Responsive Biosensors

Responsive biosensors are electromechanical devices that measure physiologically important variables, including chemicals, biological signals, and degradation products of implants, and respond in a controllable way to their own measurements. They can be designed to have a feedback mechanism to improve sensitivity or to activate a control function that affects the analyte. Achieving scientific breakthroughs at the synthetic/biological interface will require the development of a new class of multifunctional biocompatible materials and sensing devices capable of both: (1) sensing biochemical and physical phenomena such as biodegradation and cell functions at the synthetic/biological interface and (2) controlling the biochemical reaction and biodegradation rate through feedback loops. The development of responsive biosensors has a tremendous impact on treatment strategies and the health care industry.

Responsive biosensors could also integrate with current implants and serve as a novel component of "smart" implant systems. When placed in the body, biosensors are engineered to provide an electrical stimulus to facilitate the regeneration process of the tissue and trigger controlled degradation on demand when neo-tissue has advanced to an acceptable level of maturation. For example, a sensor that measures the degradation products of a biodegradable Mg implant is under development (Schulz et al. 2010). This sensor is designed to automatically adjust (slow down or speed up) the degradation rate of Mg alloys. Chemicals released in the body during degradation of an implant or device will affect the surrounding cells and tissues. Thus, controlling the degradation rate based on the cellular or tissue response can further control the functionality of an implant. Responsive biosensors can monitor molecular and cellular activities at the implant-tissue interface to ensure that the implant is safe and effective and can provide a way to control properties of implants during degradation.

Balancing the degradation rate and bone formation rate will be the key to developing biocompatible "smart" implants. Electrochemical impedance (EI) can be used to measure the kinetics of the metal's corrosion surface. The approach for controlled degradation is to use the Mg implant in the body as an electrochemical cell. In an electrochemical cell, power is supplied to stop the galvanic reaction. The Mg implant is the cathode (negative electrode) where reduction occurs. A cathodic current is impressed to inhibit corrosion of the implant. A noncorroding electrode will be used as the anode of the electrochemical cell. The controlled degradation system will comprise an in-body module that regulates the potential on the implant to slow dissolution and control hydrogen evolution. The effectiveness of controlled degradation can be enhanced if Mg alloys can be designed to have a corrosion potential closer to zero. At the corrosion potential, the current is at a minimum and corrosion is inhibited. If the corrosion potential is closer to zero, the current will have less effect on bone cells and redox reactions.

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