Zinc hybrid sintering for printed transient sensors and wireless electronics

Transient electronics, which completely degrade without harmful byproducts, offer a solution to electronic waste and enable novel bioresorbable implants. While various biodegradable materials have been proposed for these applications, fabrication remains challenging. Microfabrication techniques often employed are expensive and incompatible with temperature- and solvent-sensitive biodegradable substrates. Additive manufacturing presents a promising alternative, offering large-area, cost-effective, and low-waste production with the advantage of freeform printing on 3D surfaces and seamless integration of multi-sensing paradigms. However, additive manufacturing of transient electronics requires optimization of printable ink, deposition processes, and post-treatment methods, particularly sintering to enhance conductivity. Existing methods, such as photonic (high-power lasers or lamps) and electrochemical (acetic acid) sintering, each have limitations. Photonic sintering is often hindered by the zinc oxide layer, leading to limited conductivity. Electrochemical sintering achieves lower conductivity due to minimal particle bridging and residual binder. This work aims to overcome these limitations by introducing a hybrid sintering method combining chemical and photonic approaches to achieve highly conductive, robust, and biocompatible transient zinc traces suitable for complex devices.

The existing literature demonstrates various approaches to creating transient electronics, focusing on biodegradable metals, semiconductors, substrates, and dielectrics. Researchers have successfully developed biodegradable batteries, heaters, transistors, energy harvesters, and various sensors. However, most rely on microfabrication techniques that are costly and struggle with patterning functional layers directly onto sensitive substrates. Additive manufacturing offers an alternative, but requires optimized ink formulation, deposition, and post-treatment. Previous efforts to sinter zinc, a cost-effective and biodegradable metal, have employed either photonic or electrochemical methods. Photonic approaches face challenges due to the high melting point of the native zinc oxide layer, limiting the achieved conductivity. Electrochemical methods, while operating at room temperature, typically yield lower conductivity values due to limited particle bridging and residual binder. The need for a method that overcomes these limitations and enables the creation of complex, reliable transient devices on sensitive substrates is a significant gap addressed by this research.

The study utilized commercially available zinc microparticles (2 µm average diameter) mixed with polyvinylpyrrolidone (PVP) binder and pentanol solvent to create a printable ink. A two-step hybrid sintering process was employed: (1) electrochemical treatment via acetic acid spray coating to reduce the oxide layer, and (2) photonic sintering using flash lamp annealing to agglomerate particles. The process parameters were optimized. For electrochemical treatment, spray coating was chosen over drop coating to avoid partial dissolution of zinc patterns. Rapid drying was crucial to prevent damage from prolonged acid exposure. Multiple spray coating/drying cycles were performed to reach plateau conductivity. The PVP binder concentration was optimized to achieve the best balance between printability and conductivity. Photonic sintering parameters, including pulse energy and number of pulses, were also optimized under a nitrogen atmosphere to minimize re-oxidation. The timing between acid treatment and photonic sintering was critical to prevent oxide layer reformation. The method was tested on various substrates (polyimide, paper, PLA, PVA) and the conductivity, flexibility, and stability of the resulting zinc traces were characterized. The methodology also included the fabrication and testing of resistive strain and temperature sensors, and capacitive wireless force and pressure sensors. The fabrication of a wirelessly powered LED circuit was included to demonstrate compatibility with multilayer devices and further processing steps. Detailed characterization techniques included laser scanning confocal microscopy, SEM, four-wire resistance measurements, bending tests, adhesion tests, degradation tests in air and PBS, and capacitance and resonant frequency measurements for the sensors and circuit.

The hybrid sintering method achieved significantly higher conductivity in printed zinc layers (up to 5.62 × 10⁶ S m⁻¹) compared to existing methods. The conductivity was comparable to heat-cured copper or silver inks. The process was compatible with various substrates, including temperature- and solvent-sensitive biodegradable materials such as PLA and PVA. The resulting zinc traces exhibited good flexibility, bending to a 3 mm radius without significant conductivity loss. Encapsulation with PLA improved durability significantly. The hybrid sintering enabled the fabrication of functional transient devices: Resistive strain sensors exhibited resistance changes proportional to applied strain and returned to baseline values upon release. Resistive temperature sensors showed a linear relationship between resistance and temperature, with a TCR close to that of bulk zinc. Capacitive pressure sensors, utilizing a biodegradable elastomer (POMaC) as a dielectric, exhibited linear response to applied force. A wireless capacitive pressure sensor, leveraging a series RLC circuit, demonstrated successful force sensing with a shift in resonant frequency. A fully printed biodegradable wireless power receiver, capable of powering an LED, was also successfully demonstrated.

The hybrid sintering method presented successfully addresses the challenges of creating highly conductive, robust, and biodegradable transient electronics. The synergistic combination of electrochemical and photonic sintering overcomes the limitations of individual approaches. The high conductivity achieved is a major advancement, enabling the fabrication of more complex and functional devices. The compatibility with various substrates, particularly biodegradable materials, expands the range of applications. The demonstrated stability and flexibility of the zinc traces are critical for practical use in flexible sensors and implants. The successful fabrication of several functional sensor devices highlights the potential of this method for creating eco-friendly, bioresorbable electronics suitable for a wide range of applications, including environmental monitoring, smart packaging, and implantable biomedical devices. The results showcase the potential for additive manufacturing in creating customized, biocompatible, and environmentally friendly electronic devices.

This research successfully demonstrates a novel hybrid sintering method for creating highly conductive, flexible, and biodegradable zinc traces for transient electronics. The method addresses existing limitations of photonic and electrochemical sintering alone, achieving superior conductivity and compatibility with sensitive substrates. The fabrication of several functional devices, including resistive and capacitive sensors and a wireless power receiver, validates the practicality and versatility of the approach. Future work could focus on exploring the use of zinc nanoparticles for further miniaturization and improved resolution, investigating alternative reducing agents to simplify the process, and conducting comprehensive *in vivo* studies to assess the biocompatibility and degradation profiles of the devices in realistic biological environments.

While the achieved conductivity is high, the variability in conductivity between samples could be further reduced. The study primarily focused on zinc microparticles; further investigation into the use of nanoparticles might lead to finer resolution and different properties. The long-term performance of the capacitive pressure sensor remains to be fully assessed, particularly concerning the influence of POMaC degradation. Although a biodegradable wireless power receiver was successfully demonstrated, the range of the power transfer is limited and could be improved.