Zinc hybrid sintering for printed transient sensors and wireless electronics

The study targets the challenge of fabricating high-conductivity, patterned, transient metal interconnects on temperature- and solvent-sensitive biodegradable substrates for eco-friendly and bioresorbable electronics. Transient electronics can reduce e-waste and enable implants that resorb after use, but the direct patterning and sintering of reactive biodegradable metals (notably Zn) on sensitive substrates is difficult due to oxide shells and thermal constraints. Prior additive methods often yield low conductivity or require high-energy photonic sintering limited by ZnO shells; electrochemical sintering alone offers modest conductivity. The research hypothesis is that a hybrid process—chemical reduction of the native ZnO shell via acetic acid followed by photonic flash-lamp sintering—will synergistically enable efficient particle agglomeration at reduced energy, yielding high conductivity and robust, durable, and flexible transient Zn traces compatible with degradable substrates. The work aims to demonstrate process scalability, substrate compatibility, mechanical durability, environmental stability, and device functionality (resistive and wireless capacitive/inductive transient sensors).

The introduction surveys transient electronic materials including dissolvable metals, degradable semiconductors, and polymers, and reports prior devices (batteries, heaters, transistors, energy harvesters, pressure/strain/temperature sensors). Conventional microfabrication, shadow masks, and transfer printing have been used due to substrate sensitivity. Additive manufacturing offers digital, large-area, low-waste fabrication but requires ink and post-treatment optimization. For Zn—the prevalent transient metal—two main post-treatments exist: (1) photonic (lasers/lamps) which selectively heat metal but are limited by high-melting ZnO shells that hinder agglomeration even at >10 J cm⁻²; (2) electrochemical sintering (e.g., acetic/propionic acid) that converts ZnO to Zn²⁺ and redeposits bridges at room temperature but typically yields 1–3 × 10⁵ S m⁻¹ due to minimal bridging and residual binders. Challenges include multi-step processing that can damage reactive Zn traces and achieving robust, stable devices. This work builds on these by combining oxide reduction with flash sintering to overcome individual limitations and improve conductivity and device robustness.

Ink formulation: Zn microparticles (2 µm avg. diameter) mixed with polyvinylpyrrolidone (PVP; Mw 360k or 2000k) and pentanol. Final weight ratio 25:1:5 (Zn:PVP:solvent). Mixed in a planetary mixer (300 rpm, 30 min), stored at 4 °C; re-homogenized before printing. Substrates: Polyimide (Kapton 125 µm), biodegradable paper (Powercoat XD 200 µm), PLA (Ingeo 4032D), and PVA. PLA films: 15 wt% PLA in 1,4-dioxane, blade cast (gap 1000 µm, 2 mm s⁻¹) to ~70 µm after drying. PVA films: 25 wt% in DI water, blade cast (gap 600 µm) to ~75 µm. Oxygen plasma activation (except paper): 60 s, 200 W, 40 kHz. Printing: Stencil printing via laser-cut polyethylene adhesive tape (80 µm) or screen printing with stainless-steel meshes; ink applied with silicone squeegee; dried 1 h. Hybrid sintering process: (1) Electrochemical step: spray-coat 10 vol% acetic acid in DI water using airbrush (0.2 mm nozzle), 2 bar N₂, ~10 cm distance; followed by 2 min N₂ drying. Repeat 5 cycles. In inert process runs, samples were placed in a N₂-purged chamber (45 s purge). This step reduces ZnO and forms nanoscale conductive bridges; careful, limited deposition and rapid drying prevent pattern dissolution. (2) Photonic sintering: Flash-lamp annealing (Novacentrix PulseForge 1200) with pulse energies 3107–12867 mJ cm⁻², delivering 1–6 pulses; some experiments conducted under N₂ to limit reoxidation. Optimization studies: Assessed effect of multiple acid/dry cycles (monitoring resistance in real time), PVP chain length and concentration (optimal ~0.04 g PVP per g Zn, with constant solvent:solids). Evaluated pulse energy and pulse count under N₂. Also tested timing between acid step and flash; >1 h delay in air reduced conductivity due to re-oxidation. Characterization: Thickness and morphology by laser scanning confocal microscopy; microstructure by SEM (including cross-sections after liquid N₂ fracture). Four-wire resistance using Kelvin probes (Keysight 34401A). Adhesion by ASTM F1842-15 peel test on polyimide, paper, PLA. Bending tests on stencil-printed and optionally PLA-encapsulated lines using custom setup (various radii, cyclic bending). Durability in air (ambient) and in PBS at 37 °C with/without ~30–35 µm PLA encapsulation; continuous resistance monitoring (Keysight 34970A). TCR measurements: hybrid-sintered screen-printed Zn resistors; temperature controlled in chamber or vial (with PBS) and measured via Sensirion SHT4x; resistance recorded with DMM; TCR extracted from linear fit. Device fabrication: Resistive strain and temperature sensors on PLA using hybrid-sintered Zn; encapsulated with blade-cast PLA when required. Capacitive pressure sensors: Printed Zn electrodes and pads on PLA, then blade-cast POMaC pre-polymer (photoinitiator 5 wt%, UV cure 20 min at 365 nm), fold PLA to form parallel-plate structure with POMaC as dielectric; post-cure at 80 °C for 48 h. POMaC synthesis: maleic anhydride:citric acid:1,8-octanediol (3:2:5) melt reacted, pre-polymer purified, mixed with photoinitiator. Mechanical properties: POMaC Young’s modulus ~1 ± 0.2 MPa; εr ~6.9 ± 0.4. Capacitive sensing tests with Instron load (up to 10 N), capacitance via LCR meter at 2 MHz; cycling tests (0–4 N, 40 s period, 2500 cycles). Wireless RLC sensors: series Zn coil with Zn-POMaC capacitor; S11 via NanoVNA V2 using a silver reader coil; resonant frequency tracked by custom Python algorithm. Wireless power receiver: double-sided Zn coil separated by POMaC, vias filled with silver epoxy; assembled and post-cured at 80 °C; LED and 470 pF capacitor connected using room-temperature curable Zn-based paste (Zn ink + 10 vol% acetic acid). Inductance of coil measured; powering distance assessed relative to a driven primary coil setup from prior work.

