Preliminary processing–structure–property assessment of hydroxyapatite–halloysite nanotube composites consolidated by water-assisted cold sintering for non-load-bearing bone-defect filler applications

Hydroxyapatite (HA) is attractive for non-load-bearing bone-defect fillers because of its chemical similarity to bone mineral, but its brittleness and limited green-body strength complicate low-temperature consolidation. This preliminary study asked whether low halloysite nanotube (HNT) additions (3–5 wt%) combined with water-assisted cold sintering could improve densification, handling strength, and short-term aqueous stability while preserving the HA phase. HA97, HA96, and HA95 formulations containing 3, 4, and 5 wt% HNT, respectively, were evaluated through sequential one-factor-at-a-time screening. Pressure was varied from 200 to 600 MPa, temperature from 50 to 250 °C, and holding time from 5 to 30 min, with approximately 5 wt% water used as a transient liquid. Phase constitution, precursor thermal decomposition, morphology, compressive strength, and four-week degradation in phosphate-buffered saline were assessed by X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, mechanical testing, mass-loss measurement, pH monitoring, and calcium-ion analysis. Increasing pressure and HNT content increased relative density; HA95 reached 94.43% at 600 MPa and 250 °C. Under the selected condition of 600 MPa, 250 °C, and 15 min, HA95 showed an initial compressive strength of 54.93 MPa, compared with 50.33 MPa for HA100. After four weeks, cumulative mass loss was 6.00% for HA95 and 7.12% for HA100, while retained compressive strengths were 35.43 and 32.20 MPa, respectively. XRD showed HA as the dominant crystalline phase and broad, low-intensity HNT reflections consistent with limited long-range order. The results support HA–HNT as a promising processing concept for non-load-bearing bone-defect fillers; however, the study does not establish biological safety or clinical suitability. Differential scanning calorimetry, cytotoxicity, cell-response, and in vivo studies remain necessary.