Exploration of biomaterial-tissue integration in heterogeneous microporous annealed particle scaffolds in subcutaneous implants over 12 months.

Microporous annealed particle (MAP) scaffolds are comprised of hydrogel microparticles with inter- and intra-particle cross-links that provide structure and cell-scale porosity, making them an increasingly attractive option for injectable tissue augmentation. Many current injectable biomaterials create a substantial foreign body response (FBR), while MAP scaffolds mitigate this response and have the potential to facilitate the formation of new tissue, though this de novo tissue formation is poorly understood. Here, we leverage a subcutaneous implant model to explore the maturation of MAP implants with and without heparin microislands (µislands) over one year to identify the effect of bioactive particles on scaffold maturation. Implants were measured and explanted after 1, 3, 6, and 12 months and analyzed using immunofluorescence staining and RNA-sequencing. No fibrous capsule or significant FBR was observed, and though a significant amount of MAP remains at 12 months, we still see a volume decrease over time. Heparin µislands facilitate increased cell infiltration and recruit a wider variety of cells at 1 month than blank MAP scaffolds, although this effect diminishes after 3 months. Transcriptomics reveal a potential activation of the complement-mediated immune response at 12 months in both groups, possibly associated with pore collapse in the implants. A single 12-month sample avoided this outcome, yielding complete cell infiltration, vascularization, and substantial matrix deposition throughout. Future work will characterize the effect of implantation site and facilitate increased matrix deposition to support the scaffold and prevent pore collapse. STATEMENT OF SIGNIFICANCE: Injectable biomaterials are increasingly used clinically for soft tissue augmentation and regeneration but still face significant issues from the foreign body reaction. While some materials intentionally promote this response to stimulate collagen deposition, porous materials like MAP scaffolds can mitigate the immune response and allow for true tissue integration. However, this integration is poorly understood, particularly on long timescales, as traditional materials are dominated by inflammatory signals. In this work, we leverage a minimally inflammatory subcutaneous implant to investigate the maturation of MAP scaffolds with and without bioactive heparin-containing particles. The results presented here contribute a better understanding of the long-term material-tissue dynamics of MAP scaffolds that can inform future material design for tissue augmentation.