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. 2011 Oct;2(4):374-83.
doi: 10.1177/1947603511402320.

Acrylamide Polymer Double-Network Hydrogels: Candidate Cartilage Repair Materials with Cartilage-Like Dynamic Stiffness and Attractive Surgery-Related Attachment Mechanics

Markus P Arnold  1 , Alma U Daniels  2 , Sarah Ronken  2 , Helena Ardura García  3 , Niklaus F Friederich  1 , Takayuki Kurokawa  4 , Jian P Gong  5 , Dieter Wirz  6
Affiliations

Acrylamide Polymer Double-Network Hydrogels: Candidate Cartilage Repair Materials with Cartilage-Like Dynamic Stiffness and Attractive Surgery-Related Attachment Mechanics

Markus P Arnold et al. Cartilage. 2011 Oct.

Abstract

Background: In focal repair of joint cartilage and meniscus, initial stiffness and strength of repairs are generally much less than surrounding tissue. This increases early failure potential. Secure primary fixation of the repair material is also a problem. Acrylamide polymer double-network (DN) hydrogels are candidate-improved repair materials. DN gels have exceptional strength and toughness compared to ordinary gels. This stems from the double-network structure in which there is a high molar ratio of the second network to the first network, with the first network highly crosslinked and the second loosely crosslinked. Previous studies of acrylic PAMPS/PDMAAm and PAMPS/PAAm DN gels demonstrated physicochemical stability and tissue compatibility as well as the ability to foster cartilage formation.

Methods: Mechanical properties related to surgical use were tested in 2 types of DN gels.

Results: Remarkably, these >90%-water DN gels exhibited dynamic impact stiffness (E*) values (~1.1 and ~1.5 MPa) approaching swine meniscus (~2.9 MPa). Dynamic impact energy-absorbing capability was much lower (median loss angles of ~2°) than swine meniscus (>10°), but it is intriguing that >90%-water materials can efficiently store energy. Also, fine 4/0 suture tear-out strength approached cartilage (~2.1 and ~7.1 N v. ~13.5 N). Initial strength of attachment of DN gels to cartilage with acrylic tissue adhesive was also high (~0.20 and ~0.15 N/mm(2)).

Conclusions: DN gel strength and toughness properties stem from optimized entanglement of the 2 network components. DN gels thus have obvious structural parallels with cartilaginous tissues, and their surgical handling properties make them ideal candidates for clinical use.

Keywords: cartilage repair; gels; loss angle; modulus; suturing.

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Conflict of interest statement

Declaration of Conflicting Interests: The authors declared no potential conflicts of interest with respect to the authorship and/or publication of this article.

Figures

Figure 1.
Figure 1.
Example of the force and displacement data for an articular cartilage specimen tested in nutrition (N)–related slow sinusoidal microindentation (SSMI) to a depth resulting in cycles at 0.1 Hz with a maximum speed of 0.015 m/s. The loss angle was calculated from the time length of the phase shift between displacement and load curves in both SSMI mode and gait (G)–related fast impact microindentation (FIMI) mode.
Figure 2.
Figure 2.
Gait-related (G) mode modulus and loss angle measurement device, mounted on a stable loading frame, equipped with a load cell and laser positioning system.
Figure 3.
Figure 3.
Spherical steel indenter, 3.2 mm in diameter, mounted on a material testing system (MTS Synergie 100, MTS Systems Corporation, Eden Prairie, MN), indenting a 3-mm-thick PAMPS/PDMAAm hydrogel specimen. This configuration was used for the nutrition (N)–related slow sinusoidal microindentation (SSMI) tests.
Figure 4.
Figure 4.
Comparison of stiffness properties of PAMPS double-network hydrogels (DN gels) with different second-network components (PDMAAm and PAAm). Aggregate modulus E* and loss angle δ measured in both gait-mode (G) fast impact microindentation (FIMI) and nutrition-mode (N) slow sinusoidal microindentation (SSMI). See text for complete descriptions of test modes. For each DN gel and mode, the left bar shows box and whisker plot showing median and quartiles, and the right bar shows mean and standard deviation.
Figure 5.
Figure 5.
Tear-out forces for 4/0 (0.15-mm diameter) braided suture versus crosshead displacement. Three replicate tests for each 3-mm-thick gel specimen. Median maximum force of PAMPS/PDMAAm was 2.1 N, and median maximum force of PAMPS/PAAm was 7.4 N.
Figure 6.
Figure 6.
Pull-off force normalized by contact area versus crosshead displacement for double-network (DN) specimens attached to a glass plate using an acrylic tissue adhesive. Nominal area of contact = 3 × 10 mm. Three replicate tests for each DN hydrogel. Median maximum pull-off force of PAMPS/PDMAAm was 0.23 N/mm2, and median maximum pull-off force of PAMPS/PAAm was 0.18 N/mm2.
Figure 7.
Figure 7.
Pull-off force normalized by contact area versus crosshead displacement for double-network hydrogel (DN gel) specimens attached to porcine articular cartilage using an acrylic tissue adhesive. Three replicate tests for each DN gel. Median maximum pull-off force of PAMPS/PDMAAm was 0.20 N/mm2, and median maximum pull-off force of PAMPS/PAAm was 0.15 N/mm2.
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