Stretchable Conductive Ink Based on PolysiloxaneâSilver Composite

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Functional Inorganic Materials and Devices

Stretchable conductive ink based on polysiloxanesilver composite and its application for a frequency reconfigurable patch antenna for wearable electronics Mohamad Riduwan Ramli, Salehin Ibrahim, Zulkifli Ahmad, Intan Sorfina Zainal Abidin, and Mohd Fadzil Ain ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07671 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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ACS Applied Materials & Interfaces

Stretchable conductive ink based on polysiloxanesilver composite and its application for a frequency reconfigurable patch antenna for wearable electronics

Mohamad Riduwan Ramli1, Salehin Ibrahim1, Zulkifli Ahmad1*, Intan Sorfina Zainal Abidin2, Mohd Fadzil Ain2 1Silicone

Polymer Research Group, School of Materials & Mineral Resources Engineering, Universiti Sains Malaysia, 14300, Nibong Tebal, Penang, Malaysia.

2School

of Electrical and Electronic Engineering, Universiti Sains Malaysia, 14300, Nibong Tebal, Penang, Malaysia

KEYWORDS: polysiloxane, post-percolation threshold, Variable Range Hopping, patch antenna, radiation loss

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ABSTRACT The rapid progress in electronic applications for moveable devices requires the conductive matrix to be not only flexible but also stretchable. A simple microstrip patch antenna was fabricated based on silver ink polysiloxane composite with stretchable polysiloxane substrate at resonance frequency of 2.50 GHz. It is designed at post-percolation threshold of 35, 45 and 60 vol % conductive filler loading so as to allow a consistent conductivity at an ample range of cyclic stretching. With the presence of coupling agent and additives, the patch antenna displayed an extremely good adhesion between the ink and the substrate which prevent any local rupture during stretching. Variable Range Hopping (VRH) model verified that conductivity occurs through hopping and tunnelling mechanism giving transient optimum conductivity in the range of 10 - 70 S/cm at 10 - 20 % strain amplitude range. The fabricated prototype of microstrip patch antenna displayed a decreasing resonant frequency with strain. Of note, the radiation loss S11 and the bandwidth values are proportionally related to the conductivities during stretching. These results verified the proposed construction and destruction conductive mechanism occurring during percolation threshold system. The fabricated antenna proved the feasibility for use as stretchable device at UHF band.

INTRODUCTION Flexible conductive material is popularly used in fabrication of sensors, antenna, actuators, radio-frequency identification (RFID) health care devices and display panel

1–3.

Good flexibility

is needed so that the fabricated circuitry could be efficiently attached onto irregular surface topology. However for a stretchable platform which is perpetually exposed to cyclic motion and 2 ACS Paragon Plus Environment

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deformation, stretchability offer an advantage in providing a pervasive and unobtrusive sensing and display applications 4. Essentially, it is developed in a form of polymer composite by incorporating various conductive fillers such as carbon nanotube, silver nanoparticles, organic conductive polymers and graphene 2–4. Level of percolation threshold in this composite system is crucial in determining the onset of conductivity. Numerous work has demonstrated that beyond the percolation threshold loading, conductivity mechanism is mainly by hopping and tunnelling 5–7.

A strategy to develop stretchability is to fabricate the circuitry into various architecture

particularly horse-shoe, serpentine and wavy electrical circuit. Under these forms, a definite level of cyclic strain is allowed giving room for consistent conductivity before the material effectively stretched and finally leading to physical ruptured

8,9.

The work of Hussain et al. illustrate the

employment of copper on stretchable PDMS using twisted helical spring structure whose stress was released during twist out-of-plane without effectively stretching physically the antenna but elongation obtained through lateral restructuring of the spring 10. In this work, a stretchable conductive ink was fabricated based on polysiloxane-silver composite. By mimicking the strategy of wavy conductive circuitry, the present work developed a composite system at post-percolation threshold loading. Most composite systems were design at around percolation threshold loading as it did not involve stretching and mostly in cutting cost. Post-percolation threshold system is invariably similar with the former as it supposedly allows consistent conductivity within an ample stretching range prior to its physical failure. However, it offers a neater and simpler circuitry which is tightly adhered onto a substrate while consuming less space and avoiding complex structure fabrication process as otherwise offered using the wavy circuitry architecture. Working on antenna, it was acknowledged by Li et al. 11 that this alternative design method should fulfil high-efficiency devices while maintaining a similar level of electrical 3 ACS Paragon Plus Environment


conductivity under applied strain. It is envisaged that under this system, there is an equilibrated destruction and construction of electrical path during cyclic straining leading to a definite consistency in conductivity within a specific range of strain. A simple patch antenna was designed as a prove of the concept in the application of stretchable conductive ink in verifying the factors namely patch dimension and conductivity in influencing the performance of the antenna 12,13.

