Drug delivery systems which can control the rate of drug delivery with a possibility of site specific administration are desirable for multiple applications. Localized delivery not only increases therapeutic efficacy of the drugs but also reduces their systemic effects and toxicity. This is particularly true for intra-articular injections which are possible for many joints and are helpful for treating arthritic and painful joints. Recently, platelet rich plasma (PRP) is being extensively investigated for treatment of osteoarthritis. PRP is a concentrated assortment of growth factors and cytokines derived from the patients’ own platelets. The preparation is fairly simple and rapid and has been considered safe to use being derived from autologous protein, thus minimizing side effects . Furthermore, direct intra-articular injection of PRP has been shown to be useful in healing and reducing pain in osteoarthritis . However, an effective method to achieve sustained delivery of PRP is still required.
In this study we have developed nanoporous microgels comprised of multiarm polyethylene glycol (PEG) acrylate (PEG-Ac) with tunable degradation and controlled release characteristics for delivery of PRP-derived growth factors. The microgels were prepared by electrospraying using Michael’s addition reaction between acrylate and thiol to facilitate gelation. Microgels of size range 70-300 µm were obtained via electrospraying by manipulation of spraying parameters. The microgels had a mesh size range of 7-10 nm. The microgel physical characteristics were modulated depending upon the fabrication parameters and gelation time for PEG-Ac and dithiol crosslinker combination. The microgel degradation time ranged from couple of hours to as long as 31 days. The degradation rates were correlated with number of ester groups in the dithiol crosslinker, crosslinking efficiency, and pH used for microgel formation, thus providing easy control over the release and degradation profiles of the microspheres. Mass loss studies of unloaded microgels in simulated synovial fluid (SSF) showed complete loss of hydrogel structure at 12 days. Furthermore, we also studied the possible storage methods of these microgels including lyophilization, or freezing the microgels at -80oC. Neither of the storage methods influenced the microgel shape, size, swelling or mechanical stability.
Direct encapsulation of proteins in the microgels during electrospraying did not show any loss in their secondary structure as confirmed by circular dichroism and in vitro biological activity analysis. Release kinetics for several model proteins varying in size and hydrodynamic radii was found to be dependent on the protein size, as well as mesh size and degradation rate of the hydrogel. The amount of total protein release increased over time corresponding to increase in hydrogel mesh size. PRP loaded microgels were similar in size to unloaded particles with an encapsulation efficiency of 75%. PRP release from the microspheres showed an initial burst release followed by a sustained release for period of 10 days in serum containing media. Multiplex growth factor analysis of the releasates showed a size dependent temporal release of the growth factors (PDGF-BB, FGF-b, TGF-β) from the PRP containing microgels. The PRP releasates from the microspheres were biologically active and could induce proliferation in fibroblast over a period of 7 day.
The study shows the potential of using PEG microgels for encapsulating and delivering PRP biologically active growth factors in a sustained manner as controlled by hydrogel mesh size. The mild chemistry used for making the hydrogels ensured maintenance of growth factors’ activity and other related chemokines found in PRP. Additionally, the microgels can be fabricated in size range so as to facilitate direct injection at the site of injury, thus combining the benefits of localized targeted delivery with sustained release. This delivery device can be useful in treatments of conditions such as osteoarthritis.
 I. Andia, N. Maffulli, Platelet-rich plasma for managing pain and inflammation in osteoarthritis, Nat Rev Rheumatol, 9 (2013) 721-730.
 G. Filardo, E. Kon, R. Buda, A. Timoncini, A. Di Martino, A. Cenacchi, P.M. Fornasari, S. Giannini, M. Marcacci, Platelet-rich plasma intra-articular knee injections for the treatment of degenerative cartilage lesions and osteoarthritis, Knee Surg Sports Traumatol Arthrosc, 19 (2011) 528-535.