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An annual event which brings together this community to connect in-person. Translational Advisory Board (TAB) members, Faculty Mentors, Trainees, and Affiliate postdocs come together for professional development, seminars, business meetings, scientific presentations, and team building.

The 2020 agenda included the following:

  • Six Sigma Design of Experiments training
  • Responsible Conduct of Research training
  • Venture capital project pitches to the TAB
  • Poster presentations
  • Networking events
  • Social dinners and activities
Find out more about T32 here

Improving the Bioactivity of Polymeric Bone Regenerative Scaffolds Through Physically and Chemically Incorporated Molecules

 

Segmental reconstruction of weight-bearing long bones is particularly challenging and can result in significant disability to patients. Although autografts and allografts are widely used in the clinic, the development of artificial bone graft substitutes (BGSs) continues to be an area of intense research. The advanced understanding of bone tissue microenvironment has directed the design of scaffold-based tissue engineering strategies in regenerative medicine. Particularly, bone tissue microenvironment composed of a hybrid matrix of hydroxyapatite nanocrystals and collagen fibers, a cellular phase of osteoblasts, osteoclasts, and osteocytes, and a signaling phase of osteogenic growth factors and/or cytokines. Therefore, to apply the concept of tissue engineering in bone tissue engineering, osteogenic growth factors and/or stem cells are incorporated into artificial scaffolds for improved osteoinductivity and conductivity in the development of BGSs. In our lab, BGSs based on E1001(1k), an intrinsically osteoconductive tyrosine-based polycarbonate, and beta-tricalcium phosphate (β-TCP) nanocrystals, were custom-fabricated (abbreviated as E1001(1k)/β-TCP scaffolds) and tested in various critical size calvarial models as carriers for rhBMP-2. In Chapter 2, the comparison of human-sized E1001(1k)/β-TCP scaffolds to a clinically used β-TCP-based BGS (i.e. chronOS®) with both constructs carrying the same dose of rhBMP-2 (281 ng/mm3) in a 30 mm critical size sheep tibia model was conducted. We found that quantitative histomorphometry analyses on the explants at 17 weeks showed no significant difference between the two treatment groups, indicating E1001(1k)/β-TCP scaffolds are as good as the clinically relevant chronOS® for tibial bone regeneration in sheep. More importantly, new bone regenerated in E1001(1k)/β-TCP scaffolds demonstrated a more physiological morphology, indicating ongoing bone remodeling into compact bone tissue. Our results show E1001(1k)/β-TCP scaffolds are promising BGS candidates for segmental bone regeneration in clinical trials.

A critical factor in mimicking bone microenvironment is the osteogenic growth factor, though rhBMP-2 being the most studied growth factor in the bone regeneration field, adverse effects associated with supraphysiological doses are reported. Therefore, it is necessary to establish a reliable in vitroin vivo correlation to determine the proper dose as well as to evaluate the efficacy of the delivery system. To date, the various study design in the efforts to determine proper BMP-2 delivery in vitro leads to has precluded comparative analysis. Due to the aforementioned concerns, a systematic evaluation of experimental parameters of the most widely used in vitro rhBMP-2 activity assays is reported in Chapter 3. We found that each model cell line (i.e. W-20-17, MC3T3 and C2C12) has an optimal concentration range over which it is most sensitive to rhBMP-2 induction. In addition, a correlation between protein concentration (as measured by enzyme-linked immunosorbent assay) and protein activity (as measured by alkaline phosphatase induction from W-20-17 cells) was established. It was found that the expression system used to produce the rhBMP-2 had the most significant effect on its activity and stability in vitro.

Though E1001(1k)/β-TCP scaffolds have been successfully utilized as carriers of rhBMP-2 to assist in the healing process of critical size bone defects, the scaffold alone treatments typically demonstrate limited osteoconductivity in vivo due to the bio-inert nature of E1001(1k). In Chapter 4, E1001(1k) analog polymers were functionalized with catechol for enhanced bioactivity. Catechol being the bioinspired functional group that is responsible for mussel adhesion to wet rock surfaces, has triggered great interest in the tissue engineering field, owning to its unique redox chemistry, chelation capability with metal ions and reaction with proteins. It was found that the modified polymers were able to support mesenchymal stem cell, human osteoblast and MC3T3 cell growth. In addition, the surface anchoring catechol groups assist nano-sized silver deposition on the polymer surface by immersion-coating in silver nitrate solution, and the decorated surface has good antibacterial properties against Escherichia coli and Staphylococcus aureus.


ENGINEERING L-TYROSINE BASED POLYCARBONATE COPOLYMERS AS DEGRADABLE DEVICES FOR LONG-TERM, LOCALIZED DELIVERY OF A HYDROPHOBIC MACROLIDE IMMUNOSUPPRESSANT TO PREVENT TRANSPLANT REJECTION

 

Advances in surgical techniques and immunology have enabled transplantation of vascularized composite tissue allografts (VCAs, e.g., upper extremity, face, genitourinary tissues) to treat complex soft tissue injuries and extremity amputations.  However, more than 85% of transplant recipients develop at least 1 episode of rejection within the first year, and over 50% experience multiple such episodes. Standard treatments involving oral or intravenous delivery of immunosuppressants result in systemic levels that are far higher than needed in the graft tissue. High systemic immunosuppressant levels increase the likelihood of widespread side effects including organ toxicity and increased risk of illness.  Furthermore, the effectiveness of oral administration is dependent on patient compliance with medications requiring twice-daily dosing.

This work presents the development of polymeric local drug delivery systems to address these limitations. A biodegradable, bioresorbable tyrosine-derived polycarbonate was used a carrier matrix loaded with standard immunosuppressants. The devices were implanted at the interface between host and donor tissue during transplant surgery where they provide long-term, localized immunosuppression and reduce the incidence of rejection episodes. Engineering steps were taken to establish reproducible fabrication protocols, stabilize devices during sterilization and storage, and control the early release profile. In vitro release testing elucidated a three-stage release profile that is impacted by changes in drug distribution and polymer matrix during degradation. Release from the devices was consistent over 6 months following the transition to the late-stage release profile, providing the consistency required for continual localized immunosuppression. In vivo testing in small animal models elucidated similar release profiles correlating with in vitro testing and directed dosing in non-human primate (NHP) VCA models. Implantation of the devices in a NHP VCA model prevented rejection episodes for over 60 days with minimal exogenous immunosuppression, reducing the injected drug required over this time period by up to 83%. This is the first example of effective localized immunosuppression in a model as rigorous as NHPs.

A second generation of localized delivery devices were developed to address diffusional limitations to associated with point-source devices. Immunosuppressant-loaded tyrosine-derived polycarbonate nanospheres (TyroSpheres) were incorporated into water-soluble, biodegradable pullulan films. In vitro release testing was carried out on a variety of loading levels, focusing on the effect of drug loading in TyroSpheres and TyroSphere loading in films. In vivo the films rapidly hydrated and dissolved upon contact with the tissue bed, depositing TyroSpheres from which the immunosuppressant is released. Later explantation of the site revealed no residual material in the tissue bed.

Finally, hierarchical scaffolds are explored using a novel airbrushing method for incorporation of microfiber mats throughout 3D printed structures. This hierarchical scaffold exhibited greater cell growth in vitro and improved cellular penetration in vivo. Furthermore, they were functionalized by decellularized ECM laid down by living cells, pro-angiogenic peptide sequences through click-chemistry reaction, and cell affecting drugs through incorporation in the airbrushing solution. This scaffold fabrication approach provides a tool for increasing the bioactivity of 3D printed scaffold for locally engineering the tissue environment.