The potential of encapsulating “raw materials” in 3D osteochondral gradient scaffolds

Abstract

Scaffolds with continuous gradients in material composition and bioactive signals enable a smooth transition of properties at the interface. Components like chondroitin sulfate (CS) and bioactive glass (BG) in 3D scaffolds may serve as “raw materials” for synthesis of new extracellular matrix (ECM), and may have the potential to completely or partially replace expensive growth factors. We hypothesized that scaffolds with gradients of ECM components would enable superior performance of engineered constructs. Raw material encapsulation altered the appearance, structure, porosity, and degradation of the scaffolds. They allowed the scaffolds to better retain their 3D structure during culture and provided a buffering effect to the cells in culture. Following seeding of rat mesenchymal stem cells, there were several instances where glycosaminoglycan (GAG), collagen, or calcium contents were higher with the scaffolds containing raw materials (CS or BG) than with those containing transforming growth factor (TGF)-β3 or bone morphogenetic protein (BMP)-2. It was also noteworthy that a combination of both CS and TGF-β3 increased the secretion of collagen type II. Moreover, cells seeded in scaffolds containing opposing gradients of CS/TGF-β3 and BG/BMP-2 produced clear regional variations in the secretion of tissue-specific ECM. The study demonstrated raw materials have the potential to create a favorable microenvironment for cells; they can significantly enhance the synthesis of certain extracellular matrix (ECM) components when compared to expensive growth factors; either alone or in combination with growth factors they can enhance the secretion of tissue specific matrix proteins. Raw materials are promising candidates that can be used to either replace or be used in combination with growth factors. Success with raw materials in lieu of growth factors could have profound implications in terms of lower cost and faster regulatory approval for more rapid translation of regenerative medicine products to the clinic.

Microsphere-Based Gradient Implants for Osteochondral Regeneration: a Long-Term Study in Sheep

“…Rat mesenchymal stem cells seeded on 3D porous semi IPN scaffold secreted significantly higher amount of total GAG and collagen content and had more collagen type II when supplemented with TGF-β3 than TGF-β1 [44]. Our previous report on TGF-β3-chondroitin sulfate combination encapsulated in PLGA microspheres was found to favor chondrogenic differentiation of stem cells [20]. The rationale for these pilot Joints retrieved 1 year post-implantation were subjected to uniaxial compressive indentation and modeled with the biphasic theory, with the solid phase consisting of a biphasic material with a 'neo-Hookean' ground matrix and a spherical fiber distribution to model the collagen fibrillar matrix [33].…”

“…Hence, the current study investigated the potential of implanting gradient scaffolds engineered to recruit stem cells from the bone marrow into large defects for the purpose of regenerating bone and cartilage. We have previously investigated the potential of the microsphere-based scaffolds with gradients of growth factors, raw materials and combinations for both osteochondral tissue engineering in vitro and osteochondral regeneration in vivo in rabbits [10,20,[35][36]. This is our first report on the long-term outcome of the implant of these gradient scaffolds in critical size osteochondral defects (6 mm diameter × 6 mm depth) in a large animal model.…”

“…Instead, a 'raw materials' approach was taken whereby natural materials are leveraged both as building blocks to be incorporated into regenerating tissue and as signaling molecules to elicit desirable responses from infiltrating cells [40]. Our previous studies have demonstrated favorable cell responses to chondroitin sulfate [20,[41][42], and β-TCP is a well-known building block for bone regeneration, which is more easily integrated into regenerating bone than the hydroxyapatite we have used in our previous microsphere-based scaffold studies [10,35,43].…”

“…LFC: Lateral femoral condyle; MFC: Medial femoral condyle. centration of the TGF-β3 was estimated based on our calculations from gradient implants used for the osteochondral regeneration in critical size defects in rabbits and total volume of the defect site in sheep [20]. The gradient plugs used in group I-C received a drop-wise addition of 60 μl of DI water containing a total dose of 0.5 μg of IGF-1 per plug.…”

Mimetic Hierarchical Approaches for Osteochondral Tissue Engineering

“…In addition, therapeutic molecules can be surface-tethered to the microspheres [147]. Using "raw materials," i.e., components like chondroitin sulfate and bioactive glass, in 3D scaffolds was suggested for establishing continuous gradients of both material composition and signaling factors [148].…”