Multi-modal characterization of polymeric gels to determine the influence of testing method on observed elastic modulus

Abstract

Demand for materials that mechanically replicate native tissue has driven development and characterization of various new biomaterials. However, a consequence of materials and characterization technique diversity is a lack of consensus within the field, with no clear way to compare values measured via different modalities. This likely contributes to the difficulty in replicating findings across the research community; recent evidence suggests that different modalities do not yield the same mechanical measurements within a material, and direct comparisons cannot be made across different testing platforms. Herein, we examine whether “material properties” are characterization modality-specific by analyzing the elastic moduli determined by five typical biomaterial mechanical characterization techniques: unconfined-compression, tensiometry, rheometry, and micro-indentation at the macroscopic level, and microscopically using nanoindentation. These analyses were performed in two different polymeric gels frequently used for biological applications, polydimethylsiloxane (PDMS) and agarose. Each was fabricated to span a range of moduli, from physiologic to supraphysiologic values. All five techniques identified the same overall trend within each material group, supporting their ability to appreciate relative moduli differences. However, significant differences were found across modalities, illustrating a difference in absolute moduli values, and thereby precluding direct comparison of measurements from different characterization modalities. These observed differences may depend on material compliance, viscoelasticity, and microstructure. While determining the underlying mechanism(s) of these differences was beyond the scope of this work, these results demonstrate how each modality affects the measured moduli of the same material, and the sensitivity of each modality to changes in sample material composition.

Enzymatic Polymerization of Poly(glycerol-1,8-octanediol-sebacate): Versatile Poly(glycerol sebacate) Analogues that Form Monocomponent Biodegradable Fiber Scaffolds

“…An immobilized form of CALB, known as Novozym 435 (N435), is commercially available and has been commonly used by our group and others. − N435 is enantioselective and regioselective, enabling a one-pot synthesis of copolymerizations with multifunctional monomers (functionality ≥ 3). Furthermore, steric constraints in the active site of CALB decrease the tendency of two chains undergoing cross-linking reactions . This study builds on previous work where lipase catalysis was effective for preparing high-molecular-weight glycerol-containing polyesters .…”

“…Furthermore, steric constraints in the active site of CALB decrease the tendency of two chains undergoing cross-linking reactions. 26 This study builds on previous work where lipase catalysis was effective for preparing high-molecular-weight glycerol-containing polyesters. 27 Enzyme catalysts serve as a substitute for toxic heavy metal catalysts in polymer synthesis, while also preserving the structures of thermally or chemically sensitive monomers.…”

“…Tensile material property characterization was conducted as previously described. ,, PGOS film samples were cut into dog bone shape, according to ASTM standard D412-16 (gauge length = 30 mm), and electrospun PGOS fiber mat samples were loaded in oak-tag I-frames, to facilitate gripping and prevent damage prior to testing. A digital caliper was used to measure initial specimen dimensions (width, thickness, and gauge length) of all samples, before testing in a vertical universal testing machine (model 500LE2-1, TestResources, Shakopee, MN) with a 110 N load cell.…”

Capillary-Induced Wrinkle-to-Fold Transitions Under Biaxial Compression

“…The critical strain ε c for the onset of the wrinkle formation is given by ε c = 0.25­(3 E̅ s / E̅ f ) 2/3 , where E̅ = E /(1 – ν 2 ) is the plane-strain modulus with E being the Young’s modulus, ν the Poisson’s ratio, and the subscripts f and s denote the film and substrate, respectively . Using E f = 150 GPa, ν f = 0.38, E s = 0.04 MPa, and ν s = 0.5, we obtain ε c = 0.00003. The compressive strain ε in the film on the substrate can be estimated by measuring the change in the surface area due to wrinkle formation.…”

Mechanical Characterization of Synthetic Gels for Creation of Surrogate Hands Subjected to Low-Velocity Impacts

“…However, gel materials are challenging to characterize using conventional testing methods due to the low internal forces that these materials can withstand. A recent study found that different testing modalities do not necessarily produce the same mechanical measurements within a material [ 28 ]. This characteristic is more evident for high to very-high loading (or strain) rates in which the mechanical behavior is a function of the strain rate.…”