<p>Wearable biosensors promise proactive, precision medicine by enabling continuous biochemical monitoring, yet their clinical translation remains hindered by high-performance hardware accessibility barriers. Practical on-body operation demands stringent miniaturization, multiplexing, multi-method sensing, iontophoresis, and ultra-low power wireless operation – challenges that force researchers to remain in vitro or engineer tailored hardware. Unfortunately, prior systems rarely satisfy these requirements simultaneously, proliferating inadaptable designs that prevent reuse across applications. We introduce Biocoin, the first fully open-source, open-hardware wearable biosensing platform that unifies these needs in a reconfigurable, general-purpose framework. The 530 mm² wireless device demonstrates &gt;10 multiplexed sensor inputs, broad physicochemical sensing (amperometry, voltammetry, potentiometry, impedimetry, temperature) with benchtop-comparable performance, current-monitored iontophoresis for biofluid extraction, and extensive power optimizations enabling months-long, µA-level multimodal sensing. All hardware, firmware, and software are publicly released, providing a turnkey platform that removes long-standing hardware barriers to on-body biosensor validation and accelerates their translation into clinically actionable wearables.</p>