Printable inks for flexible healthcare sensors: From materials to device insights

Printable inks have been widely used in wearable healthcare sensors, and their functions go beyond simply creating conductive or sensing layers. Ink formulation controls printability, film formation, electrical path creation, adhesion to flexible substrates and stability of the process under mechanical strain such as bending and stretching. This review highlights recent advances in printable inks used in wearable healthcare sensing devices including metal-based, carbon-based, polymeric, and biofunctional ink systems. Their merits as well as limitations including high curing temperature, poor dispersion stability, limited conductivity, poor substrate adhesion, and difficulty of biological functionalization will be discussed. This review focuses mostly on the relationship between ink formulation, rheology, printing, printed microstructure, and sensor performance. The existing research usually studies them separately, therefore, making it difficult for comparison between different ink systems and prediction of the performance of devices in real-life situations. Rheology has been studied usually only as a limitation in printing process although it affects pattern resolution, film homogeneity, formation of conductive paths, and repeatability. The commonly used printing methods, particularly inkjet, screen printing, and flexographic printing, are also compared in terms of resolution, material compatibility, throughput, and scalability. Although wearable healthcare sensors have developed considerably, several challenges still limit their use beyond laboratory demonstrations. These include material-related limitations, poor long-term stability, mechanical degradation, biofouling, and reduction in sensor performance over time. Overall, this review highlights the importance of understanding the process-structure-performance relationships of printable inks for developing reliable, scalable, and real-time wearable sensors.