EXPERIMENTAL Materials. Sylgard 184TM(PDMS) was obtained from Dowsil Company, polydimethylsiloxane hydroxyl terminated with average molecular weight (Mn) of 110x103 gmol-1and viscosity of 50x103 cSt (OH-PDMS), vinyltrimethoxysilane (VTMS) function as coupling agent, (3glycidyloxypropyl)trimethoxysilane (ETMS), 3-(trimethyloxysilyl)propyl methacrylate (ATMS) function as additives, dibutyltin dilaurate (DBDL) and silver (Ag) powder (2-3.5 µm) were purchased from Sigma Aldrich and were used as received. Meanwhile the cyclotetrasiloxane (D4) was a gift from Penchem Technologies Sdn. Bhd. (Penang). The hexane from JT Baker was used as the solvent. Fabrication of conductive ink. The OH-PDMS (0.2 g), VTMS, ETMS and ATMS (12.5 µL each), D4 (450 µL), DBDL (5 µL) and Ag fillers (35, 45 and 60 vol %) were mixed and stirred under inert condition in nitrogen atmosphere for 24 h followed by ultra-sonication for 30 min. The formulation for conductive ink was prepared giving a viscosity of almost 50 kcSt. Meanwhile the substrate was prepared from PDMS Sylgard 184TM by mixing the two parts at the ratio of 10:1. After treating under vacuum suction to remove air bubbles, it was cured under thermal curing at 100 °C over 35 min. The dielectric constant of the substrate was 3.0 with dielectric loss 10 x 104 at 100 kHz. The conductive ink was then squeegee printed onto the PDMS substrate of size 60 x 4 ACS Paragon Plus Environment

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20 x 3 mm. The whole package was then thermally cured at 60 °C for 40 min to afford a conductive ink strip strongly adhered on the stretchable PDMS substrate.

Measurements. The adhesion property were conducted using Instron Machine Series IX by taking triplicate measurements. The resistance and conductivity measurements were made using UNI-T203 digital multimeter and four-point probe Jandel RM3000 Resistivity Meter. For the adhesion measurement the ink samples were sandwiched and cured between two PDMS substrate samples of size 50 x 10 x 2 cm at the end which cover the area of 200 mm2. Two samples with and without coupling agent were prepared for comparison. Then the samples were subjected to the tensile adhesion test using the Instron Machine. Strain gauge length was set at 50 mm and the strain rate was 5 mm/min. The scanning electron microscope (SEM) images was obtained using Field Emission SEM from FEI Tecnai model Verious 460L. The samples were coated with gold nanolayer prior to test and the measurements were taken before and after stretching.

Stretchable antenna fabrication. A rectangular microstrip patch antenna was designed, simulated and optimized in CST software before fabricated as a prototype to obtain optimised dimensions of 60 x 60 mm (substrate) and 40.2 x 30.8 mm (patch antenna) with a fixed thickness of 2 mm. The simulated design is shown in Fig 1 with top and side views (with dimensions) and the fabricated prototype. The substrate used was the PDMS while the patch antenna was the conductive ink prepared earlier. The chosen resonant frequency of this antenna design is 2.5 GHz.



Figure 1. (a) Rectangular microstrip patch antenna with dimensions of top view (b) side view (c) fabricated antenna prototype (d) Fixture to stretch the prototype antenna Testing was made by soldering a coaxial cable-type RF connector, specifically a SubMiniature version A (SMA) connector to the patch antenna at the edge of the substrate to act as a feed to the antenna. The elastic modulus of the adhesive attached between the SMA and conductive ink package is almost similar on which during stretching is able to maintain the firm attachment on the SMA and the ink. The radiative characterization was performed using a Vector Network Analyzer (VNA), from Agilent Technologies (N5245A). The antenna under test is connected to the VNA via a coaxial cable while the monitor is used to display the measurement results. The


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measured S-parameter, resonant frequency, bandwidth and directionality obtained was compared with the simulated S-parameter.

