**Pankaj Vadgama**

School of Engineering and Materials Science, Queen Mary University of London, Mile End, London E1 4NS, UK; p.vadgama@qmul.ac.uk

Received: 11 May 2020; Accepted: 31 May 2020; Published: 2 June 2020

**Abstract:** The disruptive action of an acute or critical illness is frequently manifest through rapid biochemical changes that may require continuous monitoring. Within these changes, resides trend information of predictive value, including responsiveness to therapy. In contrast to physical variables, biochemical parameters monitored on a continuous basis are a largely untapped resource because of the lack of clinically usable monitoring systems. This is despite the huge testing repertoire opening up in recent years in relation to discrete biochemical measurements. Electrochemical sensors offer one of the few routes to obtaining continuous readout and, moreover, as implantable devices information referable to specific tissue locations. This review focuses on new biological insights that have been secured through in vivo electrochemical sensors. In addition, the challenges of operating in a reactive, biological, sample matrix are highlighted. Specific attention is given to the choreographed host rejection response, as evidenced in blood and tissue, and how this limits both sensor life time and reliability of operation. Examples will be based around ion, O2, glucose, and lactate sensors, because of the fundamental importance of this group to acute health care.

**Keywords:** metabolite sensors; sensor biocompatibility; ion selective electrodes; foreign body reaction; O2; glucose; lactate
