Upconversion Photoluminescence to Monitor Local Heat Release During Femtosecond Direct Laser Writing of Bioinks In Situ

Exothermic photopolymerization releases heat into the sample environment. Using NaYF₄:Yb³⁺/Er³⁺ upconversion nanoparticle (UCNP) photoluminescence and a colinear lithography and thermometry laser configuration, we monitor thermal signatures in the focal spot during femtosecond direct laser writing in real time. A statistical short-pass filtering is introduced to reduce the standard error in temperature calibration compared to conventional Gaussian deconvolution. Thermometry performance of our set-up achieved a relative sensitivity of 0.89-1.58% K^-1 and a measurement uncertainty of 0.2-0.4 K for 2 Hz sample rates. With this, the effect of scan speed, laser power, and photoinitiator concentration on accompanying local heating could be followed. Nonlinearities and thermal runaway effects with transient temperature spikes above 120-140°C demonstrate the need for a stringent reduction of the thermal burden when writing aqueous bioinks for biomedical applications. Physiological conditions were maintained only for fast 20 µm/s scan speeds, which limited temperature quenches to not exceed physiological temperatures. This paves the way to improve process control and to optimize for laser-assisted bioprinting and other related technologies.