RESULTS AND DISCUSSIONS Percolation threshold. Attempt was initially made to establish the percolation threshold for conductivity of the fabricated system. Factors which determine this value include the type, loading concentration, geometrical shape and aspect ratio of the filler as well as structure of crosslink network, filler dispersion and interaction between the filler with polymer matrix. This can be summarised as in the following Equation 1:

   0 (V  Vc ) S

(1)

where σ, σ0, Vc and s are the conductivity of the sample, conductivity of filler, percolation threshold and critical exponent, respectively

14.

As it is dependent on a particular system, s and Vc are

determined by curve fitting of the experimental result. The onset of threshold loading found in the present system was 30 wt% with s value 1.49 beyond which there is a burst in conductivity. This behaviour is represented in Fig 2. This value was almost similar as been reported by Huang 14.



60

1.80 1.75

50

1.70 40

1.65

log 

Conductivity (S/cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30 20

y=a+b*x log \g(s) Pearson's r 0.96094 Adj. R-Squar 0.8468 Slope 1.49568 ± 0.43

1.60 1.55 1.50

10

1.45

(a)

0

10

20

30

40

50

(b)

1.40

60

-0.45

-0.40

-0.35

-0.30

-0.25

-0.20

log (V-Vc)

Filler loading (vol.%)

Figure 2. (a) Percolation threshold of conductive ink measured at several filler loading (b) linear plot to determine the critical exponent, s measured as the slope A common range of threshold loading for silver was 1 - 15 vol% as fabricated by several researchers

6,15,16.

However, they are mostly fabricated in a non-crosslink polymer matrix using

nanosized filler. The high threshold loading found in this work could mainly be attributed to the extensive crosslink network which effectively isolated filler islands in the polymer matrix. SEM scan displayed distribution of the fillers in the polymer matrix as shown in Fig 3.

Figure 3. The crosssection SEM images of the (a) conductive ink on the PDMS substrate and (b) the enlarge view of the layer 8 ACS Paragon Plus Environment

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In Fig 3 a distinct layer of conductive ink attached on the PDMS substrate showed a strong adhesion between the respective surfaces. While that of the substrate layer was a homogeneous phase, the conductive ink was in the form of interconnecting micro-globules with interstitial air voids. At microscale magnification (Fig 3b), islands of silver nanoparticles of estimated size around 0.5 μm was observed dispersed almost homogeneously in the ink matrix. Apparently, they are isolated from each other which militate against any mechanism of metallic conductivity. It is suggested that conductivity in the bulk of the ink occurred though hopping and tunnelling mechanism as will be discussed in the following section.

Adhesion analysis of conductive ink and substrate. Tear strength against strain was measured as shown in Fig 4. The adhesion strength of conductive ink containing the coupling agent was 2.5 N/m2 compared to 1.8 N/m2 without coupling agent.



Figure 4. The adhesion test of conductive ink with and without coupling agent measured in triplicate samples. The colored shadows are the standard deviation of samples Local rupture initiated at the weak point of a substarte posed a limit during stretching thus reducing the tear strength. A strategy to avoid local rupture adopted by Kubo et al. was to utilised ‘hybrid’ structure of PDMS with higher stiffness with a soft Ecoflex which allows stretchablity as substrate material during fabrication of microfluidic channel antennas 17. The strategy adopted in this present work is based on at least three factors which contributed to the good adhesion between the two phases: (a) similar type of polymer material of the ink and substrate i.e. polysiloxane which induce structural and chemical compatibility (b) role of coupling agent i.e. vinyl trimethoxysilane in forming chemical interaction with both surfaces (c) physical interaction with both layer through amphipilicity property of additives. The basic composition of conductive ink was hydroxyl rich moiety which essentialy affect the formation of siloxane bond (Si-O-Si) via condensation reaction with the coupling agent and the additives. The coupling agent in turn form a new chemical bond with the silicon hydride (Si-H) rich PDMS substrate layer through hydrosilylation reaction. Further, the additives contribute to the hydrophobicity/hydrophilicity interaction onto both layers. These interaction is summarised in Fig 5.


