Laser ablation as a rapid prototyping approach for fabricating metallic antennas on soft polymer substrates

Abstract

Quick and easy prototyping methods are beneficial for accelerated product development, allowing for concepts to be realised through quick experimentation, validating predictions based on theory and modelling. A rapid subtractive method for the fabrication of metallic antennas, using laser ablation is reported here. These antennas may be designed for a wide range of wireless power reception applications, as integral parts of a powered device. In line with typical application in sub-dermal implants, we designed the antennas in this work to operate at distances similar to that of the dermis. Inductive spiral coil designs are fabricated from gold on a biocompatible soft polymer, Polydimethylsiloxane (PDMS). PDMS has low dielectric constant of 2.32–2.40 and low loss tangent of 0.04–0.06, which is advantageous for use as antenna substrate. The fabricated inductive coils on PDMS are characterized through electrical impedance spectroscopy and tested as a wireless signal receiver, coupled with a Qi standard wireless transmitting module, at an operating frequency of 100 kHz. Importantly, the fabricated coils show resonance in the operational frequency range of the Qi standard (100 kHz to 125 kHz). This is the first instance of laser ablation defined spiral antennas on biocompatible elastomeric substrates. Although the PDMS based inductive coils have high impedance of 2800 ± 300 Ω at 100 kHz, owing to the mechanical mismatch of rigid conductor and flexible substrate, this approach shows promise. Furthermore, strategies to decrease the impedance of such PDMS-based devices are also discussed.

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Introduction

Additive and subtractive manufacturing methods such as 3D printing, laser ablation and computer numerical control (CNC) machining have often been utilized at the product prototyping phase for concept visualization and validation of products. Quick and easy fabrication methods facilitate accelerated product development.⁴ In this work, laser ablation has been employed to demonstrate a quick prototyping method for fabricating gold inductive receiver antenna coils on soft polymer Polydimethylsiloxane (PDMS) substrates with working distance equivalent to the human skin for wearable application. An antenna is an electronic device that transmits and/or receives electromagnetic waves for radio frequency (RF) applications. They transmit and receive power and/or data, connecting circuity elements for wireless operation.⁵ PDMS can be formulated to closely match the elastic modulus of human skin.⁶ The viscosity of mixed PDMS, as well as the final thickness and rigidity of cast PDMS can be modulated during the mixing and curing process by adjusting the base to curing agent ratio, filler components, and the curing temperature.⁷ PDMS, used as a soft polymer substrate in the present study, is a silicone carrier biomaterial, already used for on-body wearables and implants.⁸ In addition to biocompatibility and flexibility, PDMS also offers advantages of low dielectric constant of between 2.32–2.40 and low loss tangent of 0.04–0.06, which make it well suited as a substrate material for an antenna.^1–3 Substrates with low dielectric constant and low loss tangents are characteristics of receiving antennas with small size, high signal reception efficiency, low signal distortion and operate over a wide frequency range.^9,10 Recent developments in thin film technology and soft materials, have demonstrated PDMS-based flexible and stretchable antennas, energy harvesters, sensors and storage devices to facilitate wireless wearables and implants.^11–14 Wireless power transfer (WPT) or wireless charging enabled in wearable and implant devices provide the convenience of untethered operation.¹⁵ WPT technologies are divided into two classes: radiative, employing radio-frequency microwave as the medium, and non-radiative, charging based on magnetic coupling.¹⁶ For wearable applications, non-radiative wireless charging methods, including inductive coupling, wherein two coils in close proximity are coupled via a magnetic field, are preferred. Electric toothbrushes and electric charging of vehicles are two examples of commercialised inductive coupling technologies.^17,18

Laser ablation is a subtractive manufacturing process, considered as a 2.5D technique (i.e., in between 2D and 3D), as different depths of cut can be achieved by varying power and speed.^19,20 The process involves a laser beam being focused (often pulsed) at the target material. This high intensity light causes intense local heating, inducing cavities and material ejection.²¹ General parameters of power, speed, number of laser pulses per second i.e., frequency of laser, distance between laser lens and substrate, raster and vector engraving methods and number of passes can be varied to fine-tune ablation patterns and achieve design requirements. The choice of laser depends on the source wavelength and nature of application. Commercial lasers can be differentiated into gas lasers (carbon dioxide, argon and krypton ion, helium neon, hydrogen fluoride etc.), solid state lasers (ruby laser, neodymium:yttrium aluminium garnet (Nd:YAG), erbium:YAG laser, titanium–sapphire laser etc.) diode lasers and free electron lasers, dye lasers, metal vapor lasers (gold and copper vapor lasers).²² Depending on the material and requirement, the laser technology can also be adapted for high precision cutting, patterning through engraving, welding to join two materials together, sintering or fusing of powdered material, drilling of high-precision holes, etching and cladding. Laser ablation has a major advantage in contrast to other commonly used photolithographic methods and printing methods of patterning. Table 1 shows comparison between these methods to accommodate key features such as resolution of the patterned structure, compatibility of the process with different materials, fabrication speed and importantly environmental safety surrounding waste generation from the process.^23,24

