Microdroplet additive manufacturing: Droplet generation technologies and direct forming processes

Microdroplet additive manufacturing (MAM) enables high-precision fabrication through the controllable generation and site-specific deposition of microscale liquid entities. This review defines MAM according to the state and fate of the transported liquid: in core MAM, discrete microdroplets or droplet-derived jets serve as spatially addressable material units that directly determine local deposition and contribute to structure formation through solidification, curing, or coalescence. We first examine the governing principles and performance boundaries of atomization, continuous inkjet, and on-demand droplet generation, followed by seven representative actuation mechanisms—piezoelectric, thermal inkjet, pneumatic pulse, electromagnetic, magnetohydrodynamic, electrohydrodynamic, and laser-induced cavitation—and laser-induced forward transfer as a distinct laser-driven transfer strategy. Five direct-forming process families are then positioned within this framework: photopolymer and molten material jetting as core MAM; aerosol jet printing and binder jetting as droplet-mediated or droplet-enabled processes; and direct ink writing as an adjacent continuous-filament process. Representative advances in bioprinting, flexible electronics, and electronic packaging and precision metal fabrication are critically assessed. Key challenges include viscosity–frequency tradeoffs, stable jetting of high-melting-point metals, and multi-material interfacial compatibility. Future priorities are modular and hybrid actuation, closed-loop process control, deposition–consolidation co-design, and industrial scale-up and qualification. By integrating an operational scope definition, unified droplet physics, source-audited quantitative comparisons, and a requirements-driven technology-selection framework, this review provides a systematic basis for technology selection, process optimization, and fundamental research in MAM.