## 3 Materials and Fabrication Techniques
### Fabrication Techniques
#### 3.3.4 In-Situ Polymerization  
The OECT active layer can be deposited on the top of the source and drain electrodes also by performing an in-situ polymerization. [PERSON] et al. [117] demonstrated the possibility of depositing a layer of polyaniline between interdigitated source and drain electrodes starting from a solution containing 0.1 M aniline in 0.5 M H\({}_{2}\)SO\({}_{4}\); the electrode potential was cycled between \(-\)0.2 and +0.8 V vs. SCE. Glucose oxidase (GOx), for glucose detection, was also immobilized into the polymer matrix by electropolymerization from pH 4.0 phthalate buffer containing 0.1 M aniline and 250 units mL\({}^{-1}\) GOx. More recently, [PERSON] et al. [118] developed a procedure for the fabrication of microtubule sensors for the detection of glucose, urea and triglycerides based on electropolymerized poly(styrene sulfonate)-polyaniline (PSS-PANI) composites synthesized within the pores of track-etched polycarbonate membranes. First, two gold stripes were thermally evaporated on the opposite faces of each membrane. These stripes were then used as electrodes for the polymerization of the active layer. After deposition of the active layer, glucose oxidase was immobilized on the conducting polymer microtubules again by electropolymerization while urease and lipase (for urea and triglyceride detection, respectively) were physisorbed on the active layer. Devices were then used in a transistor configuration; the two gold stripes acting as source and drain electrodes and a reference electrode used as gate. Very recently, [PERSON] et al. [119] demonstrated that in-situ polymerization can be also performed on non-conventional substrates, such as fibers. The authors utilized Nylon PA6 yarns which were covered with a thin layer of graphene oxide (GO) which was then chemically reduced to graphene (rGO) (Figure 11).  
On such yarns, in-situ polymerization of polypyrrole (PPy) was then performed. Each OECT-based glucose sensor was then assembled by placing two yarns in a cross-junction configuration and connected by an electrolytic gel containing different glucose concentrations. The bottom yarn acted as the transistor channel while the top yarn (functionalized with GOx) acted as the gate electrode.  
Vapor-phase polymerization (VPP) has also been successfully used for the fabrication of OECTs-based glucose sensors by [PERSON] et al. [120] As reported in this paper, the authors prepared an oxidant solution containing Fe(III)-toyslate (dissolved in isopropyl alcohol) and spin-coated it on a glass substrate. The substrate was then placed inside a polymerization chamber containing a hot plate filled with a small amount of a 3,4-ethylenedioxythiophene (EDOT) solution. After polymerization (7 min at 70 \({}^{\circ}\)C), the PEDOT layer was washed with ethanol to remove the remaining oxidants. A small PDMS well was finally glued on the substrate to confine the solution to be tested (PBS at pH = 7.4, containing 3.6 mg mL\({}^{-1}\) of glucose oxidase) on the transistors channel. The transistor configuration was eventually completed by dipping a Pt wire, acting as gate electrode, into the electrolytic solution.