In Situ Upconverting Nanoparticle-Based Force Sensors for Gastrointestinal Imaging

Mechanical forces play a fundamental role in a myriad of biological processes such as stem cell differentiation, cardiovascular health, cancer progression, and digestion. The enteric nervous system is of particular interest for its neuromuscular complexity and bi-directional communication with the central nervous system, playing a significant role in neurodegenerative pathology. Quantifying colonic mechanical forces is important for understanding disease development, but current biological force measurement techniques are limited: ex vivo techniques such as atomic force microscopy and traction force microscopy do not measure forces in accurate physiological conditions, while in vivo measurement techniques like stents and catheters cause uncomfortable colon distension and risk damage or rupture. Nanoscale markers such as resonant energy transfer-based molecular tethers, oil droplets, and quantum dots also lack the robust signal to overcome a rapidly changing biological environment.<br/>Here, we demonstrate upconverting nanoparticles (UCNPs) as minimally invasive optical force sensors, delivered in polydimethylsiloxane (PDMS) pellets for in vivo gastro-intestinal tract imaging. UCNPs change their color via a ratiometric change in their two emission peaks in a linear fashion with externally applied pressure. Furthermore, they are photostable, absorb in the near-infrared tissue transparency region, can be functionalized to target specific biological structures, and do not require toxic heavy metals like cadmium for synthesis. We synthesize monodisperse 13-nm diameter cubic phase SrLuF:Yb, Er, Mn@SrLuF UCNPs, then use dual confocal-AFM microscopy to exert forces on these particles and determine their dynamic range of mechanosensitivity in the nanoNewton to microNewton force regime. We vary the Mn dopants concentration from 0% to 10% and optimize the Mn concentration to maximize color change and mechano-sensitivity. Next, we embed the UCNPs in PDMS mimics of mouse fecal pellets to create a colonic probe. We calibrate the spectral response of the UCNPs embedded in PDMS using dual confocal-AFM, demonstrating that red-to-green emission ratio changes repeatably over multiple compression and decompression cycles from 0 to 100 nanoNewtons. Finally, we deploy these pellet probes in excised mouse colons from both wild type and Etv-/- knockout mice to image and quantify forces in healthy colon pumping and irregular colon pumping, respectively. We compare the time-averaged force and location-dependent force exerted by the colons, as well as total time for pellet transition to demonstrate that our UCNP mechanosensor is able to distinguish between biological phenotypes with bright, robust optical signal that can penetrate colon wall tissue without damaging it. This platform provides a straightforward method for imaging intraluminal force dynamics in the colon and can be extended a range of other hollow neuromuscular organ systems, such as the stomach or heart cavities.