Tailoring Segmented Polyurethane Foams for Bone Tissue Engineering: Effects of Hard Segment Content and Dual-Function Microhydroxyapatite

Poly(ε-caprolactone)-based segmented polyurethanes (PUs) were developed as potential materials for bone tissue engineering, with particular emphasis on porous substitutes for cancellous bone. PUs with hard segment (HS) contents ranging from 17.6 to 32.0% were synthesized from poly(ε-caprolactone) diol, 1,6-hexamethylene diisocyanate, and 1,4-butanediol. The effects of HS content and post-synthesis incubation temperature (40 and 80 °C) on the structural, thermal, surface, mechanical, and biological properties were investigated. The polyurethane containing 25.5% HS exhibited a favorable combination of structural and mechanical properties and was selected for the preparation of porous composites containing microhydroxyapatite (μHAp) and ZrO2. The resulting composites exhibited high open porosity (70.4–73.3%). The physicochemical characteristics of μHAp support its proposed role as a predominant water-carrying ceramic phase promoting in situ CO2 generation during polyurethane crosslinking; however, because μHAp and ZrO2 were incorporated simultaneously, their individual contributions to foaming cannot be unequivocally distinguished. Extract-based MTT testing showed no cytotoxic effects under the investigated conditions. Furthermore, the 30-day stability assessment at 37 °C revealed low mass loss, preservation of Young’s modulus, and only minor changes in molecular weight parameters, supporting the short-term stability of the investigated materials. Overall, the results support further development of these porous PU/ceramic composites as materials for bone tissue engineering.