## 4 Discussion
Hence, in this study, compared to the NO-P curve, the large size of pores on surface of the SGL-P sensor allowed more glucose to reach electrode, obtaining stronger response current at low concentration. However, the response current became unstable with the increase of glucose concentration. The linearity of sensor was also poor, probably due to the insufficient oxygen. Enhancing restriction of material transfer is an attractive membrane modification method to improve linearity of the glucose sensor, which however could decrease the sensitivity. For example, when the PVDF-Nafion glucose sensor was covered by nanoparticles with great restrictive effect, R\({}^{2}\) increased from 0.776 to 0.9988, but the signal intensity decreased by 29% at the highest glucose concentration [17]. In previous studies, a PU/epoxy-enhanced (E-PU) membrane with smooth surface without holes was designed [27; 42], which had a stronger effect on glucose restriction. When the concentration of epoxy reached 40%, the PU/E-PU could block over 70% glucose with a sensitivity of 63 nA/mM [27]. Particularly, the maximum linear detection range could expand to 40 mM with the increase of epoxy concentration, but the sensitivity decreased to 6 nA/mM [42]. Although the linear detection range could be enhanced by restricting glucose transmission [33], there are still difficulties to collect effective signal and to produce the sensors at low cost. Moreover, the successive accumulation of H\({}_{2}\)O\({}_{2}\) in the reaction would result in electrode poisoning and the leakage would stimulate surrounding tissue [5; 29; 33]. An oxygen regeneration system prepared by immobilizing both [PERSON] and Cat on sensing surface was proposed by [PERSON] and [PERSON] et al. [30; 31], H\({}_{2}\)O\({}_{2}\) can be rapidly decomposed, which generates oxygen when Cat is sufficient; the influence of oxygen deficiency can be partly relieved because the oxygen demand is halved. However, the sensing area of typical sensor electrode is limited, leading to the insufficient immobilized GOD when Cat is deposited on the same surface. Thus, the sensor sensitivity and lifetime need to be optimized [43]. [PERSON] and [PERSON] et al. [29; 41] proposed an oxygen regeneration system, which immobilized GOD and Cat on different layers through layer-by-layer assembly approach and improved the sensitivity to 70 nA/mM. However, the function of signal outer membrane was simple, and it is difficult to obtain the optimal oxygen-glucose transport ratio. The SHPM-OS film proposed in this study provided a great strategy, which adjusted the morphology and distribution of pores with changed PEG concentration on different layers. Particularly, the pores of the porous PU inner film with a high proportion of PEG were large in diameter and clearly visible, which can enhance driving force for glucose and oxygen diffusion inward [41; 44] and the speed of analyzing glucose change [45], thus shortening the response time of the glucose sensor. On the contrary, the pores of the porous PU outer membrane with less PEG were small and restricted glucose transmission, maintaining the high linearity of sensors. The response current had a linear change in the range of 0-20-mM glucose concentration (R\({}^{2}\) was 0.9989). The comparison of different amperometric sensors for the glucose determination was shown in Table 1. The SHPM-OS sensor exhibited similar linear range as the other sensors in Table 1. However, it could continuously release oxygen from sandwich layer and thus, its sensitivity was increased by three times. However, compared with other commercial sensors, the linear range of SHPM-OS (0-20-mM) was relatively narrow. Thus, it is necessary to combine SHPM-OS with the AiTi health management platform (developed by our group) in the future, which would send an early warning to users, family members or private doctors when BG concentration exceeds 20 mM.