# Material Design in Implantable Biosensors toward Future Personalized Diagnostics and Treatments
## 2 Implantable Biosensors and Their Design
The present solutions are designed around using two types of wireless communication tools: (i) passive devices (based on electromagnetic transmitters or readers) and (ii) chip-integrated devices (based on the measurement of electrical signals sent from the chip) [21; 22]. However, the choice of devices should be meticulously selected depending on the intended use. The passive component-based devices are more favored due to the flexibility, self-healing ability, biocompatibility, and biodegradability of the applied substances and their lack of need for a hard and voluminous chip nor an inner power source (Figure 2) [2; 3]. However, passive sensors have limited temporal resolution and poor performance when measuring multiple or complex parameters, limiting their use as implantable sensors in many medical applications. They also suffer from measurement frequency where they take minutes to acquire data compared to active sensors especially in situations where continuous or several measurements per second are necessary, as in the case of orthopedic monitoring [23]. Besides the concept of material design and device architecture, the insertion of the device into the body is one of the serious concerns for researchers in this field. There are huge numbers of complaints about conventional insertions by surgeries which cause physical suffering and infections, long recovery times, high cost, and possible psychological problems. To avoid such complaints, minimally-invasive surgery (MIS), such as catheter-based treatment and laparoscopic surgery, with considerably fewer side effects have been introduced as standard medical procedures [24]. The inspiration by the positive feedback of MIS has led to the introduction of an advanced version of the technique called the minimally invasive insertion of implantable devices. This approach consists of syringe-injectable devices that can be inserted into the body by MIS (Figure 3) [25; 26; 27; 28]. For instance, [PERSON] et al. reported inserting mesh-like electric materials constructed with an epoxy-based negative photoresist into a living mouse via an MIS procedure (Figure 3A). The mouse's brain activity was effectively assessed, indicating the potential of this approach  
Figure 2: Representation of the properties desired in materials aimed for implantation applications. for clinical applications [27]. Another successful study by [PERSON] et al. described the insertion of a flexible mesh made from shape-memory polymer for tissue delivery [28]. This elastic scaffold was made using a UV-cross-linkable and biodegradable elastomer known as POMaC (poly(octamethylene maleate (anhydride) citrate)) which was able to deliver cardiac tissues to the heart via MIS. Other than syringe-injectable devices, inflatable balloon catheters are considered the best option in many clinical operations [29; 30]. These devices showed some significant features, including MIS-based insertions using tiny incisions, controlled inflation, small size due to the cylindrical form when it is deflated, and optimal application due to the soft contact with surrounding tissues [30]. However, the lack of functionality in these devices limited their practical applications. To overcome this lack, adding functionality to the catheters by integrating electronics was considered [29]. In the developed version, a balloon catheter is used as a base for the incorporation of sensors, heaters, etc. For instance, [PERSON] et al. proposed tube-based medical treatment as a sustainable therapeutic delivery approach (Figure 3C) [28]. There are some other splendid examples of minimally invasive insertions. For example, [PERSON] et al. described the use of a minimally invasive insertion for implanting a one-step fiber-based optogenetics system in a human body (Figure 3D) [31]. This simple and multifunctional approach exhibited the potentiality of optogenetics implementation due to the ease and cost-effectiveness of the approach.