Additive Manufacturing of Thermochromic Chitosan Hydrogels for Battery-Free Colorimetric Thermal Indication

Battery-free, at-a-glance thermal readouts are attractive for point-of-care triage and low-resource settings. However, three-dimensional thermochromic hydrogel printing remains challenging because delayed gelation can compromise linewidth control and multilayer structural fidelity. In this study, a syringe-pump-driven extrusion-based direct ink writing platform was integrated with a temperature-controlled coagulation bath to induce immediate in situ gelation during thermochromic chitosan hydrogel printing. Systematic tuning of the nozzle gauge, printing speed, and flow rate, combined with rapid thermal gelation in the heated coagulation bath, mitigated die swelling, with the die-swelling ratio, which ranged from 2.1 to 3.7 at low printing speed, decreasing to approximately 1.0 at intermediate speeds. This enabled parameter-dependent control of strand diameter and reproducible multilayer fabrication. The mechanical properties of the printed hydrogels were further tuned by varying the coagulation bath temperature and chitosan concentration; gelation at 60 °C yielded the highest ultimate tensile strength (0.172 MPa), and a 4.0 wt% chitosan formulation provided the most balanced combination of strength and extensibility. Adjusting the thermochromic pigment composition modulated chromatic separation across temperature states in CIE L*a*b* color space over 26–54 °C. Formulation tuning broadened the separation of color states within an intermediate range of 31–38 °C, which spans normal skin temperature and the febrile range relevant to fever detection. The colorimetric response remained stable over 50 heating–cooling cycles, with color differences below the threshold of visual perceptibility. A multilayer construct further demonstrated layer-resolved colorimetric readout within a single 3D structure, indicating the potential of this passive hydrogel indicator for battery-free, spatially resolved thermal indication in this temperature range.