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Figure 5. Diagram illustrated the various chemical interaction involving condensation and hydrosilylation involving coupling agent between conductive ink and PDMS substrate

The package was able to be stretched to at least 75 % of the original without inducing any local rupture while mantaining a good adhesion onto the substrate. Twisting and rolling to at least 5 mm radius were achievable without affecting light illumination from a LED as displayed in Fig 6.



Figure 6. Several stretching and rolling performed onto the conductive ink package while maintaining illumination on LED

Stretchability and mechanism of conductivity. Fig 7 shows the effect of strain on conductivity. It shows that there is an initial transient increase in conductivity followed by a plateau upon further straining particularly at high filler loading.


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70

Conductivity (S/cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

35 vol.% Ag 45 vol.% Ag 60 vol.% Ag

60 50 40 30 20 10

60

Conductivity (S/cm)

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(b)

35 vol.% Ag 45 vol.% Ag 60 vol.% Ag

50 40 30 20 10 0

0

20

40

60

80

0

20

40

60

80

100

Deformation cycle (times)

Strain (%)

Figure 7. The conductivity of conductive ink during (a) stretching at several strains (b) cyclic deformation of conductive package at 40 % strain Several works on stretchable conductive ink showed that stretching affect a decrease in conductivity at the onset of stretching

18.

It needs to be noted that these works involved filler

loading prior to or around the threshold concentration. Under this condition, stretching affect separation of conductive filler further from each other which result in decrease in conductivity. This is known as piezoresistive effect. Under this condition, band gap changes which disfavour electronic excitation into the conduction band. The work of Tang et al. demonstrated the importance of stabilising changes in conductivity during stretching by performing several air inflation cycles into the fabricated gold-coated MWNT polysiloxane patch antenna which could improve alignment and packing of conductive particulates. It was appreciated by these workers that pre-percolation loading of conductive filler would result in wider bandwidth compared to simulated result due to poor particulated electrical conductivity

19.

Under post percolation

condition the effect of piezoresistivity does not arise in our work. It is established that stretching affect otherwise at least at initial range of stretching. For all samples in the present work, there is a general trend of initial transient maximum in the range 10 - 20 % strain whose level of 13 ACS Paragon Plus Environment


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conductivity is proportional to filler content followed by plateau beyond this strain range. During this transient period there is virtually no significant changes in conductivity implying an almost consistent conductivity within this range of strain. This initial transient optimum was similarly observed in similar sample geometry by Wang et al. 20 as well as those in wavy configuration of conductive circuitry

8,9.

This prove the viability of the strategy of post-percolation filler content

composites for use as conductive ink. The conductive behaviour across the strain range could be attributed to the process of destruction and construction during stretching

21.

During stretching,

dislocation of conductive filler breaks the conductive pathways but at the same time this affect a new reorientation and positioning of the filler which established a new electrical pathway. Assisted by Poisson's contraction with reduction in width in transverse direction and the length increase along the tensile direction, the reorientation and alignment of silver filler affect a closer contact to each other giving rise to a transient optimum in conductivity. The mechanism of construction and destruction in this system can be further elaborated by considering the sample undergoing four distinct stages namely globular, elongation, fibrillation and rupture as depicted in SEM images in Fig. 8. Initially, the nano-globular polymer matrix began to elongate inducing fillers which form islands in the matrix began to disperse apart from each other and destruct conductivity path. However, as the result of Poisson strain, the displaced filler becomes reoriented uniaxially along the axis of stretching and laterally aligned near to each other. Such a transformation induced relocation of particles so that they introduce a new electrical pathway. The dynamic of this equilibrium approached maximum construction in the electrical path which leads to the transient optimum in conductivity as achieved at about 12 % strain amplitude as shown in Fig 7a. Further stretching lead to detachment of the filler from each other resulting in decrease number in conductive pathways hence in conductivity

22.

