Hydrogels are highly hydrophilic polymers with very high water content, typically from 70% to 99 %. The large water content of hydrogels makes them extremely biocompatible and as a result there is great interest in developing hydrogels for numerous biomedical applications, from tissue engineering to substrates for cell culture. When developing materials for cell culture and for bio adhesives it is important to be able to tune the mechanics of the gels to control their physical performance but also their interactions with cells and tissues. Hence, characterisation of the mechanics of these materials using accurate and robust protocols is essential. Numerous methods have been used to characterise the mechanics of soft hydrogel materials, ranging from compression testing to characterise the bulk properties to localised testing methods such as nanoindentation[1]. However, methods used to characterise bulk properties can show discrepancies with methods to characterise local mechanical properties. It has been shown that mechano-transduction between cells and their surrounding substrate plays an important role in determining cell behaviour[2],[3]. As a result when studying the effect of substrate mechanics on cell behaviour it is important that the substrates can be characterised in a way that gives data representative of what the cells will sense.
Hydrogel materials, such as carboxymethyl cellulose (CMC), are used frequently as dental adhesives. The mechanics of CMC gels have been shown to be tuneable by introducing ionic crosslinks by adding ionic salts[4] however this process has not been thoroughly quantified. The effects of ionic binders, and other crosslinking methods, on other polysaccharides have been extensively studied and these methods will be utilised to try and make tuneable CMC gels[5]. Furthermore CMC gels are extremely heterogeneous and finding methods to characterise the heterogeneity of these gels is of great interest when trying to develop novel dental adhesives.
In this project, different testing methods were explored to characterise the bulk and local mechanical properties of gels and other soft materials such as PDMS. Specifically, materials were tested by indentation, compression, rheology and AFM. Adhesion to these materials was also investigated. Discrepancies between testing methods were examined in particular. AFM was used to study the heterogeneity of samples and also their adhesion to other surfaces.
Importantly, we find that interfaces are the main source of discrepancy between the different testing protocols used. Controlling the surface chemistry of the geometries of the instruments used for these measurements allowed us to identify these effects and quantify them. Hence the contribution of interface chemistry and heterogeneity contributes both to the nanoscale mechanical properties of hydrogels and biomaterials, but also to the way bulk mechanics is sensed and probed (via rheology for example).
Figure 1 Carboxymethyl Cellulose dental adhesive failure under tensile loading monitored by optical microscopy
References:
[1] Oyen, M.L. Mechanical characterisation of hydrogel materials. International Materials Reviews 59, 44-59 (2014).
[2] Trappmann, B. et al. Extracellular-matrix tethering regulates stem-cell fate. Nature Materials 11, 642-649 (2012).
[3] Wen, J.H. et al. Interplay of matrix stiffness and protein tethering in stem cell differentiation. Nat Mater 13, 979-987 (2014).
[4] Clasen, C. & Kulicke, W.M. Determination of viscoelastic and rheo-optical material functions of water-soluble cellulose derivatives. Progress in Polymer Science 26, 1839-1919 (2001).
[5] Lee, K.Y. & Mooney, D.J. Alginate: Properties and biomedical applications. Progress in Polymer Science 37, 106-126 (2012).