Table 1 Comparison of different fabrication methods

	Feature resolution (µm)	Material compatibility	Speed	Environmental safety	Ref.
Laser ablation	<1	Metal, polymers, rubber, wood, textile, ceramic, biological cells and tissues depending the type of laser	1–20 mm min^−1in vivo applications while material applications of >300 mm min^−1	Involves high energy processes	25–28
Inkjet printing	30–50	Metal, metal oxides or polymer can be used.	1–500 m min^−1	Toxic and carcinogenic volatile organic solvents for ink formulation, hazardous waste disposal from ink and cleaning solvents along, nanoparticle release from inks, high energy during drying	29
Screen printing	30–50		50–150 m min^−1		29,30
E-jet	1		<1 m min^−1		29
Dispense	10–1000		Single stroke		29
Offset	10		1000 m min^−1		29
Gravure	10–50		1000 m min^−1		29,31
Flexo	45–100		500 m min^−1		29
Gravure-offset	30–50		1–10 m min^−1		29
Reverse-offset	5–20		0.01–1 m min^−1		29
Nanoimprint	0.1	Silicon, quartz and polymers are used as mold while thermoplastic and UV curable polymers are used as resists.	Slow	Nanoparticle release and waste disposal of toxic reagents used	32
Photolithography	0.01	Silicon, silicon dioxide substrate and positive or negative photoresists	Slow	Requires toxic chemical developers and etchants	33

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Here, fabricated inductive coils are tested as wireless signal receivers through coupling with Qi standard transmitting device. The Qi standard is an open-interface inductively-coupled charging method, utilized by smartphone manufacturers including Apple and Samsung, for wireless charging of smartphones. The system uses an electromagnetic inductive link for wireless power transfer between a transmitting and receiving unit, by the coupling of inductive antenna coils.³⁴ As a control prototype, coils using the same process were also fabricated on rigid substrate of poly(methyl methacrylate), PMMA. PMMA is a commonly used substrate material for antennas and has low dielectric constant of ∼2.55, similar to that of PDMS but with a substantially higher Young's modulus of 2.07 GPa, as compared to PDMS's 2.97 MPa.^35–37 In conjunction with its biocompatibility properties, gold was chosen as a conducting material for constructing the inductive coil. Gold was sputter coated onto the PDMS and PMMA polymeric substrates before being selectively removed to leave an inductive spiral coil structure, using the laser ablation process. Sputter coating has merits for controlled and uniform thickness of deposited films with options of depositing a variety of different metals and non-metals.³⁸ Gold possesses high electrical conductivity which allows its fit to tailored applications and frequencies. Along with being compatible with most manufacturing processes, gold is also corrosion resistant and well suited for wearable applications.

Spiral inductive coils, or spiral antennas, generally consist of a wire that is wound in a spiral configuration to enhance inductance. Depending upon design requirements, the spiral structure of an antenna can be constructed in an Archimedean or logarithmic form.³⁹ The geometric scaling of these spiral coil antennas have advantages of both broadband and frequency independent radiation behaviour with nearly constant gain and electrical output.⁴⁰ The spiral structure allows the antenna to transmit and receive electromagnetic signal over a range of frequencies, creating multiple frequencies for resonance.⁴¹ In general terms, the resonance frequency the impedance of the antenna matches most closely with the feed or transmission line contributes to the higher efficiency of power transfer with minimal loss. As such, this is the frequency at which the antennas can most effectively convert electromagnetic signals into electrical, and vice versa.⁴² Each loop of the spiral antenna can function as a resonant component within the network giving rise to multi-resonant pathway with operation over a range of frequencies. Spiral antennas belong to a class of ‘frequency independent’ antennas due to these features.⁴³ These antennas simplify the hardware requirement of devices and are practical for biomedical or wearable devices that may require energy transmission for different applications like wireless power or data transfer. Due to its versatile behaviour, it can be used for both low frequency telemetry as well as high frequency data communication.⁴⁴ This attribute of spiral antennas makes the design ideal for testing different antenna fabrication methods. In this work, logarithmic spiral coil antennas are fabricated on PDMS substrates from gold and used as Qi standard frequency antenna. These antennas can be potentially used in conjunction with wearable and implantable sensors for monitoring various biomarkers including pressure, temperature and glucose.^45–47 They can also be integrated for wireless closed loop neural stimulation purpose⁴⁸ and smart wound healing application for real time monitoring of physiological biomarkers to induce rapid healing.⁴⁹

Results and discussion

The fabricated inductive coils were characterised by SEM, optical profilometry and mechanical tests to determine their physical attributes. Electrical properties of the coil were measured through their wireless signal reception coupled with a Qi transmitter and impedance analysis. SEM of the surface of sputter coated gold on both PDMS and PMMA substrates with thickness of 100 nm are shown in Fig. 1(a) and (b), an image of the as-cast PDMS substrate is available in the S3 and Fig. S3(a) and (b). To aid manual handling, PDMS was coated on a rigid backing plate. The top of the PDMS layer was treated with (3-mercaptopropyl) trimethoxysilane (MPTMS) as an adhesion promoter before sputter coating with gold according to the process demonstrated in ref. 50. MPTMS is a silane chemical agent that contains a (–SH) mercapto group and (–OCH₃) methoxy groups. The methoxy group binds to the surface of the PDMS and thiol group modifies the surface of PDMS from its general hydrophobic nature to make it hydrophilic.⁵¹ This was confirmed through the contact angle measurements which show that contact angle decreased from 109 ± 5° to 21 ± 15° after the PDMS was treated with MPTMS. Adly et al. also confirmed the MPTMS bonding to PDMS surface via the presence of (–SH) peak at 2550 cm in the FT-IR spectrum along with the presence of sulphur detected with EDX for MPTMS treated PDMS samples.⁵⁰ The mercapto group can bond with gold surface and its methoxy groups form bonds with the PDMS surface to promote adhesion.⁵¹

Fig. 1 (a): SEM image of PDMS surface with sputter coated gold (scale:10 µm). (b) SEM of the surface of PMMA with sputter coated gold (scale:10 µm). (c) Image of the surface profile for sputter coated gold on PDMS substrate. (d) Image of the surface profile for sputter coated gold on PMMA. (e) Comparison of roughness profile of gold on PDMS and PMMA.

