Mechanoresponsive Hydrogel Particles as a Platform for Three-Dimensional Force Sensing

Abstract

We introduce a novel concept for mechanosensitive hydrogel microparticles, which translate deformation into changes in fluorescence and can thus function as mechanical probes. The hydrogel particles with controlled polymer network are produced via droplet microfluidics from poly­(ethylene glycol) (PEG) precursors. Förster resonance energy transfer donors and acceptors are coupled to the PEG hydrogel network for reporting local deformations as fluorescence shifts. We show that global network deformations, which occur upon drying/rehydration, can be detected via a characteristic fluorescence shift. Combined characterization with confocal laser scanning microscopy and atomic force microscopy (AFM) shows that also local deformation of the particles can be detected. Using AFM, the mechanical properties of the particles can be quantified, which allows linking strain with stress and thus force sensing in a three-dimensional environment. Microfluidic material design allows for precisely varying the size of our hydrogel microparticles as well as their mechanical properties and polymer network structure with regard to the choice of the macromolecular precursors and their functionalization with fluorophores. Thus, concomitant changes in mechanical properties and mechanosensitivity qualify these hydrogel microparticles as an adjustable material platform for force sensing in structural mechanics or cell culturing.

A hyperelastic model for simulating cells in flow

“…In this section, we compare our simulations to axisymmetric calculations using the commercial software Abaqus and validate our cell model with further experimental data for bovine endothelial cells from (Caille et al 2002) and very recent data for hydrogel particles from (Neubauer et al 2019). Fig.…”

“…To demonstrate the need for this more complex elastic model, we carry out extensive FluidFM Ⓡ indentation experiments for REF52 (rat embryonic fibroblast) cells at large deformation up to 80% (Alexandrova et al 2008). In addition, our model compares favorably with previous AFM experiments on bovine endothelial cells (Caille et al 2002) as well as artificial hydrogel particles (Neubauer et al 2019). Our model provides a much more realistic force-deformation behavior compared to the small-deformation Hertz approximation, but is still simple and fast enough to allow the simulation of dense cell suspensions in reasonable time.…”

“…Therefore, a common approach to characterize the elasticity of cells utilizes the Hertzian theory, which describes the contact between two linear elastic solids [ (Johnson 2003), p. 90-104], but is limited to the range of small deformations (Dintwa et al 2008). Experimental measurements with medium-to-large deformations typically show significant deviations from the Hertz prediction, e.g., for cells or hydrogel particles (Neubauer et al 2019). Instead of linear elasticity, a suitable description of cell mechanics for bioprinting applications requires more advanced hyperelastic material properties.…”

Microscale Interrogation of 3D Tissue Mechanics

“…With more information about bead deformations, either through surface tracking (Lee et al, 2019) or full deformation tracking using submicron tracers incorporated in the beads, similar to what is done in TFM (Mohagheghian et al, 2018), all types of stresses experienced by the sensor can be probed, including compressive, tensile, and shear stresses, albeit with increasing computational costs. Additionally, Förster resonance energy transfer (FRET) fluorophore pairs have been recently incorporated to PEGbased microbeads, which exhibit a characteristic fluorescence shift upon global or local tissue deformations (Neubauer et al, 2019). Nevertheless, substantial calibration is required before any quantitative information on 3D stresses can be derived, especially when compared to FRET pairs that are conjugated to elastic 2D substrates (Kong et al, 2005).…”

“…Furthermore, the direction of molecular tension can be detected when combined with fluorescence polarization microscopy (Brockman et al, 2018). These synthetic tension probes can be applied to virtually any surface including stiff glass that is not suitable for TFM, and have potential applications in 3D systems when functionalized to ECM fibers or incorporated into elastic microbeads as discussed earlier (Neubauer et al, 2019).…”