3D Stretchable Devices: Laser‐Patterned Electronic and Photonic Structures

Realizing three‐dimensional stretchable structures of functional materials with a minimum footprint on Silicone polymer is highly desirable in soft robotics, stretchable electronics, and photonics. However, material processing on a stretchable substrate requires a sophisticated deposition system with integrated substrate cooling facilities, delamination of materials from the stretchable substrate due to stretching‐releasing cycles, and coating the functional materials. Here, a methodology to address these challenges using in situ graphitization within silicone polymer, referring to transforming the material into graphite‐like structures using three‐dimensional laser printing is reported. In this case, the graphitization process occurs due to the interaction of the material with a spatially controllable, tightly focused femtosecond laser beam in the confined region within the polymer. Three‐dimensional printed embedded, stretchable electrodes and varifocal lenses of thickness 1/20th compared to the epidermis layer thickness of human skin, which can contribute to achieving compact, highly sensitive wearable sensing and imaging systems are demonstrated and characterized. This process will open a new door for forming non‐metallic stretchable three‐dimensional conductors and photonics with minimum exposure to atmospheric conditions and a pathway to interface with thin films to develop low‐dimensional devices. These graphitized three‐dimensional structures can make them integral to intelligent skins, e‐textiles, and implantable devices.


Introduction
[3][4] An electronic version of human skin known as e-skins mimics the multifunctionality of the skin by using sensing units to detect multiple stimuli. [5]On the other hand, photonic skin-based standalone technology has a higher potential than its electronic counterpart due to its non-invasive performance, sensitivity to external stimuli, and resistance to electromagnetic interference since photons are the data transmitters. [6,7]Therefore, even though the interest in this technology can be dated back to the past few years, it has already found applications like optical interconnects, [8] optomechanical tuning, [9,10] epidermal monitoring, [11] implantable applications, [12][13][14][15] photo therapeutics, [16,17] displays, [18][19][20] imaging, [21] sensing, [22][23][24][25][26][27][28][29][30] and conformal photonics. [31]hese next-generation smart skins demand micro and nanoscale non-volatile components, which become more critical with applications like human-machine interfaces, particularly neural chips. [32][35] Stretchable devices based on soft silicones are an ideal candidate for this concept. [36,37][40][41][42][43] Nevertheless, these materials are limited by the degradation resulting from the chemical or thermal treatments during the development of electrical contacts, leading to less durability of the devices. [44,45]irect laser writing (DLW) or three-dimesnional (3D) laser printing is an approach to overcome this problem due to the spatial confinement in the fabrication area, which results only in minimal material damage during the formation of circuits. [46][54][55][56][57] Besides, the processes can lead to issues such as bubble formation (as shown in Figure S1, Supporting Information). [58]Further, the resulting treated PDMS films can be moisture-exposed and require other encapsulating layers, such as parylene. [59]62] Previously, PDMS has been used as a protective layer to preserve the quality of reduced GO patterns. [63]Further, GO-PDMS composites were developed to provide higher mechanical stability, dielectric response, and nonlinear conductivity than PDMS films.Those composite films are demonstrated for sensing and anticorrosion layers. [64,65]The quality of the devices has improved, but the durability continues to be an issue due to the exposure of the contact electrodes to atmospheric conditions.Our work focuses on developing a method to overcome this problem.
There are significant developments in 3D printing techniques, especially using inks. [66]Most of these are additive approaches to a surface. [67]There are a few rare examples of subtractive printing approaches where materials are modified and dissolved. [68]We demonstrate laser-printed, high-resolution 3D structures within a polymer thin film matrix of PDMS and GOs using a predefined 3D scanning of fs-laser beam, which induces minimal heat leading to a highly confined area for the fabrication in the defined focal plane of electrical and optical applications in comparison to ns-laser beam treatment on the thin film. [53]Our technique starts with a bulk substrate and defines the structures within this material.So, it is neither additive nor subtractive and is a material transformation.Through this process, the carbon-rich material in PDMS makes it conductive.The synergistic effect of reduced GO and laser-treated PDMS leads to the formation of a 3D structure guided by the laser's patterned movement.
We generate a Computer-Aided Design (CAD) layout to produce an electrical circuit and input it into the laser fabrication system.The system follows the specified stage movement based on the CAD design.The laser beam tracks along designated coordinates, with the focused beam transforming the GO-PDMS material into reduced GO and carbon-rich PDMS material.This controlled laser movement across the thin film results in the formation of a 3D circuit.Additionally, we can fabricate structures at varying depths within the thin film by defining the desired height.As the transformed material stays embedded in its matrix material, it is not exposed to the atmosphere, leading to retaining the flexible and stretchable properties of its primary material, GO-PDMS thin film.This reduces electrode damage due to external stimuli and can lead to the development of various on-chip and integrated electronics, optical and optoelectronic devices.
Among different wearable optical components, the stretchable flat lens is an attractive proposition for miniaturizedminiaturized imaging systems.Still, it is limited to tuneable focal lengths for single wavelengths and complicated fabrication steps involved in the development process. [68,70]Recent reports on varifocal broadband stretchable flat lenses struggle with durability due to improper encapsulation. [71]Our device fabrication method avoids the requirement of transfer and additional encapsulation methods due to the minimal exposure of the physically changed area to moisture. [72]Furthermore, the approach can result in durable electrical and optoelectronic devices with extended lifetime. [73]