EDX images were taken at these


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particular globular spots and compared to those in elongated strand presumably without the presence of silver particles. The EDX scan does reveal a high silver concentration at globule spot compared to the elongated strand (Fig 8i and 8j). This observation established that the proposed construction and destruction mechanism does indeed occurring. The maximum strain level in this study was 80 %. Material become very stiff at further straining which could result in sample rupture. This is one disadvantage of having post threshold system whereby the filler content is relatively high compared to some other fabrication strategy e.g. wavy circuitry. Cyclic straining was also performed at 40 % strain. The change in conductivity was noted as in Fig 7b. It can be seen that conductivity was increased at 10 cycles followed by a decrease before reaching plateau. This was due to the effective formation of electrical pathways during stretching at this strain as similarly observed during static deformation. However, under viscoelastic deformation, thermal energy in the form of hysteresis was developed leading to fatigue failure and rupture in crosslink network during cyclic straining. This observation is mostly prominent at higher deformation cycle whose mechanism of construction and destruction equilibrium becomes less prominent. This is consistent with the convergence of conductivity of samples regardless of filler content towards higher cyclic straining. The conductivity around the 10th cycle was relatively large for 60 vol % as compared to that of 35 and 45 vol % filler content. This represent a higher rate of construction compared to destruction of the conductivity pathway at this cycle of straining since the particulate density is highest. Within this rate of straining the composite network is able to behave viscoelastically and hence allows for an efficient reversible alignment in forming electrical pathways. However, at higher cyclic straining cycle, the fatigue failure becomes increasingly severe whose frictional forces between fillers becomes dominant. This result in the failure for the



system to maintain the viscoelasticity leading to the drop in conductivity to the level as observed in samples of lower filler content.


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Figure 8. SEM images and schematic representation of stages of silver particles realignment in polymer matrix during stretching (a, e) globular (b, f) elongation (c, g) fibrillation and (d, h) rupture. The EDX at region of rupture with high concentration (i) and low concentration (j) of silver particles. Pink sphere represents silver fillers and orange arrow lines represent electrical pathways

Since the filler are surrounded by polymer matrix which are dielectric, conductivity is expected to occurs through a characteristic temperature dependence which differs from Arrhenius law normally anticipated for a thermally activated process and excitation of carriers across an energy band-gap. In amorphous and disordered materials, this led to the high possibility of conducting mechanism through hopping and tunnelling. The estimated inter-particle distance is expected to be in the range of 1 - 5 nm to achieve possible hopping and tunnelling mechanism 23. Monte Carlo simulations indicate that the cut-off tunnelling distance is found to be about 1.8 nm

24.

The

temperature dependence of hopping conductivity, can be described by means of Variable Range Hopping (VRH) model given as 25.  T   (T )   0 exp   0   T



(2)

where the parameter σ0 can be considered as the limiting value of conductivity at infinite temperature, T0 is the characteristic temperature related to Boltzmann constant that determines the thermally activated hopping among localized states at different energies and considered as a measure of disorder

26,27

and the exponent 𝛾 is related to the dimensionality d of the transport

process via the equation 𝛾= 1/(1 + d), where d = 1, 2, 3 5. A linear relationship in the plot of ln σ 17 ACS Paragon Plus Environment


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vs. T-1/4 in Fig 9 was obtained which verified that conductivity occurred through hopping mechanism. Tunnelling efficiency is related to conductivity as given by 28

[

𝜎1 = 𝜎0𝑒𝑥𝑝

4𝜋 2𝑚𝜙 ℎ

]

(𝑠1 ― 𝑠0)

(3)

where, σ0 and σ1 are the conductivity before and after stretching, h is Plank’s constant, ϕ is the height of the tunnelling potential barrier, m is the mass of an electron, and s0 and s1 are the average interparticle distance between fillers before and after stretching respectively. According to this equation, in order to maintain the conductivity after mechanical deformation, one can either reduce the tunnelling potential barrier ϕ or minimize the interparticle distance s1 during stretching. Under post percolation threshold filler content, essentially both of the tunnelling potential barrier ϕ and distances between silver particles will be reduced, which would enhance the initial conductivity but also mitigate the conductivity change upon stretching.


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Figure 9. Linear relationship of conductivity with temperature for 10, 20, 40 and 60 % strain for 60 % vol. of Ag samples

Stretchable conductive ink for frequency reconfigurable antenna application. The performance of an antenna is principally determined from the impedance matching of the sourced electrical frequency as applied onto the antenna loading. Under this condition maximum power can be transmitted affecting an optimum in the return loss S11, resonant frequency and the bandwidth. Based on Warburg model, charge carrier concentration and sample dimension independently contribute to the factors determining the impedance in a conductive material 29.

