The development of biocompatible and mechanically flexible electronic devices has opened new avenues in applications such as electronic skin and biomedicine. However, the field of transient electronics, which allows for complete biodegradation or dissolution after use, remains relatively unexplored. Nitric oxide (NO) is a crucial signaling molecule in the human body, playing a vital role in maintaining health, while its transformation into nitrogen dioxide (NO2) contributes to environmental pollution. Existing NO sensors often suffer from limitations including rigidity, low sensitivity, poor selectivity, high power consumption, and lack of biodegradability. This research addresses these limitations by presenting a flexible and bioresorbable single-crystal silicon nanomembrane (SC-Si NM)-based NOx sensing system operating at room temperature. The study focuses on detailed electrical response studies under various conditions, mechanical properties analysis with theoretical modeling, and in vitro assessments for potential use in disposable environmental monitors and bioresorbable medical implants.

Previous research on NO sensors has explored various high-performance channel materials such as graphene, carbon nanotubes, polymers, and metal oxides. Modifications to these structures have been attempted to increase the surface-to-volume ratio. However, challenges remain in areas such as rigidity, low sensitivity, poor selectivity, high power consumption, and the lack of biodegradability, which hinder their use in wearable and implantable electronic systems. This work aims to overcome these challenges.

The fabrication process began with thinning single-crystal silicon nanomembranes (SC-Si NMs) on silicon-on-insulator (SOI) wafers to ~100 nm thickness. Phosphorus doping created highly doped regions for electrical contacts. Undercut wet etching released the SC-Si NMs, enabling transfer printing onto a PMMA/diluted PI-coated temporary substrate. Reactive ion etching defined active areas, followed by deposition of interdigitated electrodes (IDEs) using magnesium (Mg). Wet etching and e-beam evaporation created patterned dielectric layers and contact pads. A mesh-type bridge geometry facilitated device lifting and transfer printing onto biodegradable substrates. For humid conditions, a polydimethylsiloxane (PDMS) semi-permeable membrane was added. Electrochemical properties were measured using an electrometer under a DC bias. Experiments were conducted in a temperature-adjustable chamber with controlled gas flow. Sensitivity was determined by measuring resistance changes upon exposure to target gases. In vitro evaluations used RAW 264.7 cells to assess biocompatibility and functionality in a wet environment. NO production was measured using a DAF-FM assay, and immunofluorescence microscopy visualized iNOS expression. Mechanical stability was assessed through bending and tensile tests, with results compared to finite element analysis simulations.

The developed biodegradable NOx sensor exhibited exceptional performance. It achieved a record-high sensitivity of 136 Rs (13,600%) for 5 ppm NO2 at room temperature, surpassing existing sensors based on carbon nanotubes, graphene, polymers, and metal oxides. The response and recovery times were fast (~30s and ~60s, respectively). A detection limit of ~20 ppb was estimated. The sensor demonstrated at least 100-fold selectivity for NO and NO2 over other gases. The sensitivity showed an inverse relationship with temperature, suggesting potential for applications at various temperatures. The integration of temperature and humidity sensors allowed for compensation of environmental effects. Large-scale (5x5) sensor arrays on a mechanically pliable configuration showed negligible performance degradation under bending and stretching (up to 40% strain), consistent with finite element modeling. In vitro studies confirmed the sensor's functionality and stability in a wet environment, suitable for biomedical applications. The bioresorbable materials (Mg electrodes, SC-Si NMs, SiO2, and PLGA substrate) completely dissolved in a PBS solution (pH 7.4) at 37°C via hydrolysis, producing non-toxic byproducts. The dissolution rate of Mg electrodes was faster than other components.

The results demonstrate the successful development of a high-performance, biodegradable, and flexible NOx sensor based on SC-Si NMs. The exceptional sensitivity, selectivity, and fast response times make it suitable for various applications including transient environmental monitoring and bioresorbable medical implants. The ability of the sensor array to withstand significant mechanical stress ensures its applicability in flexible and wearable devices. The biocompatibility and complete biodegradability address crucial limitations of existing technologies. The inverse temperature dependence of the sensitivity is an unexpected but potentially advantageous feature, offering possibilities for temperature-compensated sensing. The findings significantly advance the field of transient electronics and offer promising solutions for healthcare and environmental monitoring.

This study successfully demonstrated a high-performance, biodegradable, flexible NOx gas sensor based on SC-Si NMs. Its superior sensitivity, selectivity, biocompatibility, and mechanical flexibility offer significant advantages over existing technologies. Future research could focus on further miniaturization, integration with wireless communication systems for remote monitoring, and exploring diverse applications in personalized medicine and environmental monitoring.

While the sensor demonstrated excellent performance, further studies are needed to evaluate long-term stability in vivo. The impact of potential variations in body fluids on sensor performance should also be investigated. Although the materials are considered biodegradable, the long-term degradation products and their potential biological effects require further investigation.