- Hybrid sintering (acetic acid spray + flash-lamp) yields printed Zn conductivities up to 5.62 × 10⁶ S m⁻¹ (5.62 × 10⁴ S cm⁻¹), approximately one-third of bulk Zn (16.6 × 10⁶ S m⁻¹), surpassing prior printed Zn reports. - Electrochemical spray treatment alone plateaus at ~1980 S cm⁻¹ after 5–6 spray/dry cycles, validating oxide reduction and establishing a foundation for photonic sintering. - Photonic parameters: Conductivity improved by 1–2 orders of magnitude vs. acid-only; optimal around 3 pulses at ~6559 mJ cm⁻² under N₂. Excessive energy (≥7299 mJ cm⁻²) caused damage and low yield (~25%). Conductivity gains plateau after 2–3 pulses at given energy. - Substrate compatibility: High conductivities achieved on degradable substrates—PVA: 2.61 × 10⁴ S cm⁻¹; PLA: 2.43 × 10⁴ S cm⁻¹; paper: 2.00 × 10⁴ S cm⁻¹—comparable to polyimide. Good adhesion on polyimide, paper, and PLA. - Thickness effects: Screen-printed (17 µm) lines exhibited >2× higher conductivity than thicker stencil-printed (41 µm) lines (2.55 × 10⁴ S cm⁻¹), consistent with stronger sintering in upper layers. Cross-sections show cohesive agglomeration to ~5–10 µm depth. - Atmosphere and timing: N₂ atmosphere improved conductivity (~50% higher) by limiting oxidation. Delays >1 h between acid and flash sintering in air reduced conductivity due to re-oxidation; residual acid presence favored stable performance. - Nanoparticle feasibility: Preliminary 500 nm Zn particles achieved 3.3 × 10⁶ S m⁻¹ at lower pulse energy (5.1 J cm⁻²), indicating compatibility with nanoparticle-based printing. - Durability in air: Hybrid-sintered Zn on PLA without encapsulation showed only ~1.4× resistance increase over 31 days at ambient. Acid-only traces degraded nonlinearly, reaching ~10× resistance in 300–500 h. With ~35 µm PLA encapsulation, average conductivity loss was ~12% over 31 days. - Durability in PBS (37 °C): Unencapsulated Zn degraded by ~10× in resistance within ~46.6 h on average. With ~30 µm PLA encapsulation, ~90% conductivity loss occurred in ~28 days, highlighting encapsulation as key to service life. Estimated full resorption of 40 µm Zn in PBS ~48 days at reported corrosion rate. - Flexibility: Hybrid-sintered Zn endured bending to 3 mm radius without conductivity loss. Under 1000 bending cycles at 0.5 Hz, 10 mm radius: encapsulated lines increased resistance by ~9%, while unencapsulated more than doubled, consistent with micro-crack formation. - Sensors: Strain sensor showed ~±0.2% reversible resistance changes under tensile/compressive bending. Temperature sensors displayed linear response with TCR = 0.00316 °C⁻¹, close to bulk Zn (0.00385 °C⁻¹) and platinum-based standards; stable operation in PBS around 37 °C. - Capacitive pressure sensors (Zn/POMaC/PLA): Initial C ~15.5 ± 3.4 pF; sensitivity 71.4 fF N⁻¹ (relative 0.00035 kPa⁻¹), linearity R² = 0.997 up to 10 N; robust cycling over 2500 cycles with minimal drift (max capacitance variation ~0.36%, baseline ~0.27%). - Wireless RLC sensing: Series Zn coil + Zn-POMaC capacitor yielded resonant frequency ~154.7 ± 3.3 MHz; frequency shift ~320 kHz N⁻¹ under compressive load, enabling chipless force/pressure sensing. - Wireless power receiver: Double-sided Zn coil (L ~3.2 µH) on degradable substrate powered an LED up to ~60 mm distance using a driven primary coil; assembly achieved using room-temperature curable Zn paste.