Adhesion peel tests carried out with liquid deposition and hydrolysis of MPTMS have been reported to impart strong interfacial bonding for gold on PDMS substrates.^51–53 As seen in Fig. 1(a), PDMS substrates have a distinct continuous and uniform film of gold with wrinkled appearance, owing to the expansion and contraction of the polymer sheet in the vacuum chamber of the sputter coater.⁵⁴ The surface of PDMS coated with gold had a matte appearance while the gold film on PMMA had a smoother finish. Photographs of these samples, directly after being removed from the vacuum chamber (i.e., prior to manual handling) can be found in Fig. S1(a) and (b). No cracks are observed in the gold layer on either substrate. The surface of the sputter coated gold on both PDMS and PMMA substrates were also evaluated by optical profilometry, with the results shown in Fig. 1(c) and (d). As expected, based on the aforementioned visual analysis, significant texturing is observed for the sputtered coated gold on PDMS whereas gold appeared to a smoother finish on PMMA. A line scan of the roughness profiles compared in Fig. 1(e) shows that the gold deposited on PMMA is smoother in comparison to the gold on PDMS. Gold on PDMS measured a surface roughness of 0.43 ± 0.07 µm while the gold on the PMMA measured 0.04 ± 0.01 µm.

Fig. 2(a) shows the electrical resistance of sputtered gold on substrates PDMS and PMMA, measured for 25, 50, 75 and 100 nm thicknesses. For measurement of these electrical resistances, rectangular strips of gold coated PDMS were cut in dimensions with width 5 mm and length 15 mm and manually measured using a multimeter. S2 and Table 1 includes data with corresponding electrical resistances for both the substrates at varying metal thicknesses and S2 and Fig. S2(a) includes the schematic used for measurement. To aid durability, pre-stretching the substrate before coating with metals and alignment of polymer chains has been reported and may be employed in order to produce more robust devices.⁵⁵ Along with this, some literature reports also suggest use of serpentine structures to accommodate mechanical deformation and even strain distribution across the length of the device.⁵⁶ The thickness of PDMS may vary depending on application.

Fig. 2 (a) Measurement of electrical resistance for different thickness of sputtered gold on PDMS substrate. (b) Graph plotting the electrical resistance of 100 nm sputtered gold on PDMS substrate in response to applied strain. (c) Photograph of the gold surface on PDMS substrate having being strained to just below before electrical failure and allowed to relax.

In this case, similar electrical resistance was observed for both undisturbed PDMS and PMMA, with the coated PMMA measuring a marginally lower electrical resistance. At 100 nm thickness, gold samples on PDMS and PMMA show an averaged electrical resistance of 9.3 ± 1.8 Ω and 6.9 ± 1.7 Ω respectively. We note that two-terminal measurements include contact resistance, which may slightly bias the absolute values reported in Table 2; however, the relative trends between samples remain valid. While a four-probe measurement technique is necessary to isolate the true film resistance, the relative difference in resistance between the PDMS and PMMA samples is a valid indicator of the influence of substrate morphology. As observed by SEM and optical profilometry, the greater surface roughness and wavy morphology of the gold on PDMS (compared to the smooth film on PMMA) likely contributes to the higher overall resistance. Thickness of the PDMS and PMMA used here are 1.0 ± 0.2 mm and 1 mm (manufacturers specification for PMMA) respectively. An image of the as-cast PDMS substrate is available in the S3 and Fig. S3(a). S2 and Fig. S2(b) shows the electrical resistance plotted against 1/thickness of gold layer. A linear trend with the same slope was measured for 50 nm, 75 nm and 100 nm samples. This suggests that at 25 nm the gold layer is not fully continuous but rather an interconnected network of islands. Furthermore, the results from thicker samples indicate that the difference in the measured resistances arise from contact, and not the gold layer itself. In order to evaluate the integrity of the gold films on the soft PDMS substrate, electrical resistance of the gold film was plotted while subject to uniaxial stretching, as depicted in Fig. 2(b). The gold coated PDMS samples were strained at a rate of 1 mm min⁻¹ with initial electrical resistance at 10 Ω and measured ∼500 Ω at elongations of approximately 2%. After this point, sample presented with electrical failure. Akogwu et al. show that PDMS films sustain >100% strain.⁵⁷ Thick elastomer substrates may increase the mechanical strain tolerance to these devices, however, are unlikely to be uniform in thickness due to limitations of the casting method. Electrical resistance of 100 nm gold on PDMS remains stable until 0.6% strain after which a dramatic rise is recorded. After 0.6%, a rise in electrical resistance is observed owing to cracking throughout the gold layer. Furthermore, the robustness of the samples is limited by the mechanical mismatch between rigid metal gold and soft polymer PDMS.²³ An image of a gold coated PDMS sheet, after being subjected to uniaxial strain to just below electrical failure and relaxed, is shown in Fig. 2(c), and depicts the significant permanent cracks that formed throughout the coated gold layer after tensile testing was performed. In the current study, mechanical tests are limited to uniaxial strain measurements. However, cyclic tests with subsequent electrical resistance measurement can be carried out for mechanical durability of the device in conditions better approximating the real world. While our tests were limited to uniaxial strain, literature-cited cyclic tests on similar gold–PDMS systems provide guidance on device lifetime.⁵⁷ Cyclic tests carried out by Akogwu et al. for 100 nm e-beam deposited gold with chromium adhesive layer on PDMS can be used as a guide for subsequent measurements. In Akogwu et al.'s work, cyclic measurements for 100 cycles show that after each cycle goes between 0% unloading to 100% loading there is significant cracking of the gold after each cycle. Based on these reports, it can be anticipated that devices operating under lower strain ranges will degrade more slowly. Wearable devices can be designed to accommodate expected strain ranges, including strategies such as encapsulation, material layering, and pre-straining to mitigate cracking and enhance mechanical robustness.^58,59 These approaches allow device performance to be maintained while ensuring wearer comfort. The electrical resistance measured initially is 100 Ω and after each cycle increases and measured a 1300 Ω after 100th cycle.⁵⁷ Depending on application, strain loading can be determined. It can be anticipated that lower strain cycling ranges will result in slower degradation. Many applications, such as cochlear implants, are unlikely to experience strains in the range of 100%, suggesting viability of ablated gold on PDMS. In Kim et al.'s work, bending fatigue of metal electrodes on polymer substrates reported stable electrical resistance over 100 000 cycles at operating at a low strain amplitude of 0.7%.⁵⁸ This confirms that device lifetime is highly dependent on the operational strain window and that low-strain cycling can significantly extend durability. By operating within a controlled, acceptable sub-percent strain window, the device can maintain electrical functionality over an extended lifetime. Further cycling, to relevant strain levels, should be performed for comprehensive assessment. Wearable devices can be ergonomically designed to accommodate strains and to ensure device performance and wearer comfort through methods like material layering for strain distribution and encapsulation of device.^59,60

