Cell encapsulation devices are expected to be promising tools that can control the release of therapeutic proteins secreted from transplanted cells. The protein permeability of the device membrane is important because it allows the isolation of transplanted cells while enabling the effectiveness of the device. In this study, we investigated free-standing polymeric ultra-thin films (nanosheets) as an intrinsically semi-permeable membrane made from polydimethylsiloxane (PDMS). The PDMS nanosheet with a thickness of 600 nm showed intrinsic protein permeability, and the device fabricated with the PDMS nanosheet showed that VEGF secreted from implanted adipose tissue-derived stem cells (ASCs) could be released for at least 5 days. The ASC encapsulation device promoted angiogenesis and the development of granulation tissue 1 week after transplantation to the subcutaneous area of a mouse. This cell encapsulation device consisting of PDMS nanosheets provides a new method for pre-vascularization of the subcutaneous area in cell transplantation therapy.