Mechanical testing of hydrogels in cartilage tissue engineering: beyond the compressive modulus

Abstract

Injuries to articular cartilage result in significant pain to patients and high medical costs. Unfortunately, cartilage repair strategies have been notoriously unreliable and/or complex. Biomaterial-based tissue-engineering strategies offer great promise, including the use of hydrogels to regenerate articular cartilage. Mechanical integrity is arguably the most important functional outcome of engineered cartilage, although mechanical testing of hydrogel-based constructs to date has focused primarily on deformation rather than failure properties. In addition to deformation testing, as the field of cartilage tissue engineering matures, this community will benefit from the addition of mechanical failure testing to outcome analyses, given the crucial clinical importance of the success of engineered constructs. However, there is a tremendous disparity in the methods used to evaluate mechanical failure of hydrogels and articular cartilage. In an effort to bridge the gap in mechanical testing methods of articular cartilage and hydrogels in cartilage regeneration, this review classifies the different toughness measurements for each. The urgency for identifying the common ground between these two disparate fields is high, as mechanical failure is ready to stand alongside stiffness as a functional design requirement. In comparing toughness measurement methods between hydrogels and cartilage, we recommend that the best option for evaluating mechanical failure of hydrogel-based constructs for cartilage tissue engineering may be tensile testing based on the single edge notch test, in part because specimen preparation is more straightforward and a related American Society for Testing and Materials (ASTM) standard can be adopted in a fracture mechanics context.

FIG. 1.

Venn diagram emphasizing the distinct fields of (1) hydrogels in tissue engineering, (2) cartilage biomechanics, and (3) fracture mechanics. The purpose of this review is to identify the common ground for these distinct fields, more specifically, to understand how to best identify fracture mechanics methods most suitable for evaluating both hydrogels and cartilage. The urgency for identifying this common ground is high in light of the advanced state of hydrogels in cartilage regeneration, where fracture is ready to stand alongside stiffness as a functional design requirement. Color images available online at www.liebertpub.com/teb

FIG. 2.

Different modes for testing fracture toughness of cartilage summed up by Ahsan and Sah: (A) Mode I—Opening mode; (B) Mode II—Shearing mode; (C) Mode III—Tearing mode. Modes I and III have been the preferred methods used for evaluations of cartilage toughness. Color images available online at www.liebertpub.com/teb

FIG. 3.

Modified single edge notch (MSEN) test for cartilage (Mode I). Note that the cartilage remains affixed to the bone. The crack made before testing extends through the bone and continues a fixed distance into the cartilage, providing a rigid gripping point with the bone. Color images available online at www.liebertpub.com/teb

FIG. 4.

SEN test (Mode I). Note that the cartilage is not affixed to bone, unlike the MSEN test. Here, the cartilage is gripped directly. Color images available online at www.liebertpub.com/teb

FIG. 5.

Loading mode III—Trouser tear test: The grips grab the bone parts to tear through the cartilage, where the bone part is the area above the dot-dashed line and the cartilage part is the area below the dot-dashed line. The dotted line indicates the initial crack from the bone into the cartilage. The tearing force is applied only on the bone section. Color images available online at www.liebertpub.com/teb

FIG. 6.

A penetration or fracture defect in the surface of intact cartilage, which is attached to the underlying subchondral bone, is created by a small conical indenter. Color images available online at www.liebertpub.com/teb

FIG. 7.

(Top) Standardized rectangular shape for a trouser tear test with a hydrogel: w=5 mm, L=50 mm, h=7.5 mm, the length of the initial notch is 20 mm; (Bottom) trouser tear test: F is the tearing force, V_p is the pulling velocity, and V is the crack velocity. Color images available online at www.liebertpub.com/teb