-20

-5

-22

-10

-24

-15 -20 -25 -30 -35

0% stretch (simulated) 10% stretch (simulated) 20% stretch (simulated) 0% stretch (measured) 10% stretch (measured) 20% stretch (measured)

(a)

simulated measured simulated measured

2.50 2.45

-26

2.40

-28 2.35

-30 -32 -34

2.30

(b)

2.25

-36 2.0

2.2

2.4

2.6

2.8

3.0

Frequency (GHz)

0

S11 (dB)

S11 (dB)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

Frequency (GHz)

10

20

Strain (%)

Figure 10. (a) The simulated and experimental S11(return loss) of the rectangular microstrip patch antenna at original length, 10% and 20% stretched (b) Comparison between simulated and measured S11 and resonant frequency of antenna at several strains



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Fig 10 shows the variation of return loss S11 as well as the effect of strain on S11 and resonance frequency. The simulated unstretched resonant frequency was set at about 2.50 GHz, a frequency that is allocated for applications using the Wireless Local Area Network (WLAN) spectrum as well as for wearable electronics 30. The measured value at 2.50 GHz is almost similar to that of the simulated value. It is observed as in Fig 10a that stretching of the antenna induced shift to lower resonant frequency. At 10 % stretched, simulated antenna was 2.35 GHz as against the measured antenna radiating at around 2.40 GHz. At 20 % stretched, both simulated and measured resonant frequency reduce further to 2.29 GHz and 2.28 GHz respectively. The effect of stretching on resonant frequency is highly dependent on the fabricated system. Song et al.

31

observed an

increased in resonant frequency with stretching in a fabricated microstrip patch antenna using silver nanowire embedded in PDMS. Likewise a work by Kim et al. 32 for a compressed wavy AgNW-based monopole patch RF antennas showed an increasing resonant frequency during stretching. They implied that during stretching the Ag nanowire can still maintain connectivity in conductivity compared to Ag particles. On the other hand, the resonant frequency could increased or decreased with stretching depending on the distance of electrical pathway that arise from the respective patch layer and the ground plane and their ratio thereof. By working on wavy geometry, Zhu et al. 33 showed that for a highly dense conductive meshed patch layer, stretching effectively led to a decrease in resonant frequency. This is according to the equation: 𝑐

𝑓 = 2𝐿

𝜀𝑐

(4)

where f is the resonant frequency, L is the patch distance and 𝜀𝑐is the dielectric constant of the dielectric layer. However, the tensile strain applied to the ground plane leads to an increase in the resonance frequency. Further works also demonstrate a decrease in resonance frequencies with 20 ACS Paragon Plus Environment

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stretching 19,34. Still the work by Hussain et al. 10 found that a fabricated stretchable antenna using a copper/polyimide system retained all its essential properties namely the resonant frequency and bandwidth on stretching. This latter peculiarity partly belies on the claim of stretchability of their system whereas polyimide is flexible rather than stretchable. Apparently, the effect of stretching on resonant frequency is at least dependent on dimension and effective conductivity in the antenna package. During stretching, Poisson strain affects the elongation of the patch with simultaneous decrease in width. This will induce realignment and dislocation of the microstructure of the material which in turn affect the interplay between the capacitive impedance and the inductive impedance. The overall electrical size of the antenna decreases and the wavelength to increase, consequently lowering the frequency. In the present system, the increased in the physical dimension during stretching affect the surface current flow throughout the electrical length of the patch hence establishing the impedance matching of the antenna to that of 50 Ω towards the lower resonant frequencies. Comparison between simulated and measured S11 and resonant frequency of antenna at several strains is shown in Fig 10b. It could be seen in Fig 10b that the simulated return loss S11 at 0, 10 and 20 % stretching were -23.5, -34.6 and -21.6 dB for simulation and -19.8, -29.8 and -17.0 dB for measured antenna respectively. This trend of values is notably consistent with the stretching conductivity as shown in Fig 7a whereby there is a transient optimum of conductivity at 10 % strain compared to those at 0 and 20 % strain. This finding lends credence to the proposed construction and destruction mechanism occurring under the designed percolation system. The effect of low return loss at 10 % stretching could be related to the mechanism of elongation and fibrillation of silver microparticles which effectively induced their close alignment which otherwise lead to a dislocation of the particles further apart from each other prior or beyond this stretching level. The low return loss, 21 ACS Paragon Plus Environment


S11, displayed here shows that almost 99 % of the signal transmitted by the antenna is efficiently radiated well into the atmosphere.