Use of Graphene Oxides with PDMS
The schematic of the fabrication steps to develop GO-PDMS films is shown in Figure 1a.The process started with mixing graphene oxide powder of different concentrations with PDMS solution, then drop-casting the GO-PDMS solution on a substrate (glass or silicon).Then, further solidification of the solution happened at 60 °C for 3 h.Finally, the solidified thin film was removed from the substrate with the help of O 2 plasma, resulting in a standalone thin film that can be used for 3D laser printing of the electrodes at different focal planes within the thin film.
Several factors influence the laser beam interaction with the material, including transmission at a considered wavelength for the interaction and surface roughness. [71]For in situ laser beam interaction with the material, several factors influence the process, including the transmission of the material in the required wavelength and the surface roughness of the material.The lower laser beam transmission through the thin film can lead to the wrong detection of the desired focal plane, resulting in feature formation at undesired regions. [67,74]Further, the material roughness at different focal planes influences the laser beam scanning speed, leading to incomplete structure formation. [75]Therefore, the percentage of GO in the PDMS must be controlled as a higher amount can lead to reduced transmission (Figure 1b).In our studies, different amounts of GO powder were added to the PDMS solution by varying the concentrations from 0.05-0.45%.The increase in the quantity of GO in the thin film leads to a decrease in transmission (Figure 1b,c).Due to the above-considered factors, it was identified that the thin films having a concentration of GOs between 0.2 and 0.3% were ideal for fabrication using our custom-built laser printing setup due to the formation of adequate covalent bonding between GO and PDMS, leading to higher mechanical strength to withstand high laser beam power density and also ideal transmission intensity through the thin film in comparison to PDMS thin film.3D laser printing in GO-PDMS thin-film.
A schematic of the 3D laser printing custom-built setup used for the fabrication of the embedded electrodes is provided in Figure S2a (Supporting Information).The optimization process for the laser beam irradiation conditions within different depths of GO-PDMS thin film was conducted using different scanning speeds from 1 to 10 mm s −1 and laser beam fluences from 0.042 to 0.25 J cm −2 with an oil objective of numerical aperture (NA) 1.4 and 532 nm femtosecond (fs) laser beam with a pulse repetition rate of 70 MHz.As a result, the best 3D structure fabrication condition was observed for an average beam power of 0.13 J cm −2 and a low scanning speed of 2 mm s −1 , where the fabricated area changed colour, as given in Figure S2b (Supporting Information).
Even though the modification in the polymers due to laser beam exposure is from a linear absorption process, the involvement of an fs laser beam will induce a nonlinear absorption in the material due to the high energy focused in a confined volume, leading to changes in the atomic structure. [68,76]Besides, it was reported that the GOs present in the thin film undergo a nonlinear absorption during the interaction with the fslaser beam. [77]Therefore, the colour change observed within the confined areas of the thin film due to the tightly focused laser beam exposure can be attributed to the nonlinear absorption process.
The Raman studies were conducted in the laser beam-treated region in the thin film under optimized fabrication conditions to understand the phase change (Figure 2).It can be seen in Figure 2a that the formation of two distinctive peaks at 1350 and 1580 cm −1 corresponds to the D and G peaks in the graphitic carbon in the thin film for the features formed using specified designs. [78,79]In addition, the formation of the peak at 2700 cm −1 corresponds to 2D peaks in the graphitic carbon, [78] along with the reduction of significant peaks of PDMS at 2900 and 2980 cm −1 contributed by the symmetric and asymmetric vibrations of C-H 3 , methyl groups present in native PDMS. [36]Further, the I G /I D ratio formed in the laser beam-treated region using different laser fluences is shown in Figure 2b.It can be seen that the maximum intensity ratio was 0.5 for the 3D structures formed at different depths from 100 to 200 μm within the GO-PDMS thin film, which indicates the formation of sp 2 C═C bonds due to the breakage of sp 3 C─O bonds and confirms a graphitization domain in the laser-treated region. [79]

