Microengineered smart material probes to measure local 3D tissue stiffness In situ

摘要

Introduction: The stiffness of the extracellular matrix influences various cellular functions including cell development and differentiation. The extracellular matrix can be remodelled by cells, an important process in disease progression. Current techniques to measure extracellular matrix stiffness are limited to bulk measurements which cannot resolve local variations within a tissue, or are limited to two-dimensional surface measurements via AFM. These techniques are often destructive and limited to measurement of stiffness in a single direction, which does not capture the mechanical complexity of biological systems. Here, we develop distributable, biocompatible, microscale sensors that can be embedded in cell-laden tissues to monitor local mechanical stiffness. N-isopropylacrylamide (NIPAAm) hydrogels exhibit a significant change in shape when cooled below the lower critical solution temperature of 32°C. However, the force stroke of this expansion is small and poorly defined. We characterized these small forces by monitoring temperature-induced deformation of embedded microgels in stiffness-tunable hydrogel matrices (Fig. 1), and developed a mechanosensor to monitor local mechanical properties in live tissues with high spatial and temporal resolution. Materials and Methods: pNIPAAm hydrogel beads were synthesized by free-radical polymerization in an oil/water two-phase system using established protocols. A functionalized fluorescent molecule was incorporated into the hydrogel polymer backbone to enable optical measurement of bead sizes when embedded within engineered tissues. pNIPAAm beads were then embedded in polyacrylamide hydrogel matrices, and fluorescent microscopy was used to measure differences in hydrogel size as a function of temperature. Results and Discussion: When pNIPAAm hydrogel beads were cooled from 37°C to room temperature, the hydrogel beads expanded in the matrix, (Fig. 1). Expansion of the hydrogel beads was limited by the stiffness of the surrounding matrix, (Fig. 2C). and the concentration of NIPAAm and BIS, (Fig. 2B). The hydrogel beads showed a greater expansion with increasing concentration of NIPAAm and decreasing concentration of crosslinking agents. A greater expansion provides greater sensitivity in measuring stiffness. This mechanosensor provides high sensitivity in measuring the stiffness of matrices below 1 kPa, with a significant reduction in sensitivity in stiffer matrices, (Fig. 2C). Conclusion: Temperature-sensitive hydrogel smart material microbeads are able to exert sufficient forces to deform soft 30 matrices. This approach is hence extremely promising for mapping tissue mechanics by temporarily reducing the culture temperature, and monitoring bead deformation. The change in shape that results from the cooling of these hydrogel beads is reversible, allowing repeated measurements to be made from a single sensor. In addition, monitoring bead shape in addition to size may provide unique insights into anisotropic stiffness changes within 3D matrices.