Table 1. Summary of results of the proposed antenna Strain (%) 0 10 20

Resonant Freq. (GHz) Simulation Exp. 2.48 2.50 2.35 2.40 2.29 2.28

S11 (dB) Simulation Exp. -23.5 -19.8 -34.6 -29.8 -21.6 -17.0

Bandwidth (MHz) Simulation Exp. 280 105 240 135 320 135

Table 1 summarised the simulated and measured radiation features of the fabricated antenna. Trend displayed by the bandwidth showed that measured values are lower than those of simulated. Notably, the stretching at 10 % proved the higher bandwidth particularly for measured values. This observation can be related to the high conductivity value at this stretching level. It is known that generally a patch antenna design has a narrow band response as the bandwidth at sub 6 GHz is usually limited as compared to at mmWave frequency. However, the range of bandwidth in this work is comparable to other literatures 12,34–37.


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dBi 10 5 0 -5 -10 -15 -20 -25 -30 -35 -30 -25 -20 -15 -10 -5 0 5 10

0 330

30

300

0% stretch (simulated) 10% stretch (simulated) 20% stretch (simulated) 0% stretch (measured) 10% stretch (measured) 20% stretch (measured)

60

270

90

240

120

210

150 180

Figure 11. Simulated 2D directivity of the unstretched rectangular microstrip patch antenna in comparison with measured directivity of stretchable antenna at 10 and 20 % stretch

The radiation behaviour of the patch antenna displayed excellent forward directionality during simulation and experimental. Both simulation and experimental radiation pattern and beamwidth are in good agreement as shown in Fig 11. The simulated directivity shows a slight downward trend whereby the directivities are 6.987 dBi, 6.875 dBi and 6.757 dBi for 0%, 10% and 20% stretched respectively (measured at 2.48 GHz, 2.35 GHz and 2.29 GHz respectively). However, some changes of directivity were noted during actual measurement which shows a slight decrease. The measured directivity also shows a slight downward trend (similar in the case of simulated) whereby the directivity are around 5.08 dBi, 4.43 dBi and 3.48 dBi for 0%, 10% and 20% stretched respectively (measured at 2.5 GHz, 2.4 GHz and 2.28 GHz respectively). The fabricated prototype shows slightly reduced but comparable directivity with the simulated design. 23 ACS Paragon Plus Environment


It is worth to note that the antenna design can be further improved such as modifying the ground plane or utilizing slot feeding regardless of the amount of the dimension being stretched. Additionally, the manual fabrication process could also be enhanced to offer higher accuracy and better precision in the future.

CONCLUSIONS A stretchable conductive ink was developed based on polysiloxane matrix and silver particles as the conductive filler under post-percolation threshold loading. The adhesion property of both conductive filler and PDMS matrix were improved by adding the vinyl-silane coupling agent and functionalised epoxy and acrylate additives. Stretching result in a transient increased in conductivity followed by a plateau. Using VRH model it was established that conductivity occurred by means of hopping and tunnelling. A proof-of-concept prototype microstrip patch antenna was designed which resonated at 2.5 GHz with return loss well below -15 dB. The bandwidth is also consistent with 105 MHz unstretched and 135 MHz for both 10% and 20% stretched respectively. The radiative behaviour of the fabricated antenna lends credence to the proposed mechanism of construction and destruction in conductivity during stretching. The fabricated antenna can undergo mechanical deformation such as stretching, rolling, or twisting without breakage. It would be an attractive candidate as frequency reconfigurable and stretchable patch antenna for applications such as wearable electronics, implanted medical devices, RF sensing and interactive gaming.


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AUTHOR INFORMATION Corresponding Author *e-mail: [email protected] (Z.A.) e-mail: [email protected] (M.R.R)

Author Contributions The manuscript was written through contributions of all authors. M.R.R., S.I., Z.A., did the synthesis and fabrication of the stretchable material while I.S.Z.A and M.F.A did the antenna fabrication part. All the authors contributed equally. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors wish to thank the FRGS Grant no: 6071352 and USM short term grant no: 304/PELECT/6315294 for the financial sponsorship of this work.

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