Table 2 Impedance measurements for frequency responsive laser engraved gold coil on PDMS and PMMA at 100 kHz

		Laser engraved gold coil on PDMS substrate	Laser engraved gold coil on PMMA substrate
Impedance	|Z|	2.95 kΩ	13.30 Ω
Series resistance	R	2.79 kΩ	0.86 Ω
Series reactance	X	961.70 Ω	13.27 Ω
Series inductance	L	1.53 mH	21 µH
Quality factor	Q	0.34	15.4

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Laser engraved gold Qi inductive receiver coils

Structurally, antennas are simple electronic devices which consist of a conducting layer on a dielectric substrate. They are used for transmission and reception of electromagnetic wave signals at different frequencies depending upon application.⁶¹ Spiral coil antennas, in particular, are frequency independent antennas and have a wide band of frequencies with each loop contributing as a resonating element and with operation over a range of frequencies.⁴³ As such for prototyping using the laser ablation technique, a spiral antenna coil design was chosen. Modelling of the coil was based on the coupling resonant wireless power transfer (WPT) system given in.⁶² While “off the shelf” antennas might be a convenient option considering the cost and availability, their application is limited when it comes to operational frequency range for specialized applications, long term durability, as well as regulatory compliance relating to wearable and communication systems. Also, as modifications cannot be done for off the shelf products, long term costs outweigh the initial savings. However, for initial stages of design and development, they can be used to set up the test bench for characterization and testing of general antennae. The inductive coil pattern drawn in MATLAB is shown in Fig. 3(a). In order to produce such a structure, the pattern was inverted, (creating a negative mask) such that the subtractive laser engraving process removed the unwanted gold, leaving the desired coil. The fabrication strategy for producing the Qi based gold inductive coils is shown in S4 and Fig. S4. As previously mentioned, prior to the deposition of a 100 nm thick gold layer onto PDMS, an adhesion promoter, (3mercaptopropyl) trimethoxysilane (MPTMS), was applied. The methoxy group (–OCH₃) in MPTMS binds with PDMS while the (–SH) group binds with gold helping the gold adhere better to PDMS.^50,63 The PMMA sample was directly coated, without the adhesion promoter. Gold was coated in house onto both PDMS and PMMA for more control over the fabrication process, however depending on requirement elastomers like PDMS with conductive coatings can be purchased from specialized suppliers for saving time and reducing cost. In this research work, the gold was deposited via sputter coating method to allow flexibility in determining an appropriate coating thickness of the metal layer.

Fig. 3 (a) Coil designed using MATLAB simulation software and negative coil antenna mask prepared for the engraving process using CorelDraw. (b) Photograph of the laser engraved antenna coil with the optical profile of the fabricated coil showing the track width and track spacing.

