Upconverting Nanoparticle and Polymer Composites for Bioimaging Forces in Cartilage

Physical forces play a major role within our bodies, from kiloNewton scale bite strength to picoNewton scale cell-cell interactions; however, current methods for measuring these forces, including traction force microscopy, atomic force microscopy, ultrasound imaging, and pressure-sensitive stents and catheters, are bulky and ex vivo, low resolution and accuracy, or painful and invasive. Upconverting nanoparticles (UCNPs) present a promising solution as force-sensitive optical probes. Ceramic UCNPs are known for their photostability and relative biocompatibility, and their near-infrared excitation allows deeper tissue penetration than visible or UV excited fluorophores. Furthermore, SrLuF:Yb, Er UCNPs have been shown to change integrated red to green emission ratio as a function of externally applied force. When packaged into polymers of various mechanical properties, UCNPs provide a minimally invasive platform to probe biological forces such as those in cartilage.<br/>Here, we enhance UCNP force-dependent colorimetric emission by incorporating manganese in the UCNP-polymer composite system, either as a core dopant along with Yb and Er, or as a crosslinking ion in the polymer matrix surrounding the UCNPs. We hypothesize that the Mn-Er interionic distance more drastically modulates the energy transfer between the Er red and green emission states. In the first method, SrLuF:Yb, Er, Mn @ SrYF UCNPs with Mn concentrations between 0 and 4.2 at. % are packaged into two different polymers, rubbery polydimethylsiloxane and stiff epoxy resin. In the second method, SrYbF @ SrYbF:0.20Er @ SrYF:0.10Mn UCNPs are packaged into alginate hydrogel crosslinked with Mn2+ or Ca2+, with the UCNPs designed for energy transfer to external ions.<br/>We examine the optical effect of Mn concentration on UCNP emission with lifetime/decay measurements. We characterize the spectral force response of all UCNP-polymer thin films using a homebuilt simultaneous atomic force and confocal microscope setup. By integrating the red and green peak areas of the Er3+ emission spectrum, we calculate the change in red to green emission ratio (%ΔR:G) in the films with uniaxial compressive force applied as compared to the free-standing film. We identify that two UCNP-polymer combinations that give the greatest %ΔR:G slope, SrLuF:Yb, Er, 0.013Mn @ SrYF in epoxy resin (about 10x slope enhancement from 0% Mn UCNPs) and SrYbF @ SrYbF:0.20Er @ SrYF:0.10Mn in Mn-alginate (about 1.5x slope enhancement from UCNPs in Ca-alginate), and which best mimic cartilage mechanical properties of Young’s modulus, cyclability, and low friction. We then insert these UCNP-polymer composites between the bones of a chicken wing to demonstrate the facility of this platform for minimally-invasive optical force-sensitive imaging in situ. For future applications, with the diversity of UCNP architectures and polymer properties, we hope to extend UCNP force sensors for imaging a wider range of biological systems, including colonic gastrointestinal forces in relation to the gut-brain connection and cell to cell immune system interactions.