Universal Cryogenic Transfer of Liquid Metal Particles in Polymers for Wafer-Scale Stretchable Integrated Circuits

Integrating liquid metal patterns into soft electronics poses a significant challenge due to issues such as poor adhesion, surface oxidation, and delamination. Traditional methods for transferring these patterns often fail to achieve high yield or broad applicability across various polymer substrates. These limitations can result in increased resistance and reduced performance over time. To overcome these challenges, we introduce a novel cryogenic transfer process that leverages the mechanical and chemical property changes of materials at low temperatures. This method addresses the limitations of conventional techniques by enhancing adhesion and minimizing oxidation effects, thereby achieving a more reliable and efficient pattern transfer. Our method involves depositing a liquid metal pattern (LMP) onto a silicon (Si) substrate, then spin-coating polymer films and immersing the substrate in liquid nitrogen. This cryogenic treatment leverages three factors: polymer glass transition at low temperatures, LMP solidification and expansion, and changes in bonding energy. These factors enhance LMP adhesion to the polymer matrix. To understand this process, we conducted ab initio molecular dynamics (AIMD) simulations and X-ray diffraction (XRD) analyses, which show that LMP crystallization at cryogenic temperatures significantly improves transfer efficiency and bonding. This method achieves near-perfect transfer across various polymers and is effective for different gallium-based liquid metals. It enables the production of high-performance, stretchable printed circuit boards (PCBs). We tested the electromechanical properties of the transferred LMP patterns under various deformations. Unlike traditional methods that fail under strain, our cryogenically transferred patterns exhibit stable electrical conductivity and resistance, even under significant deformation. The low sheet resistance and strain-insensitive properties across different polymers demonstrate the versatility and potential of our approach for stretchable and wearable electronics. In summary, our cryogenic transfer process offers a significant advancement in integrating liquid metal patterns into soft electronics, enabling high-yield, universal pattern transfer and paving the way for durable stretchable electronic devices.