**3.3. Wearable metabolite biosensors**

An indisputable trend in biosensor technology is on-body continuous monitoring of metabolites using wearable devices (**Figure 4**) [1]. Wearable biosensor applications aim to transform centralized hospital-based care systems to home-based personal medicine, reducing healthcare cost and time for diagnosis. Electrochemical transducers offer many benefits as wearable sensors for physiological monitoring, and can be easily integrated onto textile materials or directly on the skin.

Sweat-based wearable sensors, although mostly focused on a small number of physical or electrophysiological parameters, can yield crucial information about the health status of a patient based on levels of vital metabolites [73]. Wearable biosensors can be either textile/plastic-based or epidermal (tattoo)-based systems [74]. Epidermal biosensors supply better contact with skin but commonly exhibit shorter lifetimes than the textile-based tools. Such biosensors were first developed in 2009 by Kim et al. for continuous monitoring of physical parameters [75] and, shortly thereafter, Jia et al. combined this route with biorecognition elements to generate the first printed tattoo-based biosensor [76]. A screen-printed electrode on no permanent tattoo paper was investigated with carbon and silver (Ag)/AgCl serving as the working and reference electrodes, respectively. The working electrode was also modified with carbon nanotubes carrying a mediator together with lactate oxidase for endlessly monitoring lactate in sweat during exercise [76].

CPs are specially beneficial for wearable sensor technology owing to their compatibility with production on flexible solids [77]. In a very interesting way, Pal et al. investigated PEDOT:PSS electrodes on flexible fully biodegradable silk protein fibroin supports using a simple photolithographic process and an aqueous ink composed of the CP and carrier proteins (**Figure 4B**) [78]. In an almost identical route by the same scientific group, silk proteins including fibroin and sericin were modified with photoreactive methacrylate groups for use as substrate inks for water-dispersible PEDOT:PSS that was micropatterned to investigate a biodegradable bioelectrode for glucose sensing *in vitro* [79]. This pathway presents a new trend for generating an entirely organic and free-standing system with controllable biodegradability including scalability and processability, leading to applications in wearable or implantable bioelectronics [80].
