretch-recovery ability within a small strain range. Figure 8c shows a primary

tensile stress/strain curve of the PI fabric. Under strain up to 14%, the plot stays good linearity

below 4% strain, representing 4% elastic range whereas plastic deformation occurreds above this

value. The excellent electrical conductivity of LIG was also found through a 4-wire resistance

test which is shown in figure 8d. The sheet resistance can be as low as 20 /sq when the laser

fluence was 22 mJ/cm2. Futher increase in laser fluence began to partially burn out the fabric

without noticeable reduction on sheet resistance.

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Figure 8. Tensile stress/strain curve of PI fabric (a) in parallel direction; (b) in diagonal

direction; (d) Tensile stress/strain curve of PI fabric (14% strain); (c) Sheet resistance of LIG

with various laser fluence.

In this study, the sensitivity, strain threshold, repeatability and response to different strain and

strain rate were examined by sensors fabricated at 20.5 mJ/cm2 of laser fluence. In fact, laser

fluence was considered to be the most relevant factor affecting sensitivity when regarding to the

laser-prepared active materials for strain sensors. The electrical properties of LIG are dominated

by the crystallization quality and defects during laser processing, which has been fully studied by

previous researches14. The LIG with better electrical properties will be more sensitive to minor

change in external strain. The curves of the resistance change versus strain of sensors

synthesized with different laser fluence are shown in figure S5, which explains this relationship

between the quality of LIG and the sensitivity of the LIG fabric sensor. In addition, it is worth

mentioning that the laser fluence may also affect the sensitivity of this fabric sensor by refining

the size of carbon particles grown on the edges of fibers. Figure S6 (An x15000-magnified SEM

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photo) proves that nano-sized carbon particles can be generated by controlling the laser fluence

without damaging the substrate. This will lead to a significant increase of the contacting and

overlapping areas among carbon particles, which may amplify the strain change and contribute to

the high sensitivity.

As shown in figure 9a, the sensor with a rectangle pattern (30 mm x 10 mm) shows a linear

response up to 4% strain elongation. To characterize the sensitivity of the sensor, the gauge

factor (GF) can be calculated on average as 21.4 (GF max=27) through the equation (1).

Although this value is inferior to some previously reported strain sensors which used elastomer

polymer as substrates, such as a laser-carbonized PDMS strain sensor reported by Rehim14 with a

GF up to 20000, a graphene-coated strain sensor based on glass fabric/silicone composite with a

GF up to 11352 and a graphene-on-polymer strain sensor which can reach GF up to 100053. This

large-area fabric-based LIG strain sensor owns a numbers of advantages, including low detection

limitation, high stability, a simple fabrication procedure technique and etc. On the other hand, the

sensitivity of this LIG fabric strain sensor is still competitive compared to conventional fabric-

based strain sensors. The reason is that the traditional fabric strain sensor cannot achieve large

sensing area through coating or deposition methods. Table S2 listsed a the comparison of

fabrication details and performance with various recent fabric strain sensors.

Basically, the mechanism of strain sensors can be grouped into two catalogues: one is the crack-

controlled design which depends on the destruction and recovery of the structure during the

stretching process54. The other one is the change in the number of carrier channels which is

caused by the variation of the contact area of the active materials55. Figure S7 show illustrates the

SEM images of the fiber surface morphology in two different directions under stretching. Based

on the mechanism of strain sensors, the stretching and recovery properties of the strain sensor

can be considered to mainly depend on the relatively stable 3D network structure of active

materials during stretching. The choice of the substrate weaving direction (diagonal direction)

directly determines the stretching range of the fabric strain sensor. On the other hand, the loss of

electrical properties during the stretching cyclse can be attributed to the partial detachment of the

active particles at the edge of the fiber. The zoom-in graph of the resistance change response

within the strain range of 0.1%, as plotted in Figure 9b, represents the small detection limitation

of strain which can be as low as 0.08%. This may be due to the carbon nanoparticles grown on