Fabrication of 3D Electrically Conductive Structures
To study the feasibility of embedded structures in the GO-PDMS thin film, three different 3D designs of electrodes of size 20 × 10 × 10 μm 3 were fabricated (Figure 3) using a scanning speed of 2 mm s −1 and a laser beam fluence of 0.13 J cm −2 at various depths from 100 μm from the surface to 200 μm within the thin film.The video files for forming these 3D electrode designs are provided in Video S1 (Supporting Information).

Electrical Characterization of Stretchable 3D Electrodes
The electrical conductivity and stretchability of electrodes are essential features of flexible optoelectronic devices. [80]The electrical performance of the fabricated 3D electrodes using an optimized average laser beam fluence of 0.13 J cm −2 based on design-3 (Figure 4) at different depths from 100 μm to 200 μm within the GO-PDMS thin films was studied.The I-V characterization using the two-electrode setup, as shown in Figure 4a, and an applied voltage between −2 and 2 V was utilized.The optimum electrical conductivity of 3D electrodes was measured to be 4.5 mS m −1 for the 3D electrodes fabricated at a depth of 100 μm (Figure 4b).The obtained electrical conductivity for the laser-patterned 3D electrodes is comparable to other GO and polymer mixture-based electrodes reported. [81]It was observed that as the depth of the fabrication increased to 200 μm within the thin film, the quality of the electrodes was dropped and was confirmed by a decrease in the electrical conductivity up to 2.6 mS m −1 .This can be attributed to the lower sp 2 to sp 3 conversion rate of GOs due to the influence of optical losses like scattering and heat formation that the laser beam undergoes during the electrode fabrication process as the depth of the focal plane increases within the thin film.
Further, the mechanical tuning of the fabricated embedded structures (design 3) within GO-PDMS thin film was studied using the same setup by stretching and relaxing cycles with respect to distances in the lateral direction.Each measurement was repeated ten times to understand the change in resistance of the electrodes measured at different strain percentages, as shown in Figure 5a.These studies contribute to understanding the reversible nature and influence of stress concentration of interconnects in the performance of the device.
Besides, the influence of strain on the electrical conductivity of the elastomeric embedded structures was performed by mechanically stretching the thin film for distances in the lateral direction (Figure 5b).For each additional 20% strain, voltage-currentvoltage (I-V) curves were measured to calculate the change in resistance.For strains up to 100%, the electrical conductivity decreased to 30% from 4.5 mS m −1 .[83][84] The measurements were repeated ten times, and an average electrical conductivity was reported.In addition, it was observed that the current increased linearly with the applied voltage under different strain conditions, indicating good ohmic contact between the fabricated 3D electrodes and the contact electrodes. [85]

Fabrication of Embedded Photonic Structures
The potential of this work is not limited to electronics applications but also can be extended to photonics.Utilizing 3D laserpatterned graphitized GO-PDMS thin film in varifocal lenses presents the potential to transform a range of applications, from intraocular lenses for living organisms to compact lens systems in soft robotics, paving the way for groundbreaking advancements in optical technology. [86,87][90][91] A conceptual schematic of the embedded graphitized lenses within GO-PDMS thin film is demonstrated in Figure 6a.

Refractive Index Modulation
A numerical simulation (COMSOL Multiphysics) [92] was conducted to understand the laser beam intensity-dependent refractive index modulation (Δn) at different depths within embedded GO-PDMS film (Figure 6b).When a laser beam intensifies, the resulting noise across cross-sectional intensity distribution can be amplified, damaging the beam.The dielectric exhibits pulse intensity dependent index of refraction that is given by: [91,93] n =n o +I where n o is the nominal refractive index,  is the nonlinear refractive index coefficient, and I is the laser intensity.
A pulsed Gaussian laser beam of wavelength 532 nm, 70 MHz repetition rate, 100 fs (Figure 6c) was applied along the zdirection of the film from the surface.An eigenvalue equation for the electric field, E, is derived from the Helmholtz equation and solved for the eigenvalue  = −j.
where E is the electric field, c o is the speed of light, n is the refractive index,  is the angular frequency, and  is the propagation constant.
A coarse mesh was used because the electric field envelope has a slower spatial variation than the electric field.Since the laser beam propagates essentially in one direction, it is assumed that the electric field, E, of the wave, can be written as where E 1 is a slowly varying field envelope function.
Inserting this electric field formulation into Maxwell's equations results in the following wave equation for the envelope function, where μ r is the relative permeability, and c 0 is the speed of light.It can be observed that the laser beam passing through the GO-PDMS thin film leads to a refractive index modulation of the order of 103 along the path of the irradiation at different depths.

