Researchers from the Leibniz Institute of Polymer Research Dresden (IPF), Germany have developed and applied an atomic force microscopy (AFM)-based nanoindentation method to quantify the local stiffness of a sponge-like biohybrid hydrogel material made of synthetic star-shaped poly(ethylene glycol) and the highly sulfated, naturally occurring glycosaminoglycan heparin (termed starPEG-heparin cryogels). The reported approach provides new insights into the physical characteristics of those cell-instructive biomaterials.
Tissue engineering relies on three-dimensional cell carriers (scaffolds) to recapitulate key features of natural extracellular matrices (ECM) for replacing diseased or injured tissues in the human organism. In addition to the contained biomolecular signals, the morphological and mechanical properties of these scaffolds play an important role for the efficacy of the therapeutic concept. A recently introduced type of biohybrid cryogel scaffold (P. B. Welzel et al., Biomacromolecules 13 (2012) 2349-2358) used in this study was previously demonstrated to be advantageous due to its spongy, macroporous structure, its tunable physical and biomolecular characteristics and its toughness.
Cells colonizing this scaffold sense the local stiffness of the cryogel struts they attach to. As the cryogel struts are small (about 10 – 30 µm width) compared to the macropores (several ten to several hundred µm in diameter) and constitute only few percent of the scaffold volume, the experimental determination of their stiffness is challenging. The IPF team introduced an analytical approach based on visualizing the microarchitecture of very thin sections of the swollen cryogels before performing AFM-based nanoindentation measurements. This allowed for the exact positioning of the AFM sensor directly on the struts.
Comparing the strut stiffness of cryogel samples with data for non-spongy reference bulk hydrogels the researchers show that the cryogel formation at sub-zero temperatures results in an elevated stiffness of the cryogel struts due to the locally increased concentration of the polymeric gel precursor molecules. While this ‘cryoconcentration effect’ has been assumed in previous publications it has never been experimentally proved so far.
In sum, a new analytical strategy for the physical characterization of spongy cryogel materials was introduced and applied. The method is expected to become instrumental for adjusting cell-instructive biomaterials for regenerative therapies as well as for mechanistically exploring the formation of macroporous materials.
