Small 3D printed femur models (d = 3.20 mm, h = 2.03 mm; n = 3) were sequentially sterilized by UVA irradiation and 70% ethanol for 15 min each. After washing 3 times in PBS, they were functionalized with a fibronectin-derived arginylglycylaspartic acid (RGD) peptide (China Peptides, N-C: CGRGDS) at 10 mM in PBS (pH 7.94) to promote cell attachment. Samples were incubated in this solution at 37 ˚C overnight before washing 3 times with PBS to remove unreacted RGD. For 3D culture, a collagen type I hydrogel was prepared from an 8.91 mg mL -1 stock solution (rat-tail, Corning) as described by Shin et al. Briefly, the collagen stock solution was mixed with 10% of 10× PBS and cells in osteogenic medium to obtain a final collagen concentration of 1 mg mL -1 and a cell concentration of 3 × 10 6 mL -1 .