The reliability of soft and flexible implantable bioelectronics is crucial for developing functional devices with thin form factors and tissue adaptability. Chronic implants require protection from the body environment to prevent damage, short circuits, and performance degradation. Thin-film encapsulations (TFEs) using organic (polyimide, parylene C, liquid-crystal polymers) or inorganic films (aluminum oxide, silicon nitride, silicon dioxide, silicon carbide) or hybrid multilayers offer hermeticity, flexibility, and microfabrication compatibility. However, water permeation through TFEs remains a significant challenge, leading to short circuits, corrosion, and delamination. Water permeation is typically quantified using the Water Transmission Rate (WTR), but in-situ quantification on real bioelectronic implants is difficult. Traditional methods are bulky, expensive, and lack sensitivity. While a recent method using corroding magnesium (Mg) as a sensing element shows promise, its application to flexible substrates and real implants has not been demonstrated. This article presents a novel, highly sensitive wireless platform using radio-frequency backscatter to quantitatively monitor TFE WTR in situ and in real time, leveraging Mg thin film corrosion and frequency modulation to provide precise WTR measurements.

The literature extensively covers the need for reliable encapsulation of bioelectronic implants to ensure long-term functionality. Various TFE materials and fabrication techniques have been explored, each with its advantages and limitations concerning permeability. Existing methods for measuring water permeation, such as the calcium test and traditional permeation cells, suffer from limitations like bulkiness, high cost, and low sensitivity. The use of corroding Mg as a sensing element has been previously proposed, but its application to flexible substrates and in vivo conditions remained largely unexplored prior to this study. This study builds upon this prior work by integrating Mg-based sensing with a wireless backscatter communication system to overcome the limitations of existing methods.

This research developed a wireless, battery-free platform for real-time water permeation monitoring. The platform utilizes backscatter communication, where a tag (the implant) modulates an externally generated carrier wave by altering its impedance. The tag incorporates microfabricated Mg thin-film sensors. Water permeation causes Mg corrosion, changing the sensor's resistance, which in turn modulates the backscattered signal's frequency. The system uses a tunable backscatter circuit, a square-wave oscillator controlled by the Mg sensor's resistance, and a frequency-modulated backscatter signal. A flexible dipole antenna enables wireless data communication. The fabrication process for the Mg sensors involves sequential thin-film deposition, patterning, and etching. The system's calibration involves a parallel configuration for the set resistance, allowing precise control of the oscillator frequency. The WTR is extracted using an analytical model based on the observed frequency shift over time. The platform was tested in vitro with various encapsulations (PI, Parylene C, hybrid organic-inorganic multilayers) at different temperatures. An implantable version (i-WPS) was developed, incorporating wireless powering and tested ex vivo in chicken meat and in vivo-like settings using mouse cadavers. The study involved characterization techniques such as XRD, SEM, AFM, and EDX to analyze the Mg sensor morphology and corrosion process. The system's power consumption was measured, and the antenna's performance was evaluated through electromagnetic simulations and measurements. The study compared the WTR values obtained with different encapsulations and temperatures.

The study successfully demonstrated a novel wireless, battery-free platform for real-time WTR monitoring. The platform accurately measured WTR values for different encapsulations (PI, Parylene C, hybrid organic-inorganic multilayers), showing sensitivity to variations in barrier properties and temperature. In vitro testing showed that the WTR decreased over time due to the formation of a Mg(OH)2 barrier layer. The activation energy for water diffusion was determined for different encapsulations. Ex vivo testing in chicken meat showed similar WTR values to the in vitro tests, suggesting the ex vivo tissue is a more aggressive environment than PBS solution. In vivo-like tests using mouse cadavers demonstrated the platform's functionality in a realistic implantation scenario, achieving a WTR of approximately 2 × 10⁻⁶ g m⁻² day⁻¹. The system showed robustness to changes in the animal's body orientation and position. The platform design demonstrated scalability and adaptability, with miniaturization allowing for use with small animal models. The study also explored the influence of Mg sensor design parameters on the corrosion process, showing that sensor geometry does not affect the initial WTR but does influence the duration of corrosion.

The results demonstrate the feasibility of a wireless, battery-free, and real-time monitoring system for water permeation in TFE. The system addresses limitations of previous techniques by providing in-situ and continuous monitoring, overcoming the challenges associated with bulky equipment and low sensitivity. The successful in vivo-like demonstration highlights the system's potential for applications in bioelectronics research and quality control of implantable devices. The findings contribute significantly to advancing the long-term reliability of implantable bioelectronics by enabling a more accurate and practical assessment of TFE barrier performance. The platform's flexibility and miniaturization potential broaden its applicability to various implantable devices and animal models.

This study successfully demonstrated a novel wireless platform for real-time monitoring of water permeation through thin-film encapsulations. The platform combines Mg-based sensing with backscatter communication to achieve battery-free operation and in-situ measurements. The findings validate the platform's accuracy, robustness, and scalability, opening up new possibilities for advancing the reliability of implantable bioelectronics. Future research could focus on further miniaturization, integration with other functionalities, and full in vivo testing in freely moving animals. The development of a more sophisticated design with multiple Mg sensors could provide a map of encapsulation reliability, offering a comprehensive assessment of barrier integrity.

The study primarily focused on subcutaneous implantation, and further research is needed to investigate the system's performance for deeper implants where signal attenuation may be significant. The in vivo-like tests were performed on mouse cadavers, and further in vivo experiments with live animals are required to confirm the results' generalizability. While the system is robust to body movement, further optimization is needed for high-data-rate applications. The current analytical model could be refined to account for more complex diffusion phenomena.