Tissue and organ regeneration by culturing and/or transplanting cell or tissue inside engineered scaffolds has become one of the most promising techniques for surgical therapy and biomedical research. The scaffolds support cell/tissue attachment, define the ultimate shape of the regenerated functional tissue, and guide tissue growth.1
A number of synthetic polymers have been used as matrix materials for scaffolds because of the capability of controlling the properties such as surface chemistry, biodegradability, and mechanical strength. However, biocompatibility and cell-tissue interactions of these materials are less optimal as compared to natural biomaterials. To overcome these main limitations, combinations of synthetic and natural materials have been developed in various tissue engineering applications including skeletal tissue regeneration. Hydroxyapatite (HA), a major inorganic compound of human hard tissue, is considered an excellent candidate to enhance mechanical strength and osteoconductive properties.2–7
Although HA nanoparticles can strengthen the polymer matrix, the composite’s mechanical properties depend mainly on the matrix. Furthermore, HA composition in the composite may not exceed a certain value to prevent disadvantages such as brittleness of the composites and poor torsional resistance. Therefore, it is crucial to choose a matrix material for achieving desired mechanical strength and modulus for load-bearing applications.
Poly(propylene fumarate) (PPF), an unsaturated linear polyester that can be crosslinked in situ
to provide suitable mechanical properties, is one of the promising materials for load-bearing tissue regeneration.8
PPF has been used to form composites with enhanced mechanical strength and osteoconductive properties by adding calcium phosphates such as β-tricalcium phosphate (TCP).9,10
By incorporating HA nanoparticles, we have developed a series of crosslinkable nanocomposite disks and demonstrated that crosslinked PPF/HA nanocomposites have sufficient mechanical strength for bone tissue engineering, increased hydrophilicity and protein absorption on their surfaces with increasing HA contents, and enhanced 2D attachment and proliferation of pre-osteoblast in vitro
Besides material properties, independent control of pore microstructure is a prerequisite for increasing cell growth into the three-dimensional (3D) scaffold and enhancing new tissue integration with the host. Several conventional techniques, such as gas foaming/particulate leaching,12
have been successfully used to fabricate polymer/HA nanocomposite scaffolds. However, these methods have limited controllability over the bulk properties of scaffolds such as porosity and average pore size, by varying parameters such as porogen size and content, temperature, and applied voltage. As an alternative method, solid freeform fabrication (SFF), including 3D printing,16
selective laser sintering,17
and indirect sacrifice mold technique,18
has improved the capability of controlling the microstructures of scaffolds by using computer-aided design (CAD). These studies have shown that controlling of individual pore size, shape, arrangement and interconnectivity within the scaffold is essential for 3D cell ingrowth and proliferation.
In this work, we have fabricated crosslinked PPF/HA nanocomposite scaffolds with two pre-designed pore structures from CAD models using SFF technique. For comparison, crosslinked PPF/HA nanocomposite scaffolds with random pore structures were fabricated using the conventional NaCl leaching technique. The structure, morphology, and mechanical properties of crosslinked PPF and PPF/HA nanocomposite scaffolds have been characterized using scanning electron microscopy (SEM) and mechanical testing. Pore interconnectvity of each scaffold was measured using micro-computed tomography (micro-CT) and image analysis. To investigate in vitro cellular responses, MC3T3-E1 mouse pre-osteoblasts were seeded on the scaffolds and cultured in a rotating-wall-vessel bioreactor for 4 and 7 days. Cell morphology, viability, ingrowth depth, and density were examined.