The pattern is laser engraved into the substrates using a 10.6 µm CO₂ laser source operating at 18 W at 800 mm s⁻¹ and 1000 PPI (pulses per inch) resolution in raster scanning mode. Fig. 3(b) shows an image of the laser engraved inductive coil fabricated on PDMS substrate with its optical profile. When gold is ablated to fabricate the inductive coils, there was no visible sign of degradation to the substrates observed. Some features were however seen along the ablated path which might be attributed to the residual gold due to missed scan of laser. Residual gold along the ablated path is observed after a single laser pass can affect coil performance. However, as they are isolated gold they do not affect the antenna performance. They can also be easily removed by carrying out multiple passes along the track. Optimization via multiple passes or other process modifications is outside the scope of the current study and is suggested for future work. For PDMS based substrates with thickness 1.0 ± 0.2 mm, inductive coils measured track width and spacing of 257 ± 16 µm and 500 ± 80 µm respectively. Samples engraved on

PMMA substrate with thickness 1 mm (as provided by manufacturer) measured coil track width and spacing of 269 ± 12 µm and 500 ± 20 µm respectively, suggesting slightly better control over the pattern when a more rigid substrate is employed. The initial electrical resistance of PDMS and PMMA after deposition of 100 nm of gold are 9.3 ± 1.8 Ω and 6.9 ± 1.7 Ω measured using a multimeter with rectangular strip dimension of 5 × 15 mm. However, after laser ablation process for the fabrication of spiral coil antennas, ablated path causes localized heating with thermal expansion of the base. Though thermal conductivity of PDMS 0.16 W mK^−1 64 is similar to PMMA 0.19 W mK⁻¹,⁶⁵ thermal expansion of PDMS 309.6 ppm °C^−1 66 is higher than PMMA 193.6 ppm °C⁻¹.⁶⁷ This results in localized stress and higher electrical impedance of the PDMS based spiral coil antennas in comparison to the PMMA based coils. As spiral coil antennas, the coil antennas are frequency responsive and have a complex impedance in the form of R + jX where R is the series resistance and X is the equivalent series reactance. Impedance measured for these frequency responsive spiral coils based on both PDMS and PMMA at 100 kHz are tabulated in Table 2. In addition to higher series electrical resistance, PDMS based coils also have 70-fold higher inductance value compared to PMMA based coil as shown in the table. To compare with geometric expectations, we estimated the inductance of the 10-turn, 30 mm outer diameter planar coil using a Wheeler-type formula, yielding L ∼15 µH.^68,69 This is comparable to the measured value for the PMMA-based coil (∼21 µH). The much larger inductance reported for the PDMS-based coil (∼1.53 mH) is likely due to fabrication and measurement artifacts, including microcracks from laser ablation, or contact issues during impedance measurement. Thus, the PMMA coil reflects the expected geometric inductance, while the PDMS coil highlights the challenges of flexible-substrate fabrication. The quality factor (Q = X/R) at 100 kHz was estimated from the impedance data in Table 2, yielding Q ≈ 0.34 for the PDMS-based coil and Q ≈ 15.4 for the PMMA-based coil. The much higher Q of the PMMA sample reflects significantly lower series losses, in agreement with the smoother surface morphology and lower dielectric loss of PMMA relative to PDMS. While full frequency-dependent Q spectra were not acquired, the impedance trends (Fig. S5) show consistent behavior, confirming the superior performance of PMMA as a substrate for low-loss inductive coils. We note that two-terminal measurements include contact resistance, which may slightly bias the absolute values reported in Table 2; however, the relative trends between samples remain valid.

The higher electrical resistance values for PDMS based samples is attributed to the immediate cracking of gold layer while handling and transferring of thin coil tracks. As such, inductive coils fabricated on PDMS were spin coated with a layer of PDMS for protection against further cracking while handling. Encapsulated coils were easier to handle, however if stretched excessively during handling, irreversible damage could still occur. Other than PDMS encapsulation, other biocompatible polymers, (polyethylene glycol) (PEG), poly(lactic-co-glycolic acid) (PLGA), small organic molecules or inorganic coatings (silica) can also be used. These methods can be used in parallel with drug delivery and cell therapy to reduce adverse effects like inflammation and more strongly rejection.⁷⁰

Impedance measurement and wireless signal reception measurements for the laser engraved gold coil antenna

Impedance measurements were carried out with a 1 V AC with 0 V DC offset and are shown in Fig. 4(a) and (b) respectively with the schematic of the measurement set up shown in S5 and Fig. S5(a). The impedance (Z) in black and phase angle (θ) plots in blue were examined to determine the resonant operating frequency range. For coils fabricated on PDMS, a range between 4 kHz to 2.5 MHz was observed, where the phase angle was positive (Fig. 4(a)). For inductive coils fabricated on the PMMA, positive phase angles were observed for frequencies between 160 Hz and 10 MHz, as shown in Fig. 4(b). Fig. 4 shows measurements performed over the full frequency range of the impedance analyzer. The transmitter module operates in the 100–125 kHz band, where the positive phase indicates near-field resonance; this does not imply full Qi standard compliance.³⁴ These frequencies including intermediate values are inclusive of resonant frequency range for PDMS and PMMA-based devices. Capacitive effects are dominant outside the resonance range and decrease the overall reactance of the device.⁷¹ As mentioned, wide resonant frequency ranges are a characteristic feature of spiral coil antennas.⁷² The fabricated coils exhibit resonance within the 100–125 kHz range explored in our experiments, as evidenced by positive phase in Fig. 4(a) and (b). These measurements demonstrate near-field inductive coupling suitable for Qi-like frequency ranges; however, this does not constitute full compliance with the Qi standard, which allows wider operating windows depending on power class and dynamic negotiation. The inductive coils on PMMA have wider frequency range compared to the ones on the PDMS, stemming from the lower electrical impedance at 18 ± 5 Ω of the device, which in turn comes from well matched mechanical strength and thermal expansion properties of the conducting gold with the rigid PMMA. Inductive coil fabricated on PDMS were measured to have electrical impedances of 2800 ± 300 Ω. The high impedance on PDMS is attributed to the cracking within the tracks owing to the mechanical mismatch of metal layer over the flexible substrate. Promisingly, impedance measurements show both engraved inductive coils (on PDMS and on PMMA) are resonant across the frequency range defined in the Qi specifications (100–125 kHz). The resonant frequency range and key impedance fitting parameters are tabulated in Table 3. Resistance and capacitance plots for fabricated inductive coils are given in S5 and Fig. S5(b) and (c).

