Neural injuries such as spinal cord injuries， traumatic brain injuries， or nerve transection injuries pose a major health problem. Neurotrophins such as nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF) have been shown to improve the outcome of neural injuries in several pre-clinical models， but their use in clinics is limited by the lack of a robust delivery system that enhances their bioavailability and half-life.