Designing of Stretchable Embedded Graphitized Lens
Further, a Fresnel plate lens was designed as an optical application using the detour phase technique. [93,94]The method was introduced in 1966 to modify the complex amplitude distribution of a wavefront by slightly changing the locations of apertures and widths.The theory considered an infinite 1D structure like grating made of periodically repeating apertures.When a light beam passes through two adjacent apertures, it will be diffracted at each aperture and produce secondary wave sources.The first diffraction orders of each secondary wave source form a plane wavefront with parallel ray directions, marked as red arrows.The phase modulation occurs when a spatial shift is introduced to the apertures, which provides an effective method to modify the relative phase value of the first-order diffraction field aperture by the aperture.A guide for restructuring the diffracted light beam wavefront using the spatial location shift of the aperture can be defined as the function of the desired phase value: where Λ is the period of the initial grating, and (x) is the desired phase value at point x.
Based on the above equation, a line structure for a converging planar lens is designed to shape the wavefront as an off-axis converging wavefront.Then, a converging lens with a symmetrical concentric ring structure can be obtained by rotating the line structure out of the plane along the edge of the line structure.Finally, the light beam undergoes constructive interference to form a symmetrical focal spot.Figure S3 (Supporting Information) shows the phase distribution along the radial direction of a converging lens.For the discretized paraxial phase profile, different periodicities of the initial grating lead to other aperture locations, and thus, different lens designs can be obtained.
The discretized paraxial phase distribution profile,  i (r), with the radial position of the n th ring (n th slit) of the converging lens has the following form: where r is the radial distance from the center of the lens, and D is the diameter of the lens.Following Equation ( 6), we can define the radii of each ring.For focal length tunability, the paraxial converging phase distribution of  2 /( 1 f 0 ) is considered where f 0 is the focal length and  1 is the working wavelength.When the uniaxial stretchability of the lens is taken into the effectiveness, the radii of the rings become R ′ n = R n (r) where  is the stretch ratio of the lens.The phase profile of the lens becomes: The Relationship Between the Focal Length and the Stretch Ratio can be Written as where f′ the focal length when the lens is stretched does not depend on the incident wavelength.Meanwhile, the theory was extended and optimized the focal length for different wavelengths in the visible region.For a given design, the angle of the first diffraction order for wavelength  1 is defined as  1 and for another incident wavelength  2 , the diffraction angle will be changed to  2 .The relationship between  1 and  2 can be defined as ) sin  1 ].The phase profile distribution for the diffraction order with  2 will be the same as in Equation 3 using wavelength,  2 .Then the phase modulation generated by the lens for wavelength,  2 can be defined as Therefore, the initial lens structure designed for incident wavelength  1 with a focal length, f 0 has a focal length for an arbitrary wavelength given by Based on this relationship, the lens design based on the detour phase method can result in broadband focusing capability without phase distortion.
Our initial lens design considers the central wavelength at 532 nm, and the central radius of the lens was 3.3 μm.The lens consists of 20 rings with a width of 1.5 μm, and the distance between each ring is 5 μm.The lens radius was 470 μm with a thickness of 5 μm.The focal length of the lens was ≈940 μm at 532 nm wavelength before stretching.The numerical aperture (NA) of the lens was 0.5 and was calculated using the equation: The embedded graphitized lens was laser patterned within the GO-PDMS thin film made with optimized GO concentration in a PDMS matrix of 0.25% using an oil objective of NA 1.4 and 532 nm femtosecond (fs) laser beam with a constant pulse repetition rate of 70 MHz, a laser fluence of 0.13 J cm −2 and a scanning speed of 2 mm s −1 .The phase modulation of the lens is defined using, where d is the thickness of the lens which contributes to the optimum focusing efficiency of the lens, n is the refractive index of the GO-PDMS material, and  is the wavelength of the incident light beam (Figure 7a).This approach ensures that the lens fabricated is standalone and avoids the requirement for the substrate.Figure 7b is the image taken for the lens 4f microscope, which is within the GO-PDMS thin film.As imaging optics need to penetrate the thickness of PDMS, better resolution images with sharper definition are not possible.