Fig. 4 (a) Impedance and phase plots for laser engraved inductive coils fabricated on PDMS in the frequency range of 0 Hz–10 MHz (b) Impedance and phase measurement for engraved inductive coil fabricated on PMMA between 0 Hz–10 MHz. (c) Oscilloscope measurement of the transmitted (black line), received signal by the coil antenna on PDMS (red line) and the received signal by coil antenna on PMMA (in blue). (d) Frequency-dependent voltage transfer magnitude of the fabricated coil antennas measured at a 5 mm transmission distance under aligned coil configuration.

Table 3 Summary of impedance measurements for the laser engraved inductive coils fabricated on PDMS and PMMA at Qi frequency 100 kHz and the resonant frequency range

		Laser engraved gold inductive gold coils
a Limit of potentiostat used to run test.
Substrate		PDMS	PMMA
Thickness of deposited gold		100 nm	100 nm
Low frequency limit	θ1	4.05 ± 0.13 kHz	168 ± 6 Hz
High frequency limit	θ2	2.51 ± 0.16 MHz	10 MHz^a
At 100 kHz frequency,
Impedance	|Z|	2.8 ± 0.3 kΩ	18 ± 5 Ω

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Heat dissipation resulting from ohmic losses⁷³ is always a consideration for user safety. As thermal dissipation occurs near the skin interface it can lead to localized temperature increases. In order to minimise the effects of this (in the extreme, this includes tissue damage, but even in more moderate cases, it causes discomfort), this work uses pulsing signal frequency as opposed to continuous signal as the transmitted signal using the Qi transmitter system as shown in Fig. 4(c). Pulsed operation effectively limits heating, ensuring that the coil temperature remains within safe operating limits under the experimental conditions. Fig. 4(c) shows traces for the signal provided by the Qi transmitter module and responses from the inductive coils. These measurements demonstrate capability of the laser engraved inductive coils to serve as receivers, at a working distance of 5 mm, which is similar to the thickness of the human dermis. The transmitted signal from the module, shown in black with a peak-to-peak value (VPP) of 6.5 V, was compared against the received signal from the fabricated coils. For the coils on PDMS, a VPP of 3.4 ± 0.3 V was received, while VPP observed on the PMMA based coil was 4.4 ± 0.3 V. These values correspond to qualitative reception efficiencies of 52 ± 4% and 68 ± 4% respectively for stimulation at 100 kHz and 5 mm working distance. Depending upon the material used for fabricating the coil, the distance between the transmitter and receiver and alignment, Qi charger based reception efficiency can be varied. While these values provide a useful qualitative measure of power transfer, quantifying the actual delivered power and its uncertainty under these conditions was not performed and is a limitation of the current study. Section S7 provides a brief note to quantitatively estimate received power from measured VPP. Yu and coworkers have recently demonstrated a receiver efficiency of 71.9% using Qi platform through a metasurface enhanced WPT system.^74,75 A video showing wireless power reception by the fabricated PDMS based coil to blink an LED is shown in the S6 and Fig. S6 “Qi_powered_LED.mov”.

Fig. 4(d) presents the measured voltage transfer magnitude of the fabricated coil antennas as a function of frequency, recorded at a transmission distance of 5 mm with the coils in aligned configuration. The voltage transfer magnitude for a receiving antenna, measured in decibels (dB), refers to the effective signal reception capacity of the antenna compared to a standard reference antenna in a specific direction. Both the fabricated inductive coils have a negative voltage transfer magnitude at −11.7 dB for fabricated coil on PDMS and −35.4 dB for fabricated coil on the PMMA at 100 kHz frequency. It should be noted that the quantities in Fig. 4(c) and (d) are not directly commensurate. Fig. 4(c) reports the experimentally measured received voltage (V_pp), which depends on both the inductive coupling efficiency and the absolute voltage applied at the transmitter. In contrast, Fig. 4(d) shows the normalized frequency response (voltage transfer magnitude) obtained from impedance measurements. The lower voltage transfer magnitude observed for PMMA reflects its higher dielectric loss at high frequencies, whereas the higher received V_pp in Fig. 4(c) arises from slightly greater mutual inductance under the specific measurement setup used for PMMA.

Inductive coils at 30 mm as fabricated in this work has an electrically small size calculated as:⁷⁶

At the operational frequency of 100 kHz used in the experimentation, power transfer is governed by magnetic coupling (mutual inductance and coupling coefficient k). Because the separation distances used (<10 mm) are well within λ/2π, the structure behaves as an inductive coupler rather than a radiating antenna. The observed increase in voltage transfer magnitude at frequencies >1 MHz (as shown in Fig. 4(d)) is a result of improved coupling efficiency. These antennas can benefit from impedance matching network as well as ferrite shielding techniques for boosting magnetic coupling, minimizing system impedance and signal reflection for comparable translation to state-of-the-art Qi-compatible devices.^77,78

