A critical challenge in bioelectronics is creating effective interfaces between electronic devices and living tissues for direct biological system assessment. Plant tissue-integrated sensors offer valuable data on plant health and the surrounding environment. However, fibrotic tissue formation hinders long-term sensor operation. This research explores the use of biocompatible materials and fabrication techniques to address this challenge. Previous work using substrates like PET or PEN resulted in plant tissue necrosis, limiting long-term monitoring. Soft electronics, particularly cryogels (hydrogels formed by freeze-thaw cycling), offer a promising alternative. Cryogels possess tunable mechanical properties and biocompatibility, making them suitable for tissue engineering and drug delivery. Polyvinyl alcohol (PVA) cryogels, in particular, are cost-effective, biodegradable, and biocompatible. Incorporating electroactive materials, such as PEDOT:PSS, into hydrogels presents challenges in balancing conductivity and mechanical properties. PEDOT:PSS, known for its biocompatibility, chemical stability, and aqueous processability, is a suitable candidate. While previous studies explored self-adhesive conductive polymer composites and self-healing conducting polymers, challenges remain in long-term in-vivo implantation due to substrate-induced tissue formation. This study aims to develop implantable printed electronics using PEDOT:PSS traces in PVA cryogel matrices to create robust, biocompatible, and long-lasting devices for plant tissue monitoring.

The development of electronics that integrate with living tissue has advanced significantly, with applications in e-skin, tattoo-like sensors, and implantable transient electronics for health monitoring and treatment. Similar devices have shown promise in monitoring plant physiological parameters, offering benefits to agriculture and environmental monitoring. Organic electrochemical transistors (OECTs), organic ion pumps, and bioristors are used to interface plant tissues with electronics. However, conventional substrates like PET, PEN, or yarns can cause plant tissue necrosis and limit long-term monitoring. Soft electronics, including cryogels, are emerging as a viable alternative. Cryogels' tunable mechanical properties and biocompatibility make them attractive for tissue engineering and drug delivery. PVA, a cost-effective, biodegradable, and biocompatible polymer, is frequently used to create cryogels. Previous research demonstrated methods for incorporating electroactive materials like PEDOT:PSS into hydrogels, but balancing conductivity and mechanical properties remains a challenge. Studies have also explored self-adhesive conductive polymer composites and self-healing conducting polymers for applications in electromyography and other biomedical devices, but long-term in-vivo implantation in plants still faces significant hurdles due to the potential for substrate-induced tissue reactions. This work builds upon these previous efforts to create a more robust and biocompatible system for long-term in-vivo plant monitoring.

The fabrication process begins with inkjet printing of PEDOT:PSS traces onto a glass slide. A PVA hydrogel layer is then drop-cast onto the printed trace and air-dried. The resulting structure is then peeled from the glass slide. A second layer of wet PVA hydrogel is drop-cast onto the sensing area, and the sample undergoes freeze-thaw cycles to create a physically cross-linked cryogel. Different device functionalities, including OECTs, capacitors, and circuits, were evaluated. Inkjet printing allows for customization of device patterns. The mechanical properties of the cryogels were evaluated using tensile testing, varying PVA molecular weight and concentration to tune Young's modulus and stretchability. Electrical resistance was examined under different strain conditions, with serpentine traces used to improve resistance to mechanical deformation. Capacitors were characterized using cyclic voltammetry, assessing specific capacitance and stability under strain. Self-healing properties were assessed by cutting a cryogel and evaluating its resistance and mechanical properties over time. For biocompatibility studies, cryogels were implanted into tomato plant stems, and cross-sections of the stems were imaged via optical microscopy and X-ray computed tomography over 120 days. Impedance spectroscopy was used to monitor ion transport in the plant stems, evaluating responses to changes in nutrient concentration and drought conditions. OECTs were also implanted to monitor ion movement in real-time. The characterization involved techniques such as SEM, impedance spectroscopy, cyclic voltammetry, and tensile testing. Materials used included PEDOT:PSS (Clevios™ F.H.C. ink from Heraeus), PVA (Sigma Aldrich), and other chemicals. A Dimatix DMP-2800 inkjet printer and an MTS Insight 2 electromechanical testing machine were utilized.

The inkjet-printed PVA cryogels demonstrated high electrical conductivity (up to 350 S/cm), a transconductance in the milliSiemens range for OECTs, and a capacitance of up to 4.2 mF/g. The cryogels exhibited high stretchability (up to 330% strain) and self-healing properties. Implants in tomato plant stems showed minimal scar tissue formation over two months, enabling long-term monitoring. Impedance spectroscopy revealed changes in stem impedance correlated with nutrient uptake (KCl) and drought conditions. OECTs successfully monitored ion transport in real-time, demonstrating responsiveness to KCl application and a delayed response to drought stress. The mechanical properties of the cryogels were tunable by adjusting PVA molecular weight and concentration, affecting Young's modulus and elongation. The use of serpentine electrode patterns improved electrical stability under mechanical stress. Capacitors displayed capacitive behavior and maintained 60% of their initial capacitance under 100% strain. The self-healing properties allowed for partial recovery of electrical resistance after cutting and rejoining the cryogel. Histological analysis showed limited lignification (scar tissue formation) compared to control cuts.

The findings address the research question by demonstrating the feasibility of long-term, in-vivo plant monitoring using a novel biocompatible and mechanically robust sensor platform. The self-healing and stretchable nature of the electronic cryogels mitigates the limitations of previous approaches that suffered from plant tissue damage and short operational lifespans. The ability to monitor ion transport using both impedance spectroscopy and OECTs provides a comprehensive understanding of plant physiological responses to environmental stimuli. The tunability of the cryogels' mechanical properties allows for optimization based on the specific plant tissue and application. The minimal immune response in the tomato plant stems suggests broad applicability across various plant species. The results are highly relevant to the field of plant bioelectronics and precision agriculture, opening avenues for real-time monitoring of plant health and environmental conditions.

This research successfully developed self-healable and stretchable printed electronic cryogels for in-vivo plant monitoring. The cryogels offer superior long-term stability and biocompatibility compared to previous approaches. Successful in-vivo testing demonstrated the potential for monitoring plant responses to nutrient availability and water stress. Future research could explore the application of these cryogels to other plant species and environmental factors, optimizing the design for specific applications and integrating more advanced signal processing capabilities.

The study focused on tomato plants, limiting the generalizability of the findings to other plant species. The long-term effects of the cryogel implants on plant health warrant further investigation. The self-healing capabilities, while promising, may not fully restore functionality after severe damage. While the design was shown to be robust, the impact of extremely harsh environmental conditions (e.g., extreme temperatures, high salinity) is not fully investigated.