Characterization of Embedded Graphitized Lens
A numerical simulation for the electrical field distribution of incident wavelength using the embedded graphitized lens (COM-SOL Multiphysics) was conducted to investigate the theoretically calculated focal length for incident wavelengths at 450, 550, 650, and 750 nm.As shown in Figure S4a (Supporting Information), the intensity distribution using the fabricated lens demonstrated a focal spot at 940 μm independent of the incident wavelengths.Furthermore, the observation was supported by the electrical distribution enhancement at 940 μm, as given in Figure S4 (Supporting Information).A custom-built stretchable device characterized the fabricated embedded graphitized lens for mechanical tuning, enabling practical zoom imaging and broadband focusing capability on the visible region from 450 to 700 nm (Figure S5, Supporting Information).We used 3D laser patterned letters "FMM" of size 35 × 25 μm2 fixed on a uniaxial stage as the object image for the stretching lens measurements from a stretch ratio of  of 1.00-1.21,corresponding to a strain up to 21%.A broadband LED source (Thorlabs) and 30 nm bandpass optical filters were used.The focused image from the embedded graphitized lens was collected using an objective of 100×, 1.4 NA, and a CCD camera for the visible region wavelengths.
It can be seen in Figure 8a that the resultant image from the CCD camera was clear enough to be read without distortion at a minimum focal length of 0.94 mm.Moreover, at different stretching ratios otherwise strain percentages for stretching in lateral direction, a focal length shift upto 0.3 mm was obtained for different incident visible wavelengths from 450 to 750 nm as shown in Figure 8b.The measurements were repeated ten times to obtain an average, and an error bar was added to the results.The results also demonstrate that these embedded graphitized lenses can be broadband varifocal zoom lenses based on the existing imaging rule. [95,96]Further, the focal shifts were calculated from the imaging measurements for different stretching ratios to confirm the theoretical studies.Finally, a numerical simulation confirmed the varifocal changes for the lateral stretch conditions, as shown in Figure S6 (Supporting Information).However, it was observed that as the focal plane depth of the fabrication of the lens was increased, the imaging quality became blurred.

Conclusion
In conclusion, we have demonstrated embedded 3D non-metallic conductive electrodes from laser-treated GO-PDMS thin film matrix with an electrical conductivity of 10 −3 S m −1 .The electrodes withstand up to 100% strain with a 30% variation in con-ductivity due to the mechanical tuning ability.Further, using mechanical tunability, we have demonstrated a standalone varifocal embedded graphitized lens with broadband focusing capability in the visible region with a tuneable focal length of up to 0.3 mm under a strain application of up to 21% using the detour phase method.Moreover, the imaging measurements concluded that the mechanical tuning does not introduce aberration and distortion, [97] which avoids the post-processing or correction device requirement.These miniaturized technologies allow direct interfacing to low-dimensional devices and prevent the requirement of metallic connectors.Further, these technologies can generate implantable devices and soft robotics without requiring additional encapsulation methods; thus, more durability can be ensured.The method provides a novel fabrication method for biocompatible, custom-built, personalized wearable, and implantable healthcare monitors and communication devices.Further, the process can find applications in renewable energy technologies such as solar cells and storage.

