Recent Advances in Supplementary Haptic Feedback for Human-Machine Interfaces in Upper Limb Assistance and Rehabilitation.

Despite the rapid technological advancements we witnessed in the last few decades, effective regaining or substituting the impaired sensorimotor function of the upper limb is still a dream for many patients and researchers worldwide. While technology-aided motor therapy and advanced human-machine interfaces have significantly evolved, the efforts to integrate supplementary sensory feedback (SSF) to promote sensorimotor restoration after neurological or orthopedic damage became relevant only in recent years. In this review, we examine emerging strategies for encoding and delivering somatosensory information to users of prosthetic, orthotic, and rehabilitation systems, highlighting advances in electrotactile, vibrotactile, mechanotactile, and neurostimulation-based approaches. We synthesize cross-disciplinary findings from neuroscience, haptics, and clinical bioengineering to outline how SSF influences embodiment, motor learning, user acceptance, and real-world performance. Despite rapid technical progress, major gaps persist, including limited long-term evaluation, narrow user representation, and a lack of standardized methods for characterizing sensations and benchmarking device performance. We discuss the scientific and translational barriers that currently constrain widespread adoption of SSF technologies and identify promising directions for future research, including unified assessment frameworks, personalization strategies, and the development of richer haptic vocabularies to enhance the functionality and clinical relevance of next-generation sensorimotor interfaces.