Rapid Stencil Mask Fabrication Enabled One-Step Polymer-Free Graphene Patterning and Direct Transfer for Flexible Graphene Devices

Fabrication of high-resolution, flexible, laser-induced graphene sensors via stencil masking

The advent of wearable sensing platforms capable of continuously monitoring physiological parameters indicative of health status have resulted in a paradigm shift for clinical medicine. The accessibility and adaptability of such portable, unobtrusive devices enables proactive, personalized care based on real-time physiological insights. While wearable sensing platforms exhibit powerful capabilities for continuously monitoring physiological parameters, device fabrication often requires specialized facilities and technical expertise, restricting deployment opportunities and innovation potential. The recent emergence of rapid prototyping approaches to sensor fabrication, such as laser-induced graphene (LIG), provides a pathway for circumventing these barriers through low-cost, scalable fabrication. However, inherent limitations in laser processing restrict the spatial resolution of LIG-based flexible electronic devices to the minimum laser spot size. For a CO2 laser-a commonly reported laser for device production-this corresponds to a feature size of ∼120 μm. Here, we demonstrate a facile, low-cost stencil-masking technique to reduce the minimum resolvable feature size of a LIG-based device from 120 ± 20 μm to 45 ± 3 μm when fabricated by CO2 laser. Characterization of device performance reveals this stencil-masked LIG (s-LIG) method yields a concomitant improvement in electrical properties, which we hypothesize is the result of changes in macrostructure of the patterned LIG. We showcase the performance of this fabrication method via production of common sensors including temperature and multi-electrode electrochemical sensors. We fabricate fine-line microarray electrodes not typically achievable via native CO2 laser processing, demonstrating the potential of the expanded design capabilities. Comparing microarray sensors made with and without the stencil to traditional macro LIG electrodes reveals the s-LIG sensors have significantly reduced capacitance for similar electroactive surface areas. Beyond improving sensor performance, the increased resolution enabled by this metal stencil technique expands capabilities for scalable fabrication of high-performance wearable sensors in low-resource settings without reliance on traditional fabrication pathways.

Fully desktop fabricated flexible graphene electrocorticography (ECoG) arrays

Objective:Flexible Electrocorticography (ECoG) electrode arrays that conform to the cortical surface and record surface field potentials from multiple brain regions provide unique insights into how computations occurring in distributed brain regions mediate behavior. Specialized microfabrication methods are required to produce flexible ECoG devices with high-density electrode arrays. However, these fabrication methods are challenging for scientists without access to cleanroom fabrication equipment.Results:Here we present a fully desktop fabricated flexible graphene ECoG array. First, we synthesized a stable, conductive ink via liquid exfoliation of Graphene in Cyrene. Next, we established a stencil-printing process for patterning the graphene ink via laser-cut stencils on flexible polyimide substrates. Benchtop tests indicate that the graphene electrodes have good conductivity of ∼1.1 × 10³S cm^-1, flexibility to maintain their electrical connection under static bending, and electrochemical stability in a 15 d accelerated corrosion test. Chronically implanted graphene ECoG devices remain fully functional for up to 180 d, with averagein vivoimpedances of 24.72 ± 95.23 kΩ at 1 kHz. The ECoG device can measure spontaneous surface field potentials from mice under awake and anesthetized states and sensory stimulus-evoked responses.Significance:The stencil-printing fabrication process can be used to create Graphene ECoG devices with customized electrode layouts within 24 h using commonly available laboratory equipment.

Plotter Cut Stencil Masks for the Deposition of Organic and Inorganic Materials and a New Rapid, Cost Effective Technique for Antimicrobial Evaluations

Plotter cutters in stencil mask prototyping are underutilized but have several advantages over traditional MEMS techniques. In this paper we investigate the use of a conventional plotter cutter as a highly effective benchtop tool for the rapid prototyping of stencil masks in the sub-250 μm range and characterize patterned layers of organic/inorganic materials. Furthermore, we show a new diagnostic monitoring application for use in healthcare, and a potential replacement of the Standard Kirby-Bauer Diffusion Antibiotic Resistance tests was developed and tested on both Escherichia coli and Xanthomonas alfalfae as pathogens with Oxytetracycline, Streptomycin and Kanamycin. We show that the reduction in area required for the minimum inhibitory concentration tests; allow for three times the number of tests to be performed within the same nutrient agar Petri dish, demonstrated both theoretically and experimentally resulting in correlations of R ≈ 0.96 and 0.985, respectively for both pathogens.