Experimental Section
Thin-Film Synthesis: Graphene oxide (GO) powder (4-10% edgeoxidized) was purchased from Sigma-Aldrich.Sylgard 184A (base) and Sylgard 184B (curer), the two parts manufactured by Dow Corning, were mixed in the ratio of 10:1 weight ratio and stirred vigorously in a mixer under vacuum and separated into a 10 mL quantities in individual beakers.Different GO concentrations varying from 0.05% to 0.45% were added to separate PDMS beakers.The air bubbles trapped in the mixture were removed by applying a gentle vacuum in a desiccator for 30 min.Next, the mixture was drop-casted on a (100) silicon wafer to obtain a 3 mm thick elastomer film.Then, it was cured at 60 °C for 3 h in a vacuum oven to form the cross-linked PDMS network and to remove any solvents.
4.0.0.1.Structure Fabrication: The 3D laser (Coherent) galvo-dithering fabrication method was implemented based on computer-based designs using a wavelength of 532 nm, a pulse width of 55 fs, and a repetition rate of 70 MHz using 100× oil objective at different focal planes from 100 μm to a depth of 200 μm for a volume of 20 × 10 × 10 μm3 for electrodes inside the GO-PDMS thin films.For the fabrication of optical lenses, a radius of 470 μm, with a thickness of 5 μm inside the GO-PDMS thin films, was considered.
Optical Characterization: UV-vis absorbance profiles of GO-PDMS thin film were measured by CRAIC 20/30 XL UV-vis microspectrophotometer.Optical imaging of the laser-treated electrodes was conducted using a custom-made 4f imaging system using a 100× oil objective with a LED light source (Thorlabs) of 632 nm.The Raman spectra were obtained using a Horiba LabRAM Evolution micro-Raman system with 9 mW, 532 nm laser (0.5 μm lateral resolution, 0.25 s exposure), and a 50× objective.
Electrical Characterization: Current-voltage (I-V) characteristics of these laser-treated 3D electrode devices were carried out in an ambient atmosphere using an Agilent B2912A source meter semiconductor characterization system for two-probe measurements using a Signatone probe station with an applied voltage between −2 and 2 V.The stretching and relaxing measurements were carried out using a custom-made setup with an elongation distance up to 50 mm.
Refractive Index Modulation Simulation: Simulations were performed using the Beam Envelope method by applying a Gaussian laser beam intensity variation across the z-direction.A perfect matched layer (PML) boundary conditions were applied to the top and bottom of the cylinder to account for the electric field distribution of the incident Gaussian beam.In addition to the PML, the cylinder's periphery was considered a perfect electric conductor.
Embedded Graphitized Lens Characterization: The imaging and focusing performance of the obtained lens was characterized by a custom-built setup with a broadband LED (Thorlabs) with illumination wavelengths from 450 to 750 nm (450, 532, 632, and 750 nm) obtained using a 30 nm optical bandpass filter as the light source and a CCD camera (Basler Ace) for the imaging using an objective of NA = 1.4.The object for imaging was mounted on a 1D piezo scanning stage (Thorlabs) and placed in front of an embedded graphitized lens.

Figure 1 .
Figure 1.Creation of embedded 3D electronic and optical structures.a) The fabrication process for forming embedded 3D graphene structures within GO-PDMS thin films.b) Photographs of PDMS doped with increasing GO concentration show a loss of transparency.c) Transmission spectra corresponding to (b) verifying transmission loss beyond the visible region to near-infrared.

Figure 2 .
Figure 2. Raman studies laser patterned GO-PDMS thin film at different depths within the thin film.a) Raman spectra of graphene 3D electrodes fabricated using a laser beam fluence of 0.13 J cm −2 at different depths from 100 to 200 μm in GO-PDMS thin film.b) Raman I G /I D ratio of graphene features in GO-PDMS thin film using laser beam fluence of 0.13 J cm −2 at different depths within the thin film.

Figure 4 .
Figure 4. Electrical Characterization of embedded 3D electrodes.a) Image of electrical measurement studies of 3D graphene electrodes in GO-PDMS thin film.b) Electrical conductivity obtained for different average fs laser beam fluences for 3D electrodes fabricated to varying depths within the GO-PDMS thin film.

Figure 5 .
Figure5.Mechanical studies on laser patterned GO-PDMS thin film using stretching in the lateral direction.a) Relative change in resistance-strain percentage graph for 3D electrodes (design 3) fabricated at a depth of 100 μm within the GO-PDMS thin film after ten repetitions.b) After ten repeated measurements, the electrical conductivity-strain percentage for 3D electrodes was fabricated at a depth of 100 μm within the GO-PDMS thin film.

Figure 6 .
Figure 6.Demonstration of the optical application using the embedded graphistised structures.a) The varifocal concept using the embedded graphitized lens.b) Refractive index modulation along the direction of laser beam interaction in GO-PDMS film.c) Electric field distribution along the direction of laser beam incident within the GO-PDMS thin film.

Figure 7 .
Figure 7. Embedded graphitized 3D laser patterned lens.a) Schematic of Fresnel lens plate fabrication using 3D laser printing of embedded graphitized structures.b) The fabricated GO-PDMS thin film embedded lens of radius 470 μm, with concentric structures visible but with limited clarity due to the depth of structures below the PDMS surface.

Figure 8 .
Figure 8. Characterization of the embedded lens.a) Images were obtained for different stretching ratios and strain percentages at different incident wavelengths.b) The calculated (dashed lines) and experimentally observed (data points) focal length shifts for different incident wavelengths for different stretching ratios.