(554f) Magnetically Responsive Gels for Enhancing Osteo-Differentiation By Controlling the Timing of Recruitment and Differentiation Factor Deliveries

Each year more than 6 million bone fractures happen in United States and lead to approximately 900,000 hospitalizations. Critical-sized bone fractures and defects traditionally require autogenous or allogenic grafting. Biomaterial scaffolds have been investigated as bone grafting materials to overcome limitations associated with autogenous and allogenic grafting (e.g., donor site morbidity, sensitivity, and limitability). While biomaterial scaffolds provide three dimensional structures for new bone growth and can be loaded with osteogenic factors to enhance that growth, traditional biomaterial scaffolds cannot adequately coordinate the sequence of events inherent to bone regeneration (bone progenitor recruitment, proliferation, and differentiation). More importantly, the timing, rate, and sequence of recruitment, proliferation, and differentiation factor deliveries needed for optimal osteo-differentiation are not yet known (and are likely different for different patients and different wounds). We hypothesized that the relatively timing of recruitment and differentiation factor deliveries could be optimized and that magnetically responsive scaffolding systems could enable flexible control over the timing of these deliveries.

Here, we investigated how the timing of osteo-differentiation factor delivery (delivery of Bone Morphogenetic Protein-2 (BMP-2)) impacted bone progenitor cell (D1 mouse Mesenchymal Stem Cells (mMSC)) population vs. time and osteo-differentiation (by measuring osteocalcein secretion Alizarin Red staining). mMSCs were plated at 1500 cells/cm² and exposed to BMP-2 at either early time points (starting on day 1) or later time points (delayed, starting on day 4) over the course of 14 days using the same integrated dose of BMP-2 (858 ng mL^-1 days). Results indicated that early delivery of BMP-2 slowed mMSC population growth, ultimately resulting in less cumulative osteocalcin secretion and matrix calcification than the conditions where BMP-2 delivery was delayed 4 days. This motivates the need for scaffolding materials capable of rapidly recruiting bone progenitors and delaying the presentation of BMP-2.

We therefore created a two-compartment biomaterial system consisting of (1) an outer-compartment made from porous gelatin that was designed to rapidly release Stromal Cell-Derived Factor-1a (SDF-1a)) to recruit and harbor mMSCs and (2) an inner ferrogel compartment capable of being deformed in the presence of hand-held magnets, delivering BMP-2 payloads to recruited mMSCs when magnetically deformed. We demonstrated that the outer compartment could be loaded with SDF-1a (0, 250, and 1000 ng per gelatin scaffold) and rapidly recruit mMSCs throughout the volume of the scaffold in vitro. Higher SDF-1a loadings resulted in more SDF-1a release and more rapid mMSC recruitment. Inner ferrogel compartments were capable of being loaded with BMP-2 (1000 ng per ferrogel), releasing BMP-2 at very low rates prior to magnetic stimulation (0.037 ng/h), and releasing BMP-2 more rapidly (0.588 ng/h) when magnetically stimulated. Additionally, we demonstrated that the timing of enhanced BMP-2 release could be magnetically regulated. Experiments were conducted where hand-held magnets were applied at day 3 or 7, revealing that enhanced BMP-2 release was generated on days 3 or 7, respectively.

In conclusion, these studies demonstrate that delayed BMP-2 delivery can result in improved mMSC osteodifferentiation and that magnetically responsive scaffolding systems can be used to delay the delivery of BMP-2 to recruited mMSCs. These magnetically responsive scaffolds will provide important tools for investigating how the timing and sequence of protein factor deliveries impacts regenerative outcome and can provide the means to clinically alter the course of therapies in real time according to updates in patient prognosis.

Contact Information: Name: Stephen M. Kennedy, Institution: University of Rhode Island, Department: Electrical, Computer, & Biomedical Engineering and Chemical Engineering, Phone #401-874-5295, Email: smkennedy@uri.edu.