Physical and Chemical Sensors on the Basis of Laser-Induced Graphene: Mechanisms, Applications, and Perspectives

Laser-induced graphene (LIG) is produced rapidly by directly irradiating carbonaceous precursors, and it naturally exhibits as a three-dimensional porous structure. Due to advantages such as simple preparation, time-saving, environmental friendliness, low cost, and expanding categories of raw materials, LIG and its derivatives have achieved broad applications in sensors. This has been witnessed in various fields such as wearable devices, disease diagnosis, intelligent robots, and pollution detection. However, despite LIG sensors having demonstrated an excellent capability to monitor physical and chemical parameters, the systematic review of synthesis, sensing mechanisms, and applications of them combined with comparison against other preparation approaches of graphene is still lacking. Here, graphene-based sensors for physical, biological, and chemical detection are reviewed first, followed by the introduction of general preparation methods for the laser-induced method to yield graphene. The preparation and advantages of LIG, sensing mechanisms, and the properties of different types of emerging LIG-based sensors are comprehensively reviewed. Finally, possible solutions to the problems and challenges of preparing LIG and LIG-based sensors are proposed. This review may serve as a detailed reference to guide the development of LIG-based sensors that possess properties for future smart sensors in health care, environmental protection, and industrial production.

Graphene Confers Ultralow Friction on Nanogear Cogs

Friction-induced energy dissipation impedes the performance of nanomechanical devices. Nevertheless, the application of graphene is known to modulate frictional dissipation by inducing local strain. This work reports on the nanomechanics of graphene conformed on different textured silicon surfaces that mimic the cogs of a nanoscale gear. The variation in the pitch lengths regulates the strain induced in capped graphene revealed by scanning probe techniques, Raman spectroscopy, and molecular dynamics simulation. The atomistic visualization elucidates asymmetric straining of CC bonds over the corrugated architecture resulting in distinct friction dissipation with respect to the groove axis. Experimental results are reported for strain-dependent solid lubrication which can be regulated by the corrugation and leads to ultralow frictional forces. The results are applicable for graphene covered corrugated structures with movable components such as nanoelectromechanical systems, nanoscale gears, and robotics.

Patterning Graphene Films by H2O-Based Magnetic-Assisted UV Photolysis

Properly cutting graphene into certain high-quality micro-/nanoscale structures in a cost-effective way has a critical role. Here, we report a novel approach to pattern graphene films by H₂O-based magnetic-assisted ultraviolet (UV) photolysis under irradiation at 184.9 nm. By virtue of the paramagnetic characteristic, the photo-dissociated hydroxyl [OH(X²Π)] radicals are magnetized and have their oxidation capability highly enhanced through converting into an accelerated directional motion. Meanwhile, the precursor of H₂O(X̃¹A₁) molecules distributes uniformly thanks to its weak diamagnetic characteristic, and there exists no instable diamagnetic intermediate to cause lateral oxidation. Possessing these unique traits, the H₂O-based magnetic-assisted UV photolysis has the capability of making graphene microscale patterns with the linewidth down to 8.5 μm under a copper grid shadow mask. Furthermore, it is feasible to pattern graphene films into 40 nm-wide ribbons under ZnO nanowires and realize hybrid graphene/ZnO nanoribbon field-effect transistors with a hole mobility up to 7200 cm²·V^-1·s^-1. The X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry analyses reveal that OH(X²Π) radicals act as a strong oxidant and that another product of H(1²S) adsorbs weakly on graphene.

Tunable Wettability of Graphene through Nondestructive Hydrogenation and Wettability-Based Patterning for Bioapplications

The wettability of graphene has been extensively studied and successfully modified by chemical functionalization. Nevertheless, the unavoidable introduction of undesired defects and the absence of systematic and local control over wettability by previous methods have limited the use of graphene in applications. In addition, microscale patterning, according to wettability, has not been attempted. Here, we demonstrate that the wettability of graphene can be systematically controlled and surface patterned into microscale sections based on wettability without creating significant defects, possible by nondestructive hydrogen plasma. Hydrophobic graphene is progressively converted to hydrophilic hydrogenated graphene (H-Gr) that reaches superhydrophilicity. The great contrast in wettability between graphene and H-Gr makes it possible to selectively position and isolate human breast cancer cells on arrays of micropatterns since strong hydrophilicity facilitates the adsorption of the cells. We believe that our method will provide an essential technique for enabling surface and biological applications requiring microscale patterns with different wettability.

Fabrication of Two-Dimensional and Three-Dimensional High-Resolution Binder-Free Graphene Circuits Using a Microfluidic Approach for Sensor Applications

In this study, a simple microfluidic method, which can be universally applied to different rigid or flexible substrates, was developed to fabricate high-resolution, conductive, two-dimensional and three-dimensional microstructured graphene-based electronic circuits. The method involves controlled and selective filling of microchannels on substrate surfaces with a conductive binder-free graphene nanoplatelet (GNP) solution. The ethanol-thermal reaction of GNP solution at low temperatures (∼75 °C) prior to microchannel filling (preheating) can further reduce the GNP andprovide a homogeneous GNP solution, which in turn enhances conductivity, reduces sheet resistance (∼0.05 kΩ sq^-1), enables room-temperature fabrication, and eliminates harsh postprocessing, which makes this fabrication technique compatible with degradable substrates. This method can also be used in combination with 3D printing to fabricate 3D circuits. The feature sizes of the graphene patterns can range from a few micrometers (down to ∼15 μm in width and ∼5 μm in depth) to a few millimeters and use very small amounts of GNP solution (∼2.5 mg of graphene to obtain ∼0.1 kΩ sq^-1 of sheet resistance for 1 cm²). This microfluidic approach can also be implemented using other conductive liquids, such as conductive graphene-silver solutions. This technology has the potential to pave the way for low-cost, disposable, and biodegradable circuits for a range of electronic applications including near-field communication antennas and pressure or strain sensors.

