## 5 Conclusions and Future Perspectives  
Thin film technology has been utilized for more than half a century and is still being evolving on a daily basis due to its key role in the 21 st development of new thin-film materials and novel thin-film devices. Specifically, it is used for facilitating the advancement of next-generation environmental-friendly devices, which are capable of environment sensing, health monitoring, \"green\" electronics, and so on. The literature review provides an overview of environmental applications based on various thin-films, mainly including inorganic materials (e.g., metal-oxide, halide perovskite, 2D) and organic materials. Before that, preparation methods of thin films were introduced simply, and then printing techniques were proposed for producing flexible, cost-effective, and environmental-friendly electronic products. Then, inorganic thin films including metal-oxide, PZT, halide perovskite, 2D thin-films, as extensively used thin-film materials, in applications of waste-gas/water sensing, energy, storage, etc. were reviewed in greater details. In light of the degradability and resorbability of partial inorganic thin films, they can be employed to fabricate the transient/degradable electronics for alleviating e-waste pollution threats. 2D material is one of the most popular low-dimension materials, which are of significance in academics and industries. Consequently, the properties and environmental applications of 2D thin films, such as degradability and compatibility as well as sensing of waste-gas/water pollution and storage of clean energy. To truly realize the complete transient/degradable electronics, except for inorganic thin-film materials, organic thin-film materials are desirable thanks to the unique advantages of recyclable degradation, resorbability, flexibility, and compatibility. Therefore, the small molecular and polymer thin-films as well as inorganic-organic thin-films applied for environmental protection and degradable devices are reviewed. After which, this paper highlights the perspective of the transient resistive-switching devices (such as transient RRAM), which provides abilities for the embedded system or Internet of Things (IoT) edge computing side, such as autonomous driving, smart sensors, and wearable devices. Furthermore, the memristor-based artificial degradable and/or compatible synapse is highly desirable for security neuromorphic computing and bio-integrated systems, which is a promising candidate for the new generation memory and computing systems in AI applications.  
Conceptualization, Y.T., and X.L. ([PERSON]); writing--original draft preparation, X.L. ([PERSON]), M.S., Y.L., and L.Z.; writing--review and editing, X.W., X.L. ([PERSON]), F.B.L.C., and Y.T.; visualization, Z.R.L., and Z.J.O.; supervision, Y.T.; funding acquisition, Y.T., [PERSON] ([PERSON]), and X.W. All the authors contributed to the work. All authors have read and agreed to the published version of the manuscript.  
This work was supported in part by the National Natural Science Foundation of China (No. 61804079, No. 61874059, No. 21671167, No. 61904087), the Natural Science Foundation of Jiangsu Province (18 KJD510005, CZ1060619001, SZDG2018007), the Science Research Funds for Nanjing University of Posts and Telecommunications (NY218110, NY217116, NY219014, BK20191202, KFJ)20170101).  
The authors declare no conflict of interest.