More to come soon...
The classical paradigm of tissue engineering has been the cell-seeded scaffold to generate a living tissue construct prior to implantation in static culture or bioreactor systems. We would like to move the paradigm towards in situ tissue engineering, meaning the modulation of tissue via an implanted biomaterial that can be cell-free but influences cellular action in situ via delivery of small molecules. This can include homing cells to the construct, thus sculpting the healing and repair response.
Scarring, a surplus of collagen deposition, threatens the integration of implanted devices and tissue constructs into host tissue by enshrouding them in an avascular fibrotic capsule which effectively sequestrates them. To curb this process we employ small chemicals, prolyl hydroxylase inhibitors that interfere with collagen synthesis and secretion. The ultimate goal is to reprogram myofibroblasts, the cellular culprits of fibrosis, into a more benign phenotype. For this purpose we have been evaluating the histone deacetylase inhibitor SAHA for its antifibrotic purposes (Wang et al., 2009).
The prolyl hydroxylase inhibitors which we employ for antifibrosis also stabilise HIF-1α, an angiogenic master switch. We are now developing these substances into advanced functional biomaterials that prevent peri-implantation fibrosis while inducing mesenchymal cells in the host tissue stroma to induce endothelial sprouting into these implanted biomaterials. Current strategies involve the chemical coupling or incorporation of 2,4-pyridine dicarboxylic acid or ciclopirox olamine into materials that have been approved for medical use such as gelfoam or hydrogels. Based on our earlier work in vitro (Raghunath et al., 2009), we have proven this concept in the rat renal pouch model (Sham et al., 2014). We have also discovered that the combination of prolyl hydroxylase inhibitors and sphingosine-1-phosphate is particularly angiogenic (IP; Lim et al., 2013).
We are currently developing this work towards an advanced wound healing product for chronic skin wounds (IMB, A*STAR), but envision this system to also be useful for cardiac patches or non-union bone fractures.
These materials either directly influence the tissue composition around them or function as a homing beacon for a variety of cell types to modulate inflammation, remodeling and repair locally. We have successfully incorporated a range of diverse active compounds simultaneously into electrospun microfibers (growth factors, vitamins, hormones) and demonstrated that their bioactivity is preserved after release (Peh et al.; in revision). Preliminary data suggest that electrospun meshes with these ingredients are able to achieve epithelial coverage in splinted skin wounds that normally would not re-epithelize.
In a MINDEF (Ministry of Defense of Singapore)-funded project we have evaluated this system for acute wounds using electrospun meshes as capture systems for bone marrow derived mesenchymal stromal cells (autologous or allogeneic) prior to implantation. We envision this system also to be very useful to be evaluated in chronic wounds. We have evaluated this system with human MSCs in immunosuppressed rats. We have implemented splinted wound model in the Zucker diabetic fatty (ZDF) rat in the Institute for Medical Biology to test this system further.
We have been able to incorporate an adipogenic induction cocktail into electrospun fiber meshes and could direct adipogenic differentiation of mesenchymal stem cells without the addition of differentiating factors into the culture medium; the data show potential for building a device for circulating or resident stem cells with a view to locally "brown" (induce a brown phenotype in) white adipose tissue. This project is in its very early stage, but very exciting as a therapeutic vision.