The hybrid sintering strategy directly addresses the central challenge of achieving high-conductivity, robust Zn interconnects on temperature- and water-sensitive biodegradable substrates. Chemical oxide reduction via acetic acid establishes conductive bridges and removes the barrier of the high-melting ZnO shell, enabling effective subsequent photonic sintering at reduced energy. The flash-lamp step promotes particle fusion into cohesive layers while minimizing thermal load on substrates, broadening compatibility to PLA, PVA, and paper. The approach yields record-high conductivities for printed transient Zn, translating into improved durability against re-oxidation in air, mechanically flexible behavior under bending, and stable temperature coefficients near bulk values. Device-level demonstrations—resistive strain and temperature sensors, soft capacitive pressure sensors, and wireless RLC circuits—validate that the enhanced electrical properties and processing robustness support multilayer assembly, UV-curing of elastomers, and integration of discrete components using a degradable Zn paste. In biomedical contexts, results in PBS confirm rapid unencapsulated degradation but show that thin PLA encapsulation substantially extends functional lifetime, aligning with application-dependent needs. Overall, the synergistic process overcomes individual limitations of electrochemical or photonic sintering alone and establishes a scalable path for additive manufacturing of transient, eco-friendly, and bioresorbable electronics.

This work introduces a two-step hybrid sintering process—acetic acid-mediated oxide reduction followed by flash-lamp annealing—that achieves state-of-the-art conductivity for printed transient Zn and enables robust, flexible interconnects on degradable substrates. The method reduces photonic energy requirements, preserves line geometry, affords good adhesion, and supports multilayer device fabrication. Demonstrated devices include resistive strain and temperature sensors with near-bulk Zn TCR, soft POMaC-based capacitive pressure sensors with stable cycling behavior, wireless chipless RLC force sensors, and a wireless power receiver coil capable of powering an LED at ~60 mm. Future directions include: (i) integration of reducing agents directly into inks to eliminate separate spray steps and improve throughput; (ii) deeper optimization for nanoparticle-based inks and finer features (inkjet/aerosol-jet/direct ink writing); (iii) improved uniformity and yield via controlled dispensing and particle size control; (iv) enhanced sensor sensitivity using micro-/nano-structured dielectrics; (v) systematic studies of long-term elastomer degradation and encapsulation strategies to tailor device lifetimes; and (vi) leveraging digital additive manufacturing for freeform, conformal, and self-powered transient sensor networks and implants.

- Process throughput: The need for multiple (≈5–6) acid spray/dry cycles may limit scalability; dispensing optimization or embedding reducing agents in ink is proposed but not realized here. - Sensitivity to timing and atmosphere: Conductivity degrades if photonic sintering is delayed (>1 h in air) post-acid treatment; inert atmosphere improves results, implying process control requirements. - Energy/yield trade-off: High photonic energies (≥7.3 J cm⁻²) cause damage and low yield (≈25%), indicating a narrow optimal window and variability in outcomes. - Variability: Conductivity variability across samples, influenced by particle size, thickness, and dispensing uniformity; smaller particles or more controlled dispensing could reduce spread. - Depth of sintering: Agglomeration is strongest in the top 5–10 µm; thicker layers show lower overall conductivity, suggesting depth limitations for very thick prints. - Environmental durability: Rapid degradation in PBS without encapsulation (order-of-magnitude resistance increase in ~47 h); device lifetime strongly depends on encapsulation choice. - Device performance: Capacitive pressure sensors show relatively low sensitivity vs. state-of-the-art micro/nanostructured dielectrics; enhancements left for future work. - Materials scope: While preliminary nanoparticle results are promising, a comprehensive nanoparticle-focused study (varied sizes/inks) is not provided. - Long-term materials behavior: Viscoelastic evolution and degradation of POMaC and encapsulants over extended times were not fully characterized, impacting long-term device stability.