# Material Design in Implantable Biosensors toward Future Personalized Diagnostics and Treatments
## 3 Classification of Implantable Biosensors Based on Material Design
### Fiber-Based Implantable Biosensors  
As mentioned before, creating a steady interface between soft tissues and rigid biosensors is one of the grand challenges in implantable biosensors. Even though nanomaterials and nanotechnology could significantly deal with this problematic issue, there are still drawbacks and further work needed to fully resolve it. One of the practical and promising solutions is using flexible implantable fiber biosensors. For example, a mesh electrode prepared by following a thermal drawing procedure and materials (such as photoresist, gold, and polymer composite fibers) were successfully implanted in brain tissues and shown low immune responses with stable neural activity even after weeks from implantation [63]. In another study, silver nanoparticles and polyurethane were used to create an elastomeric fiber composite for the development of conductive and stretchable wireless system [64]. Fiber-based implantable biosensors present various advantages compared to the planar implantable sensing platforms, such as increased compatibility towards complex tissues and organs thanks to their one-dimensional (1-D) fibrous structure. Additionally, these devices are stitched on target organs directly which makes the surgical procedures simpler. Another point is that the system does not necessitate any welding which is known as a problematic issue for stretchable electronics. These systems also boast of having a passive readout circuit that can be expanded to a time-domain readout procedure due to the self-resonance feature of the circuit. Besides polymeric fibers, carbon nanotube (CNT) fibers (CNFs) have enhanced biocompatibility, flexibility, and wide surface areas (Figure 5) [65]. The flexibility of the 1-D fibers created an opportunity to develop spirally assembled multiple fiber electrochemical biosensors for the concomitant recognition of multiple molecules [65]. For example, the implantation of calcium ions (Ca\({}^{2+}\))-glucose integrated fiber-based biosensors in a cat's vein exhibited promising results. This biosensor could record any fluctuation of Ca\({}^{2+}\) and glucose in the blood. At the same time, the obtained information was timely and precise, which provides a potential in situ biosensor candidate \begin{table}
\begin{tabular}{c c c} \hline \hline
**Nanomaterial** & **Outcome** & **Ref.** \\ \hline Tunable gold nanogap & Ultra-sensitive electrochemical impedance biosensor for detection & [57] \\  & of streptavidin. & [58] \\ \hline Si and Si/SiO\({}_{2}\) & Round diaphragm pressure sensors. & [59] \\ \hline Carbon nanofiber & Nano-implant for neural tissues monitoring, diagnosis, and treatment. & [57] \\ \hline PbS hollow sphere QDs & Early-stage cancer diagnostics and treatment of ophthalmic diseases. & [60] \\ \hline Ultrananocrystalline diamond & Implantable retinal microchip. & [4] \\ \hline TiO\({}_{2}\) nanotubes & Soft-tissue responsive sensor. & [61] \\ \hline BZT-BCT NWs/PDMs nanocomposite & Biodection of resistance with an implantable, wireless, and & [62] \\  & power-free nanosystem. & [62] \\ \hline PLGA nanoporous Si composite & Bioresorbable sensor for the brain. & [22] \\ \hline Si: silicon, SiO\({}_{2}\): silicon dioxide, PbS: lead sulfide, QDs: quantum dots, TiO\({}_{2}\): titanium dioxide, BZT-BCT: Ba(Zr\({}_{2.2}\)Ti\({}_{0.8}\))O\({}_{3}\)-(Ba\({}_{0.7}\)Ca\({}_{0.3}\))TiO\({}_{3}\), NWs: nanowires, PDMs: polydimethylsiloxane, PLGA: poly(lactic-co-glycolic acid). & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Nanomaterials in the fabrication of implantable biosensing platforms. that is light and simple compared with the traditionally complicated sampling equipment and procedures [65]. It is shown that the active surface area proportionally declines with increasing flexibility and decreasing size, which are unavoidable factors in developing novel implantable devices. The decrease in the surface area leads to an impedance increase and decreased signal-to-noise ratios which automatically decrease the sensing performance of the system [66].