Advances in Insulin Delivery: Transdermal and Needle-Free Technologies as Emerging Strategies to Improve Metabolic Control and Treatment Adherence

Insulin therapy remains essential for the management of diabetes mellitus; however, conventional subcutaneous injection continues to impose significant physical, psychological, and behavioral barriers that negatively affect treatment adherence and metabolic outcomes. Injection-related pain, fear of needles, local tissue complications, and psychological insulin resistance contribute to delayed insulin initiation, inadequate dose titration, and suboptimal glycemic control worldwide. In response, alternative insulin delivery routes (including oral, pulmonary, nasal, and transdermal strategies) have been explored to reduce invasiveness and improve patient experience. Among these, transdermal insulin delivery has emerged as a particularly promising approach due to its potential to bypass gastrointestinal degradation, provide controlled absorption, and enhance patient acceptance. Recent advances in microneedle-based systems and needle-free jet injectors have enabled effective transdermal insulin administration by overcoming the skin barrier while minimizing pain and discomfort. This narrative review synthesizes current evidence on insulin delivery technologies with a specific focus on transdermal and needle-free systems. We discuss the biological and physicochemical challenges of insulin transport, the mechanisms underlying emerging delivery platforms, and clinical evidence regarding metabolic efficacy, glycemic variability, and patient-reported outcomes. The integration of these technologies with continuous glucose monitoring is also explored. Finally, we address translational challenges and future perspectives, highlighting the role of needle-free insulin delivery as a patient-centered strategy to improve adherence and metabolic control in diabetes care.