Introduction: Hydrogels are promising biomaterials for regenerative medicine due to their flexible shape, resemblance with the extracellular matrix and capacity to deliver and release bioactive compounds. Especially, injectable hydrogels gained popularity due to their minimal invasive application . Fucoidan is a sulfated polysaccharide from the cell wall of brown algae with multiple bioactivities on osteogenesis, angiogenesis and inflammation [2, 3]. In this present study, we developed an injectable thermosensitive hydrogel to deliver fucoidan or cells in the context of bone repair. Biocompatibility and functionality were tested using human bone mesenchymal stem cells (MSC) and outgrowth endothelial cells (OEC). Both cell types represent suitable model systems to evaluate bone regeneration and vascularization .
Methodology: The thermosensitive injectable hydrogel was prepared using chitosan, collagen type I and β-glycerophosphate (β-GP). Chitosan in combination with β-GP forms a thermosensitive sol which gelates upon temperature increase . Collagen type I was integrated into the hydrogel formulation to mimic the bone tissue environment and to enhance biocompatibility. Different concentrations of commercially available fucoidan (F. vesiculosus, Sigma-Aldrich) or enzymatically-extracted fucoidan from F. evanescens  were encapsulated into the hydrogel. Basic properties such as gelation time, pH and the equilibrium swelling ratio (ESR) were determined. Further, the internal structure of the hydrogels was analyzed by scanning electron microscopy (SEM) after critical point drying. Cultures of human MSC and OEC either seeded on top of the gel (2D) or integrated into the gel (3D) were subjected to life staining and SEM to study the biocompatibility of the hydrogels.
Results: The hydrogels with encapsulated fucoidan gelated within 5 min at 37°C. Fucoidan encapsulation had only minor effects on the studied hydrogel properties. However, the encapsulation of high fucoidan amounts (500 µg/ml) caused a turbid sol and decreased the ESR. Additional experiments revealed that within two days 60 % and after six days 80 % of the fucoidan was released from the hydrogel. Life/dead stainings showed that MSC cultured on top and inside the hydrogels were vital, able to adhere and to spread after six days. OEC cultured on hydrogels with encapsulated enzymatically-extracted fucoidan appeared vital after six days and formed primitive prevascular structures.
Conclusion: This study documents the general applicability to encapsulate fucoidan into the described thermosensitive and injectable hydrogel with a release profile tested up to six days. The biocompatibility for MSC and the observed pro-angiogenic effect by fucoidan integrated into the gel are promising for future applications in bone regeneration.
- Liu, M. et al., Bone Research 5 (2017).
- Kim, B.-S. et al., J. Tissue Eng Regen Med. 12, e1311-e1324 (2018).
- Ohmes, J. et al., Mar Drugs 18 (2020).
- Kolbe, M. et al., Tissue Engineering Part A 17, 2199-2212 (2011).
- Chenite, A. et al., Biomaterials 21, 2155-2161 (2000).
- Nguyen, T. T. et al., Mar Drugs 18 (2020)