# Material Design in Implantable Biosensors toward Future Personalized Diagnostics and Treatments
## 3 Classification of Implantable Biosensors Based on Material Design
### Polymer-Based Implantable Biosensors  
The incredible nature of polymers has made them promising materials to deal with the complexity of physiological environments, where mutual interferences happen between tissues or organs and the implanted sensors. The ability of the polymer-based implantable sensor to respond to different physiological stimuli is an outstanding achievement in design and health monitoring. Based on the type of stimuli, polymer-based implantable sensors can be classified as biophysical (responsive to physical information such as pressure and temperature) and biochemical (real-time monitoring of molecular concentration such as sugar, ions, etc.) sensors. For example, [PERSON] et al. described the application of a polymeric composite consisting of a molybdenum electrode with piezoelectric poly (L-lactic acid) nanofibers (PLLA) encapsulation for the creation of a nanofiber-based piezoelectric transducer [69]. Piezoelectrical transducers are widely employed in the development of pressure sensors. This smart composition generates electricity following the distortion of the structure. After implantation into the abdomen, a miniature circuit board (PCB) is linked via the piezoelectric PLLA-molybdenum nanofiber. Afterwards, the sensor self-degrades with time. As a response to insulated saline solution into the abdominal cavity and, consequently, internal fluid pressure, the sensor generates a wireless signal that can be read in response to the depression and relaxation of the abdomen (Figure 8A,C). Figure 6: Fiber OECTs for biochemical detection of dopamine and glucose. (**A**) Representation of the OECT composition. (**B,C**) Example response towards dopamine and glucose. (**D,E**) Photograph of the OECT implanted on a mouse brain. (**F,G**) Demonstration of the OECT insertion in the brain followed through fluorescence staining. Reproduced with permission from ref. [67]. ©2020 Science China Press and Springer\(-\) Verlag GmbH Germany. Some reports about temperature sensors have been used for in vivo applications. For example, [PERSON] et al. reported the application of a polymer-based thin-film transistor temperature sensor (TFT-TS) in measuring brain temperature to understand cerebral metabolism [70]. A combination of temperature and pressure sensors were used to assess the intracranial physiological status (Figure 8D,E). In this sensor, the bioresorbable PLGA and Si-NMs enhanced the sensor's biocompatibility allowing for a straightforward assessment of the brain status. In the meantime, a commercial thermistor closely recorded the brain pressure and temperature, and a typical intracranial pressure (ICP) monitor was positioned close to the degradable sensor. The obtained results of temperature and pressure were closely comparable to the commercial sensor (Figure 8F). This bioresorbable sensor was able to differentiate between anesthesia-induced temperatures decrease and increased temperatures related to waking up, similar to the data observed in intraparenchymal tissues (Figure 8G) [22]. Figure 7: Implantable MFNPs-based module as an electronic interface for brain sensing. (**A,B**) Comparison between direct implantation of a rigid probe and a soft MFNP module in the brain. (**C**) Demonstration of a dry\(-\)MFNP composition. (**D**) Picture a wet-MFNP. (**E**) Effectiveness of elastic modules (Au wire, dry\(-\)MFNP, wet\(-\)MFNP) and mouse brain via indentation measurement. (**F**) MFNP implantation in a mouse brain. (**G,H**) Immunohistochemistrydistochemistry analysis of brain tissues implanted with an MFNP compared to a control (no implant). The yellow dashed circle shows the position of the MFNP. (**I**) MFNP\(-\)based recording of endogenous activity. Reproduced with permission from ref. [68]. ©2020 The Royal Society of Chemistry. As mentioned above, polymer-based implantable biosensors can be applied to detect biochemical substances which play essential roles in cellular activities. [PERSON] et al.