High performance with stability, easy-handling electrodes, and biofluid-flow controllable system with mechanical strength of the biofuel cell can be considered as the critical issues for future human body implant. These three challenges are sufficiently considered by using the effective platform regarding the high surface area from multi-walled carbon nanotube-conducting polymer with poly(3,4-ethylenedioxythiophene), and size/shape dependent flexible yarn electrodes for the implantation of biofuel cell. High power biofuel cell of mW cm-2 range in physiological condition (low glucose-containing phosphate buffered saline solution and human blood serum) controlling the stirring degree is also first demonstrated for future implantation in this study. Biofuel cells for future implantation in human body vitally require long-term stability and high power outputs. We have demonstrated that a high-surface area yarn-based biofuel cell retained over 70% of its initial power output after an extended 20 days period of continuous operation in human blood serum, while delivering a power density of ∼1.0 mW cm-2. Subsequently, our enhanced enzymatic biofuel cell system would be potentially used as an innovative power source for the next generation implantable electronics.