Asian Research Journal of Current Science

Needle-free injection denotes the delivery of a medicinal substance across a tissue barrier without penetration by a conventional solid hypodermic needle. Liquid-jet injectors are its most clinically developed form; particulate ballistic systems and emerging laser-, pneumatic- and electromagnetic-actuated microjets represent related but technically distinct approaches. This critical narrative review integrates engineering, pharmaceutical, clinical and implementation evidence to examine how device architecture, jet dynamics, tissue heterogeneity and formulation characteristics determine the balance between delivery accuracy and injury. Literature published from 2000 to 9 August 2026 was considered, together with earlier concepts where necessary, using targeted scholarly searching and reference tracing. Experimental studies support the importance of nozzle geometry, jet power, temporally controlled propulsion, applicator loading and tissue properties. Randomised influenza and poliovirus vaccine studies demonstrate that particular injector–vaccine combinations can achieve clinically acceptable immune responses, although local reactogenicity and delivered-dose variability remain relevant. Controlled insulin studies indicate earlier drug absorption and improved early postprandial glycaemia, but do not establish long-term superiority in diabetes outcomes. Dermatological trials support selected intralesional applications, while their small samples, heterogeneous endpoints and frequent drug–device confounding restrict extrapolation. Historical bloodborne-pathogen risks show why reusable patient-contact components cannot be regarded as safe merely because no needle is present. Preclinical microjet and nucleic-acid findings extend the design space but are not equivalent to validated human delivery. The review proposes a device–formulation–tissue–setting framework in which pharmacological effectiveness, contamination prevention, usability and affordability must be demonstrated jointly rather than inferred from needle elimination. Future advances depend on reproducible dose accounting, robust biopharmaceutical stability testing, comparative clinical endpoints and implementation research under realistic conditions.