Functional inks for direct-write inkjet printing: materials, processing, and applications

Inkjet printing has established itself as a leading additive manufacturing (AM) platform for functional electronics, enabling maskless, material-efficient deposition of picolitre scale droplets onto rigid, flexible and non-conventional substrates with precise spatial control. As illustrated in the graphical figure, this review systematically examines functional ink materials for drop-on-demand inkjet printing and the influence of process parameters on printed device performance. The rheological requirements governing jettability, including viscosity (1–30 mPa.s), surface tension (25–50 mN/m) and the dimensionless Z-number printability window are discussed alongside droplet formation dynamics, substrate wettability and film formation phenomena including coffee ring effect. Six principal ink categories are critically reviewed: conductive inks based on silver, copper, gold and carbon nanomaterials; semiconductor inks spanning organic polymers, small molecules, metal oxides and perovskite and quantum dot systems; dielectric inks including polymeric, composite and inorganic oxide formulations; magnetic inks incorporating iron oxide and nickel nanoparticles; sacrificial inks for complex three-dimensional architecture and biological inks. Post-deposition processing strategies such as thermal sintering, photonic and intense pulsed light sintering, chemical sintering and UV curing are evaluated with emphasis of substrate compatibility. The influence of key process parameters including droplet spacing, jetting waveform, substrate temperature and multi-pass strategies on feature morphology and functional performance is examined. Machine Learning and data-driven approaches for ink formulation and process optimisation are also discussed. Applications across flexible electronics, wearable sensors, energy storage, RFID and biomedical devices are surveyed. Persistent challenges in ink formulation, nozzle reliability, resolution limits, and scalable manufacturing are identified, and future research directions are outlined.