<p>Flexible epidermal sensors assume an indispensable role in the field of wearable electronics, enabling the imperceptible detection of biomechanical signals for personalized health care. As an integral part, solution-processed ultrathin nanocomposite conductors with high strain sensitivity offer cost-effectiveness and scalability for sensor manufacturing. Nevertheless, their controllable fabrication remains a challenge, and the presence of abundant polymers usually lead to high hysteresis and unsatisfactory sensitivity for low strain detection. Here, a robust, unform, and highly conductive silver nanofilm is prepared through Layer-by-Layer (LbL) assembly by combining positively charged polyurethane and uniformly sized silver nanoparticles (AgNPs) on desired substrates. Subsequent photonic sintering is used to fuse the AgNPs into a cohesive structure and mitigate uncontrollable heat-induced cracks in the Ag nanofilm owing to continuous thermal expansion from underlying elastomeric substrates. Consequently, the Ag nanofilms achieve a conductivity of 5.1×10⁴ S cm⁻¹, and demonstrate substrate-dependent electromechanical properties. In particular, the LbL assembled Ag nanofilms on oxygen plasma-treated polydimethylsiloxane (PDMS) can serve as hypersensitive sensors with gauge factors of more than 3000 at strains of less than 5%, or they are stretchable with small resistance variations to more than 50% on (3-aminopropyl)triethoxysilane modified PDMS or other thermoplastic elastomers. Mechanisms on this substrate dependent electromechanical properties are investigated, and the ultrasensitive strain sensors on PDMS are demonstrated for the detection of sound frequencies, pulses, and small forces.</p>