Rapid development of miniature, portable and highly integrated electronics increases the demand for micro-sized power sources and on-chip energy storage units; micro-supercapacitors with in-plane electrode finger arrays are developed for this purpose. This study fabricates flexible, solid-state, interdigital micro-supercapacitors with MoO3-x nanorods as cathode fingers and glucose-derived activated carbon nanospheres as anode fingers via a facile and economic screen-printing technique. A highly concentrated sodium alginate bio-hydrogel is used as the non-toxic and biocompatible electrolyte, which provides wider electrochemical stability window (∼2.8 V) than that of conventional aqueous electrolytes (1.23 V). The fabricated asymmetric micro-supercapacitors present outstanding electrochemical performances, for example, high areal capacitance of 47.20 mF cm−2, superior energy density of 21.20 μWh cm−2 (47.11 mWh cm−3) at a power density of 0.18 mW cm−2 (0.40 W cm−3), and excellent capacitance retention of 95% after 10,000 cycles. The device also exhibits good mechanical stability and can be integrated into any printed circuits, demonstrating its potential application for highly customized power systems in the Internet of Things and wearable/implantable on-chip electronics with high safety requirement.