The fabricated inductive coils with negative gains at 100 kHz result in a 180-phase reversal or out of phase signal. This results the peaks in the coupled wireless signal to be received as troughs. As such the received signal of the fabricated coils at 100 kHz are inverted signals and can be seen in Fig. 4(c). Negative voltage transfer magnitude also implies signal attenuation for the received antenna which is seen through the clipping output of the inductive coils. Clipping of the signal in the positive part is also seen for both examples of laser fabricated inductive coils. It means that the signal at that point is outside or exceeds the output of the antenna device constructed at that frequency. Clipping behaviour is generally to avoid distortion. Using an impedance matching network may solve this problem, but this is outside the scope of current work.⁷⁹ For given results, tuning capacitor was not used, however these can also be added in parallel configuration to the terminals of the inductive coils to fine tune the antenna resonance at desired frequency for improving efficiency of signal reception.⁸⁰ Tuning capacitors are variable capacitors which when attached to the antenna circuit adjusts or matches the impedance of the circuit to increase the antenna efficiency.† Simple LC circuits with and inductor (L) and capacitor (C) or on-shelf radio frequency (RF) networks can also be implemented for impedance matching in the antenna for tuning purpose and effective signal reception.⁸¹

Experimental method

Materials and reagents

PDMS was prepared using Sylgard 184 Downing kit purchased from BSC Prestons, Sydney. (3mercaptopropyl)trimethoxysilane (MPTMS) (95%) used as adhesion promoter was purchased from Sigma Aldrich. MPTMS solution was prepared in ethanol purchased from ChemSupply. Poly(methyl methacrylate) (PMMA) sheets were purchased from Cammthane, Australia.

Coil antenna design

Qi based inductive coil was designed using the MATLAB design and simulation tool. The modelling carried out based on the wireless power transfer (WPT) system as in ref. 62. The MATLAB design function createSpiral (internal diameter, outer diameter, number of turns) can be used for generating coils with defined parameters. Internal and external diameters of the inductive coil was fixed at 15 mm and 30 mm respectively. The coil was designed with 10 full turns and wire length of 0.72 m. CorelDraw software was used to add contact pads to the designed inductive coil pattern.

Polydimethylsiloxane (PDMS) sheet substrate preparation

PDMS substrates were prepared using the Sylgard 184 base to curing agent mixed in 10 : 1 weight ratio. The Planetary Centrifugal Mixer Model AE-250CE was used for mixing, the program constituting of a series of mixing and degassing steps. First the centrifugal mixer was programmed to initiate mix at 800 Revolutions Per Minute (RPM) for 30 seconds, then continuing to mix at 2000 rpm for 2 minutes followed by a degassing step at 2200 rpm. The resultant mixer was cast onto acrylic sheets using spin coater Laurell WS-650HZ-15NPP/LITE and dried at 70 °C overnight. PDMS sheets thus prepared were surface treated with 25 mM solution of (3-mercaptopropyl) trimethoxysilane (MPTMS) in ethanol as in⁵⁰. For treatment with MPTMS, the sheets were bath sonicated in ethanol for 10 min followed by immersion in MPTMS/ethanol for an hour. Sheets were then rinsed in 1 mM HCl/distilled water solution for another hour and further rinsed.

Contact angle measurement on PDMS surface

The goniometer OCA15EC from DataPhysics Instruments, Germany was used for contact angle measurement of the PDMS surface.

Sputter coating gold on the PDMS sheets and the PMMA

The surface treated PDMS and PMMA were sputter coated with 100 nm of gold using Edwards Auto 306 thermal evaporator/vacuum coating system. The sputter chamber was adjusted to 3 × 10⁻³ Pa at deposition rate of 10 nm min⁻¹ for coating. PMMA was wiped cleaned with ethanol prior to coating. The chosen 100 nm gold thickness provides a balance between electrical conductivity and substrate flexibility. To further reduce electrical losses, alternative fabrication strategies could include thicker sputtered films, electroplating on a gold seed layer, or using laminated gold foils. Thicker sputtered or electroplated layers increase the conductive cross-section without significantly affecting pattern resolution or flexibility, while laminated foils provide improved mechanical robustness and lower resistance. Each of these options can improve conductivity without compromising biocompatibility or substrate flexibility. These strategies are noted as considerations for future high-performance implementations.

Scanning electron microscopy (SEM)

JEOL 7500 scanning electron microscope operating at 10 kV and 5 mA was used for SEM imaging of the samples.

Optical profile/microscopy

Olympus LEXT OLS5100 3D Laser Scanning Microscope was used for measuring the optical profiles of the laser engraved inductive coils at 50× magnification, set at pitch of 0.4 µm and distance of 47 µm for imaging.

Mechanical tensile testing and electrical response measurements

Shimadzu EZ mechanical tester fitted with a 10 N force transducer at 1 mm min⁻¹ was used for mechanical uniaxial tensile measurements. The samples were also simultaneously connected to Keysight Technologies Truevolt 34450A Bench Digital Multimeter for measurement of electrical resistance when subject to stress.

Laser cutting and engraving coil antenna pattern

The laser engraving was carried out using ULS PLS6MW laser engraver (Universal Laser Systems GmbH, Austria). The laser system was fitted with a 10.6 µm CO₂ laser and recommended focus lens of 2.0 MW (51 mm). The average spot size or diameter for this laser configuration was 127 µm (0.005″) according to the user manual.^82,83 Single scan of the laser, with power of 18 W (60% of full power, 30 W 10.6 µm CO₂ laser), a scan rate of 800 mm s⁻¹ (40% of the maximum scan velocity) and 1000 PPI (pulses per inch), in raster scanning mode, was used for fabricating the antennas. Multiple scans may be resorted for eliminating gold remains between spiral coil traces. As the current study doesn’t clearly observe residual gold between the antenna tracks, it is recommended to use techniques such as optical profilometry, current or resistance mapping to analyse the elimination of any residual gold. It is also recommended to do this for successive passes of the laser during a multiple scan fabrication.

