Poly- -caprolactone (PCL) is a biodegradable polyester that has received great attentions in clinical and biomedical applications as sutures, drug delivery tool, and implantable scaffold material. Silica aerogel is a material composed of SiO2 that has excellent physical properties for use in drug release formulations and biomaterials for tissue engineering. The current study addresses a composite of silica aerogel with PCL as a potential bone scaffold material for bone tissue engineering. The biocompatibility evaluation of this composite indicates that the presence of silica aerogel effectively prevented any cytotoxic effects of the PCL membrane during extended tissue culture periods and improved the survival, attachment, and growth of 3T3 cells and primary mouse osteoblastic cells. The beneficial effect of silica aerogel may be due to neutralization of the acidic condition that develops during PCL degradation. Specifically, it appears that silica aerogel to PCL wt/wt ratio at 0.5？:？1 maintains a constant pH environment for up to 4 weeks and provides a better environment for cell growth. 1. Introduction Poly- -caprolactone (PCL) is one of the polyester polymers that possess several advantages including benign biocompatibility, low cost, biodegradability, and easy fabrication. Previous studies have suggested that PCL was a good candidate biomaterial for cartilage tissue engineering in terms of cell attachment, proliferation, and matrix production [1–4]. Positive effects of PCL composites on osteoblasts when using as bone graft substitute have also been demonstrated [3, 5–7]. In addition, PCL has been investigated for reconstruction of many other tissues such as skin, nerve, and retina [8]. The major drawback to the use of PCL as tissue scaffold is the production of an acidic environment during the PCL degradation process, which may influence the local microenvironment and cell viability. Aerogels are materials with extremely high porosity and a high surface area [9]. They are usually produced by supercritical extraction of a stable gel using sol-gel technology [10, 11]. Aerogels have useful properties such as high heat insulation [12, 13], low refractive index [14, 15], and dielectric constant close to gas properties [16, 17]. For the past decade, aerogels have gained increased attention in the biomedical field as a potential tool for targeted drug delivery systems [18–20]. Silica aerogel is a material composed of SiO2 with physical properties that include (1) amorphous properties with extremely low bulk density (0.003–0.35？g/cm3), (2) optical
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