Background: The architecture of the native enthesis (tendon/bone junction), consists of an organized gradient of bone, tendon and cartilage tissues. This complex tissue structure effectively transmits the interactions between dynamic muscle tissues and the rigid skeleton. Healed tendon repairs fail to regenerate the appropriate tissue architecture and composition present in the native enthesis.

Objective: To create a biologically compatible scaffold that recapitulates the transition of nano-mechanical properties found in the native enthesis.

Methods: Wild-type mouse Achilles tendon-bone composite tissues were harvested and flash frozen. The modulus of elasticity for each sample was evaluated using atomic force microscopy (AFM). Force curves were generated, and modulus values calculated using a Hertz-Sneddon model. Biocompatible inks (bio-inks) composed of gelatin methacryloyl (GelMA), and other physiologically relevant additives, such as hydroxyapatite and collagen I were fabricated to generate hydrogels of different nano-mechanical properties as assessed using AFM. The various concentrations of GelMA, additives, and photo cross-linking parameters were then adjusted to more closely replicate the transition of nano-mechanical properties found in native enthesis tissues.

Results:

The native mouse enthesis modulus for bone, tendon and cartilage tissues were 6.5 +/- 2.6 GPa, 111.8 +/- 47.7 MPa, and 36.7 +/- 29.2 MPa respectively. Several different bio-ink formulations were tested to best replicate the modulus values of the native enthesis. The trials are currently ongoing. The results to date are represented in Table 1.

Conclusions: Three distinct “tissues” were created with different concentrations of GelMa and other physiologically-relevant additives. These tissues differ in composition and nano-mechanical properties. The final bio-inks will later undergo 3D-printing to create a biomimetic scaffold with a gradient of nano-mechanical properties that replicates those of the native enthesis. Future directions include the synthesis of a composite scaffold with three distinct layers, each of which supports the differentiation of MSCs into three distinct phenotypes reflective of the native enthesis.

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