Examples of devices (biofuel cells) that harvest chemical energy from glucose in a) cerebrospinal fluid (CSF), b) blood, c) interstitial fluid (IF), and d) e) gastrointestinal fluid (GIF). a) Adapted with permission.[371] Copyright 2013, Springer Nature. b) Adapted with permission.[366] Copyright 2013, Royal Society of Chemistry. c) Adapted with permission.[362] Copyright 2018, Elsevier. d) Adapted with permission.[387] Copyright 2018, Royal Society of Chemistry. e) Reproduced with permission.[388] Copyright 2013, Elsevier.