Nondestructive Evaluation of Hydrogel Mechanical Properties Using Ultrasound

Abstract

The feasibility of using ultrasound technology as a noninvasive, nondestructive method for evaluating the mechanical properties of engineered weight-bearing tissues was evaluated. A fixture was designed to accurately and reproducibly position the ultrasound transducer normal to the test sample surface. Agarose hydrogels were used as phantoms for cartilage to explore the feasibility of establishing correlations between ultrasound measurements and commonly used mechanical tissue assessments. The hydrogels were fabricated in 1–10% concentrations with a 2–10 mm thickness. For each concentration and thickness, six samples were created, for a total of 216 gel samples. Speed of sound was determined from the time difference between peak reflections and the known height of each sample. Modulus was computed from the speed of sound using elastic and poroelastic models. All ultrasonic measurements were made using a 15 MHz ultrasound transducer. The elastic modulus was also determined for each sample from a mechanical unconfined compression test. Analytical comparison and statistical analysis of ultrasound and mechanical testing data was carried out. A correlation between estimates of compressive modulus from ultrasonic and mechanical measurements was found, but the correlation depended on the model used to estimate the modulus from ultrasonic measurements. A stronger correlation with mechanical measurements was found using the poroelastic rather than the elastic model. Results from this preliminary testing will be used to guide further studies of native and engineered cartilage.

Nondestructive Techniques to Evaluate the Characteristics and Development of Engineered Cartilage

“…We used hydrogels as surrogates for cartilage to develop correlations between mechanical properties of cartilage and SOS, density and solid and fluid volume fractions. 95, 173 Most such correlations have been identified in native, enzyme treated and osteoarthritic cartilage, 69, 117, 129, 165, 175 but it is not unreasonable to suggest that such correlations will also exist for engineered cartilage. With respect to composition, proteoglycan, collagen and water content are significantly correlated with R, and SOS, IRC and URI.…”

Quantitative Ultrasound for Nondestructive Characterization of Engineered Tissues and Biomaterials

“…As an example, in one study, the speed of sound was measured in agarose hydrogels at concentrations ranging from 1-10%. 53 Speed of sound measurements were then used to estimate the moduli of these hydrogels using both elastic and poroelastic models. 53 …”

“…53 Speed of sound measurements were then used to estimate the moduli of these hydrogels using both elastic and poroelastic models. 53 …”

Pre-clinical Characterization of Tissue Engineering Constructs for Bone and Cartilage Regeneration

“…However, fixtures may be designed to reproducibly position the instrument to minimize user variability, which has been demonstrated with an US device used to couple acoustics with mechanical properties. 125 Additionally, measurements may vary depending on the penetration depth of the indenter, so appropriate thresholds should be established before testing. 84 An alternative to contact indentation to consider is water jet indentation, which uses water instead of a physical probe for mechanical measurement, and has been evaluated in tissue phantoms, ex vivo plugs, and a rabbit osteoarthritis model with favorable correlation to contact indentation.…”