A California start-up company testing an ultra-thin zinc-polymer battery may soon be printing money by selling the flexible, rechargeable batteries to manufacturers of wearable electronics, medical devices, smart labels and environmental sensors. Christine Ho, co-founder of Imprint Energy, Alameda, Calif., collaborated with a researcher in Japan to produce microscopic zinc batteries using a 3-D printer. The batteries for most laptops and smart phones contain lithium, which is highly reactive and has to be protected in ways that add size and bulk. Zinc batteries are more stable, but their water-based electrolytes cause zinc to form branch-like structures that can grow between electrodes and short out. Ho’s solid polymer electrolyte avoids this branching problem while increasing the battery’s capacity to be recharged. The company’s tests show that while existing batteries failed catastrophically after fewer than 1,000 bending cycles, the zinc-polymer batteries remained stable.