Abstract
The conversion of various polymer substrates into laser-induced graphene (LIG) with a CO2 laser in ambient condition is recently emerging as a simple method for obtaining patterned porous graphene conductors, with a myriad of applications in sensing, actuation, and energy. In this paper, a method is presented for embedding porous LIG (LIG-P) or LIG fibers (LIG-F) into a thin (about 50 μm) and soft medical grade polyurethane (MPU) providing excellent conformal adhesion on skin, stretchability, and maximum breathability to boost the development of various unperceivable monitoring systems on skin. The effect of varying laser fluence and geometry of the laser scribing on the LIG micro–nanostructure morphology and on the electrical and electromechanical properties of LIG/MPU composites is investigated. A peculiar and distinct behavior is observed for either LIG-P or LIG-F. Excellent stretchability without permanent impairment of conductive properties is revealed up to 100% strain and retained after hundreds of cycles of stretching tests. A distinct piezoresistive behavior, with an average gauge factor of 40, opens the way to various potential strain/pressure sensing applications. A novel method based on laser scribing is then introduced for providing vertical interconnect access (VIA) into LIG/MPU conformable epidermal sensors. Such VIA enables stable connections to an external measurement device, as this represents a typical weakness of many epidermal devices so far. Three examples of minimally invasive LIG/MPU epidermal sensing proof of concepts are presented: as electrodes for electromyographic recording on limb and as piezoresistive sensors for touch and respiration detection on skin. Long-term wearability and functioning up to several days and under repeated stretching tests is demonstrated.
Highlights
The investigation of materials and processes for obtaining flexible and stretchable conductors has a long history, driven by both scientific curiosity and technological needs. It has been boosted by the requirements of novel applications in the fields of flexible/stretchable electronics, wearable sensors/devices, epidermal electronics, biointerfaces, soft robotics, prosthetics, actuators, and energy harvesting devices.[1−6] In principle, an “optimal” stretchable conductor would combine the mechanical properties of a typical elastomer material with the electrical properties of a purely ohmic conductor
medical grade polyurethane (MPU) has numerous features, making it an optimal substrate for epidermal electronics, which include, among others, reduced thickness, high transparency, stable long-term and conformal adhesion on skin, large stretchability, excellent gas permeability, impermeability to liquid water, off-the-shelf availability in large area format, and ease of manipulation/transfer on skin
The scribing of laserinduced graphene (LIG) conductive patterns was performed on top of PI sheets with a laser engraver/cutter equipped with a 30 W CO2 laser source operating in the rastering mode
Summary
The investigation of materials and processes for obtaining flexible and stretchable conductors has a long history, driven by both scientific curiosity and technological needs. It has been boosted by the requirements of novel applications in the fields of flexible/stretchable electronics, wearable sensors/devices, epidermal electronics, biointerfaces, soft robotics, prosthetics, actuators, and energy harvesting devices.[1−6] In principle, an “optimal” stretchable conductor would combine the mechanical properties of a typical elastomer material (such as silicone or natural rubber) with the electrical properties of a purely ohmic conductor (as a metal). A method is implemented for obtaining vertical interconnect access (VIA) into sensors, as desired for minimizing typical rupture problems of epidermal devices related to external wiring connectors
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