## 4 Discussion  
To date, the biggest challenge of the implantable glucose sensors is how to improve the sensitivity while maintain good linearity, biocompatibility and lifetime [5; 35]. There are several ways to solve this problem, including maximizing sensing surface of the electrode; reducing the negative effects of oxygen deficiency; balancing high sensitivity and wide linear detection range; increasing the signal to noise ratio; and, maintaining the stability of the sensor for a long time. Generally, gold [11], platinum [36; 37] and platinum-iridium [38; 39] are often used as conventional glucose electrode bases, where GOD could be directly immobilized. However, these metals have poor hardness and have to be implanted through an auxiliary device, which makes it inconvenient for diabetic diagnosis. So some researches indicated that high hardness stainless-steel electrode optimized by metal nanoparticles can effectively expand sensing area, accelerate electron transfer [10; 40] and be directly implanted by hand, improving the detection ability of immobilized GOD and minimizing the injury to body [41]. Moreover, the particles, such as size and density, could be easily adjusted by controlling reaction conditions. As a result, the sensing area is enlarged, the number of immobilized enzyme and active sites is increased and the electrical catalysis performance of electrode is optimized. As indicated in previous studies, the ratio of glucose to oxygen was more than 100 in interstitial fluid, causing an oxygen deficiency during monitor and a narrow linear detection range [17; 27; 30]. The sensor modified by nanoparticles would aggravate oxygen deficiency [17]. It has been demonstrated that biocompatible films can regulate the ratio of oxygen-glucose concentration in the reaction process [10]. Moreover, influences of sensor coatings on glucose transmission depend on the thickness, uniformity of pores distribution, surface morphology and other factors. Currently, a variety of natural, semi-synthetic and synthetic materials, such as chitosan, PEG and polyvinyl acetate, have been applied to modify outer films to optimize the biocompatibility and monitoring performance of implantable glucose sensors [8]. Specially, PEG is a strong hydrophilic amphoteric ion and is a promising modification material for sensor coating. The PEG segments gather on the membrane surface and absorb a large mass of water, generating a lot of pores on the surface after adequate hydration and drying [18]. Hence, in this study, compared to the NO-P curve, the large size of pores on surface of the SGL-P sensor allowed  
Figure 5: Stability of the relative sensitivity of the SHPM-OS sensor. (**a**) comparison of the amperometric response results of the SHPM-OS sensors versus glucose concentration at different intervals. (**b**) histogram of relative sensitivity of the SHPM-OS sensors restored in PBS. As indicated in previous studies, the ratio of glucose to oxygen was more than 100 in interstitial fluid, causing an oxygen deficiency during monitor and a narrow linear detection range [17; 27; 30]. The sensor modified by nanoparticles would aggravate oxygen deficiency [17]. It has been demonstrated that biocompatible films can regulate the ratio of oxygen-glucose concentration in the reaction process [10]. Moreover, influences of sensor coatings on glucose transmission depend on the thickness, uniformity of pores distribution, surface morphology and other factors. Currently, a variety of natural, semi-synthetic and synthetic materials, such as chitosan, PEG and polyvinyl acetate, have been applied to modify outer films to optimize the biocompatibility and monitoring performance of implantable glucose sensors [8]. Specially, PEG is a strong hydrophilic amphoteric ion and is a promising modification material for sensor coating. The PEG segments gather on the membrane surface and absorb a large mass of water, generating a lot of pores on the surface after adequate hydration and drying [18].