Impedance analysis and wireless signal measurements

The digilent analog discover 2 portable benchtop kit was used for impedance measurements with its internal waveform generator and oscilloscope connected together with a reference resistor of 10 kΩ. For wireless Qi based signal reception demonstration, fabricated inductive coils were connected to Bewinner Universal Wireless Charger Transmitter Module. Signal waveforms were recorded with the help of Rigol DS-1052E digital oscilloscope. For a comprehensive assessment of the coil antenna, quantifying key figures of merit, including the coupling coefficient (k), mutual inductance (M), and S-parameter-based power-transfer efficiency under a defined load is recommended.

Conclusions

In this work, inductive coils, suitable for use in wearable devices, were fabricated using gold and a biocompatible polymeric substrate material along with a laser ablation process. The fabrication process included sputter coating of gold on PDMS substrate and laser ablating them in inductive coil patterns. Laser ablation was carried out with a 10.6 µm CO₂ laser and 2.0 MW focus lens of (51 mm), with the spot size for this laser configuration approximately 127 µm in diameter. The laser power of 18 W, speed of 800 mm s⁻¹ and 1000 PPI (pulses per inch) in one pass raster scanning mode was used for fabricating the antennas. The performance of the fabricated inductive coil on PDMS was measured against a fabricated control inductive coil of same dimension on PMMA substrate. PDMS has reported low dielectric constant of 2.32–2.4. PMMA has a similar dielectric constant of 2.546, however, in contrast to PDMS, it is much more rigid. The substrates were tested in relation to their performance to the sputtered coated gold. With surface analysis performed by SEM and optical profilometry, it was observed that sputtered gold had a wrinkled and wavy appearance on the PDMS in contrast to the gold on PMMA which was smooth. The mechanical strength of the gold/PDMS was measured with uniaxial tensile measurements and showed stable electrical properties until 0.6% strain.

The electrical performances of the fabricated coils were also assessed by impedance measurements and their resonant frequency range determined. From these tests, it was observed that the fabricated coils showed resonance at 100–125 kHz frequency, which is the operational range of Qi based wireless transmitters and can be used as receiving antennas at that frequency. Impedance results showed high values of 2800 ± 300 Ω for PDMS based inductive coils whereas PMMA based coils had lower values of 18 ± 5 Ω at 100 kHz. Similarly, Q-factor calculated as 0.34 and 15.4 for PDMS and PMMA suggest lower loss of the coil fabricated on PMMA compared with PDMS. The high impedance of the PDMS based coil is based on the mechanical mismatch of the polymer substrate with the rigid metal coating. The stiffness of the PDMS substrates can be tuned by adjusting its base to curing agent ratio, volume of the final mixed solution, coating speed, curing temperature and filler concentration. When tested as wireless signal receivers coupled to a Qi wireless transmitter, the fabricated inductive coil on PDMS received 52 ± 4% of the transmitted signal compared to its PMMA counterpart which received 68 ± 4% of the transmitted signal. This efficiency was determined at 5 mm working distance which is comparable to dermis of skin. The inductive coils were used as it is without tuning and may achieve higher values of signal reception with a matching network. However, to provide a comprehensive and standardized assessment of a coil's performance for high-efficiency wireless power transfer, future work will include a more complete electrical characterization in the near-field regime. This will involve quantifying key figures of merit, including the coupling coefficient, mutual inductance, and performing S-parameter-based power-transfer efficiency measurements under a defined load.

Potential opportunities and medical translation

Using biocompatible materials like gold, which is widely used in medicine.⁸⁴ Additionally, USP Class VI approves the medical use of PDMS polymer.⁸⁵ As such, wireless antennas prototyped with laser ablation approach has massive opportunities of integration with biomedical devices for smarter diagnostics and real time wireless monitoring. For medical translation and development as a product, cytotoxicity and aging studies under critical conditions as per the ISO 10993-1:2018 and FDA 2016 Guidance on ISO 10993 guideline needs to be followed.^86,87

Author contributions

Grishmi Rajbhandari: conceptualization, data curation, investigation, methodology, resources, software, validation, writing – original draft, writing – review & editing. Andrew Nattestad: writing – review & editing. Xiao Liu: conceptualization, supervision, writing – review & editing. Stephen Beirne: conceptualization, supervision, writing – review & editing. Gordon G. Wallace: conceptualization, funding acquisition, supervision, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The datasets generated during and/or analysed during the current study are available at https://figshare.com/s/7e0429b6aac177c659dc?file=57175439.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5tb00699f.

Acknowledgements

This work was supported by ARC Industrial Transformation Training Center (ITTC) in Additive Biomanufacturing. The authors would like to acknowledge ARC Centre of Excellence for Electromaterials Science (ACES) and the support of the Australian National Fabrication Facility (ANFF) Materials node for access to equipment and expertise. Dr. Xiao Liu acknowledges the Garnett Passe and Rodney Williams Memorial Foundation for support of the Mid-Career Fellowship (2024_MCF_Liu).

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Footnote

† Limitation of the instrument.

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