Fabrication of High-resolution Graphene-based Flexible Electronics via Polymer Casting

In this study, a novel method based on the transfer of graphene patterns from a rigid or flexible substrate onto a polymeric film surface via solvent casting was developed. The method involves the creation of predetermined graphene patterns on the substrate, casting a polymer solution, and directly transferring the graphene patterns from the substrate to the surface of the target polymer film via a peeling-off method. The feature sizes of the graphene patterns on the final film can vary from a few micrometers (as low as 5 µm) to few millimeters range. This process, applied at room temperature, eliminates the need for harsh post-processing techniques and enables creation of conductive graphene circuits (sheet resistance: ~0.2 kΩ/sq) with high stability (stable after 100 bending and 24 h washing cycles) on various polymeric flexible substrates. Moreover, this approach allows precise control of the substrate properties such as composition, biodegradability, 3D microstructure, pore size, porosity and mechanical properties using different film formation techniques. This approach can also be used to fabricate flexible biointerfaces to control stem cell behavior, such as differentiation and alignment. Overall, this promising approach provides a facile and low-cost method for the fabrication of flexible and stretchable electronic circuits.

Patterning Graphene Film by Magnetic-assisted UV Ozonation

Developing an alternative method for fabricating microscale graphene patterns that overcomes the obstacles of organic contamination, linewidth resolution, and substrate damaging is paramount for applications in optoelectronics. Here we propose to pattern chemical vapor deposition grown graphene film through a stencil mask by magnetic-assisted ultraviolet (UV) ozonation under irradiation of a xenon excimer lamp. In this process, the paramagnetic oxygen molecules and photochemically generated oxygen radicals are magnetized and attracted in an inhomogenous external magnetic field. As a consequence, their random motions convert into directional, which can greatly modify or enhance the quality of graphene patterns. Using a ferromagnetic steel mask, an approximately vertical magnetic-field-assisted UV ozonation (B_Z = 0.31 T, ∇B_Z = 90 T · m⁻¹) has a capability of patterning graphene microstructures with a line width of 29 μm and lateral under-oxidation less than 4 μm. Our approach is applicable to patterning graphene field-effect transistor arrays, and it can be a promising solution toward resist-free, substrate non-damaging, and cost effective microscale patterning of graphene film.

A stretchable crumpled graphene photodetector with plasmonically enhanced photoresponsivity

Graphene has been widely explored for flexible, high-performance photodetectors due to its exceptional mechanical strength, broadband absorption, and high carrier mobility. However, the low stretchability and limited photoabsorption of graphene have restricted its applications in flexible and highly sensitive photodetection systems. Various hybrid systems based on photonic or plasmonic nanostructures have been introduced to improve the limited photoresponsivity of graphene photodetectors. In most cases, the hybrid systems succeeded in the enhancement of photoresponsivity, but showed limited mechanical stretchability. Here, we demonstrate a stretchable photodetector based on a crumpled graphene-gold nanoparticle (AuNP) hybrid structure with ∼1200% enhanced photoresponsivity, compared to a conventional flat graphene-only photodetector, and exceptional mechanical stretchability up to a 200% tensile strain. We achieve plasmonically enhanced photoresponsivity by integrating AuNPs with graphene. By crumpling the hybrid structure, we realize mechanical stretchability and further enhancement of the optical absorption by densification. We also demonstrate that our highly stretchable photodetector with enhanced photoresponsivity can be integrated on a contact lens and a spring structure. We believe that our stretchable, high performance graphene photodetector can find broad applications for conformable and flexible optical sensors and dynamic mechanical strain sensors.

Skin-mountable stretch sensor for wearable health monitoring

This work presents a wrinkled Platinum (wPt) strain sensor with tunable strain sensitivity for applications in wearable health monitoring. These stretchable sensors show a dynamic range of up to 185% strain and gauge factor (GF) of 42. This is believed to be the highest reported GF of any metal thin film strain sensor over a physiologically relevant dynamic range to date. Importantly, sensitivity and dynamic range are tunable to the application by adjusting wPt film thickness. Performance is reliable over 1000 cycles with low hysteresis after sensor conditioning. The possibility of using such a sensor for real-time respiratory monitoring by measuring chest wall displacement and correlating with lung volume is demonstrated.

Ultrasensitive Strain Sensor Produced by Direct Patterning of Liquid Crystals of Graphene Oxide on a Flexible Substrate

Ultrasensitive flexible strain sensors were developed through the combination of shear alignment of a high concentration graphene oxide (GO) dispersion with fast and precise patterning of multiple rectangular features on a flexible substrate. Resistive changes in the reduced GO films were investigated under various uniaxial strain cycles ranging from 0.025 to 2%, controlled with a motorized nanopositioning stage. The devices uniquely combine a very small detection limit (0.025%) and a high gauge factor with a rapid fabrication process conducive to batch production.