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the fibre framework which are observed in SEM graphs. An ultra-small particle size indicatesed

that a weak strain can also cause a sufficient amount of active material to be separated from each

other, producing a resolvable resistance response. Further evidence is shown in figure S8, in

which all the plots show similar threshould values around 0.08% under different strain at 1%,

2%, 3% and 4% respectively. Figure 9c shows the acquisition of stable signals of the sensor

device under various strain loads. It is clear that the change of resistance increases with the

growth of applied strain in an approximately linear law. In figure 9d, it can be seen that the

amplitude of signals under different strain loading rate remain consistent for several loading and

unloading cycles. Applying repeated external load, cracks in the fiber skeleton initiate, propagate

and recover, dominating the change in electrical properties. Below the elastic deformation scale,

the peak value and the bottom value of the resistance signals were are maintained within a

relatively stable range. This presents a the good signal recording performance of the fabric strain

sensor without obvious lag errors. The cycle test is depicted in figure 9e to examine mechanical

durability. It can be found that only 4% deviation is generated after 1000 cycles at ε=4%. This

means that there is around 4% of the resistance change loss after 1000 loading-unloading

cycles.There is also a difference of the normalized resistance change between the statical and

dynamical test, which is caused by patially electrical properites loss during long-term loading

and unloading process. The zoom-in curve of 4 cycles is compared with the strain cycles in

figure 9f, exhibiting small lag errors of 250 ms during long-term loading and unloading cycles.

The fatigue test of the PI fabric can be shown in figure S9 to indicate the stability of the PI fabric

substrate.

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Figure 9. (a)The resistance change versus strain plot within the strain range of 4%; (b) Zoom-in

figure a within the strain range of 0.1%; (c) response of resistance change under different strain

value; (d) response of resistance change under different strain rate at 2% strain; (e) cycle test at

4% strain for 1000 cycles; (f) Zoomed in figure of resistance change cycles comparing to the

strain cycles. (sensors for this experimental part were fabricated at a laser fluence of 20.5

mJ/cm2).

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Figure 10. Photographs showing the LIG fabric strain sensor in human motion detection. (a) The

sensor applied to the knuckle of the index finger, examing finger bending; (b) The sensor

attached to the wrist joint to detect wrist flexion; (c) The sensor attached to the bicipital muscle

to record the movement of the muscle.

3.4 Applications of LIG fabric strain sensors on human motion.

Experiments of practical response to different human motions were carried out to explore the

utility of this LIG fabric strain sensor. Devices used in this section were fabricated at a laser

fluence of 20.5 mJ/cm2 due to the highest sensitivity. Finger bending was firstly detected, as

shown in figure 10a, for 9 bending-stretching cycles. cliff-shaped peaks for each cycle were

derived from the nonlinear change of joint bending angle. Figure 10b demonstrates the resistance

change of the sensor in response to the flexion of the wrist. The steepness of signal peak in this

curve is slightly larger than the plot of finger bending, resulting from the larger separation of the

wrist joint. When the LIG strain sensor was attached on the bicipital muscle which is shown in

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figure 10c, the movements of this muscle were recorded in detail. At the top of each peak,

fluctuation of the curve can be observed which is due to the slight muscle twitch in the tense

state. The results above further prove the potential applications of the LIG fabric strain sensors to

distinguish various signals of human motion.

4. Conclusion

We have demonstrated a UV laser direct writing technique for the production of porous graphene

strain sensors directly on PI fabrics. The results demonstrated that the formation of LIG is

strongly controlled by the laser fluence, which further influencesd the sensing performance of

assembled LIG sensors. This LIG device has a low sheet resistance of 20 /sq, a maximum GF

of 27, a small strain threshold value of 0.08% and high mechanical stability of 4 % resistance

change loss after 1000 cycles. Through this sensor, various human motion signals were detected

and recorded in the form of resistance change, indicating the promising potential applications in

the health monitoring on smart wearables.

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