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Collagen/nano-hydroxyapatite scaffolds as in situ forming hydrogel for the hard tissue engineering

Development and biocompatibility of the injectable collagen/nano-hydroxyapatite scaffolds as in situ forming hydrogel for the hard tissue engineering application

Injectable hydrogels attract more attention to hard tissue engineering for the fulfilment of the defects with irregular shapes. Therefore, the researchers investigated the biocompatibility and immune response to the injectable PCL-PEG-PCL-Col/nHA hydrogels in a mouse model. The histological examination was done via H&E. The activation of the immune cells was evaluated by using antibodies against the CD68, CD4, and CD8 markers. The expression of CCL-2, BCL-2, IL-10, and CD31 genes was measured. Moreover, serum levels of the ALT, ALP, AST, and Urea were detected. The results of the chemical analysis showed that the collagen and Nano-hydroxyapatite were successfully integrated into the PCLPEG-PCL hydrogels. The histological examination revealed a delayed biodegradation rate after the addition of the collagen and Nano-hydroxyapatite. No prominent pro-inflammatory response was found at the site of the injection. There are no significant differences in the levels of the CD68 and CD8/CD4 lymphocyte ratio among groups (p > .05). The expression of the CD31, IL-10 was significantly increased in the PCL-PEG-PCL-Col/nHA hydrogel (p < .05). ALT, ALP, AST, and Urea levels were not altered preand post-transplantation of the hydrogels (p > .05). These in vivo results demonstrated that the injectable PCL-PEG-PCL-Col/nHA hydrogels are biocompatible and suitable for further research in hard tissue regeneration.

Regeneration strategies in the craniofacial region for repairing and reconstructing the damaged hard tissues including bones, teeth, and cartilage should mimic or promote the oral developmental processes by using the biomaterials to induce the tissue formation via stimulation of the specific cellular function for regaining the function and aesthetic in this area.

In general, hard tissue engineering has revolutionized the treatment of injuries by overcoming the conventional drawbacks and has become a promising approach for healing injured tissues. Bone, tooth, and cartilage are complex biomineralized hierarchical structures containing the Nanohydroxyapatite (nHA) and collagen.

Hence, the ideal alternative to the conventional reconstruction methods should mimic the distinctive properties of the natural host tissue and promote regeneration. Tissue engineering scaffolds should be biocompatible, biodegradable, and have similar composition natural bone extracellular matrix (ECM). This similarity between ECM and scaffold structure could provide a distinct niche for cell migration, adhesion, and proliferation similar to the natural microenvironment, which is described for the tissue.

It should be noted that the fabricated scaffolds should be biocompatible without the induction of the immune responses to avoid producing inflammatory responses, acute immunogenicity, or cytotoxicity for the transplant cells, tissues, and organs.1

 FE-SEM image of synthesized nanoparticles, scaffolds, and the corresponding diameter distributions (A) PCL-PEG-PCL-Col; (B) PCL-PEG-PCL-Col/nHA; (C) Nano hydroxyapatite particles. EDX; (D) synthesized nano-hydroxyapatite particles; (E) PCL-PEG-PCL-Col/nHA.
FE-SEM image of synthesized nanoparticles, scaffolds, and the corresponding diameter distributions (A) PCL-PEG-PCL-Col; (B) PCL-PEG-PCL-Col/nHA; (C) Nano hydroxyapatite particles. EDX; (D) synthesized nano-hydroxyapatite particles; (E) PCL-PEG-PCL-Col/nHA.

The sol-gel transition of PCL-PEG-PCL-Col and PCL-PEG-PCL-Col/nHA scaffolds.

Gross and histological examination of the transplantation site after 14 days (A–C). In group PCL-PEG-PCL, the transplant mass was considerably biodegraded while PCL-PEG-PCL-Col and PCL-PEG-PCL-Col/nHA exhibited a slow biodegradation rate. Bright-field imaging showed the lack of PCL-PEG-PCL substrate beneath the epidermis while remnants of PCL-PEG-PCL-Col and PCL-PEG-PCL-Col/nHA were surrounded by fibrotic tissue. These data showed the stability of PCL-PEG-PCL-Col and PCL-PEG-PCL-Col/nHA hydrogels in in vivo conditions after 14 days.

1. https://doi.org/10.1080/21691401.2021.1877153