Mechanosensitive Polymer Matrices of Biologically-Relevant Compliance Based on Upconverting Nanoparticles

Upconverting nanoparticles (UCNPs) are promising optical biomechanical force sensors due to their near-infrared excitation, low toxicity, photostability, and linear colorimetric sensitivity to micronewtons of force. Recently, a composite force sensor based on UCNPs embedded in a polystyrene microbead enabled the first real-time measurement of feeding forces in living nematodes. However, the comparatively large stiffness of polystyrene only makes it relevant to biomedical applications in a small subset of biological tissue. To facilitate deployment of UCNPs into biological tissues with a range of mechanical properties, we expand upon polymer-UCNP composite systems by embedding UCNPs in three polymer matrices with varying stiffnesses (epoxy resin, polydimethylsiloxane, and alginate hydrogels). Furthermore, to enhance these composites' mechanosensitivity, we methodically investigate using two different core-shell architectures of SrLuF-based UCNPs doped with ytterbium, erbium, and varying manganese concentrations. We calibrate polymer-UCNP composite optical force sensitivity with colocalized atomic force and confocal microscopy. Using the red to green emission ratio (Δ% I_Red:I_Green) as the force read-out, we determine that SrLuF:Yb_0.28Er_0.025Mn_0.013 @ SrYF dispersed in epoxy resin exhibits the greatest emission color change (12 Δ%I_Red:I_Green per microNewton). Finally, we map forces in the epoxy-UCNP composite on the macroscale between the joint of a chicken wing bone using a commercially available wide-field microscope, thereby demonstrating its ability to optically measure pressures in situ. This work establishes the utility and modularity of the UCNP-polymer composite system for force-sensing in geometrically and mechanically diverse biological systems.