Polydopamine constructed interfacial molecular bridge in nano-hydroxylapatite/polycaprolactone composite scaffold

Highlights

• PDA coating on the surface of nano-HAP via dopamine self-polymerization.

• Molecular bridge constructed by PDA enhanced interfacial bonding between nano-HAP and PCL.

• P-HAP/PCL scaffolds were fabricated by selective laser sintering.

• P-HAP exhibited higher mechanical reinforcing effects on PCL scaffolds than HAP.

Abstract

Nano-hydroxylapatite (nano-HAP)/polycaprolactone (PCL) composite scaffold is proved to possess great potential for bone tissue engineering application since the biocompatibility of PCL and the osteoinduction ability of nano-HAP. However, the interfacial bonding between nano-HAP and PCL is weak by reason of the difference in thermodynamic properties. Herein, nano-HAP was modified by polydopamine (PDA) and then added to the PCL matrix to enhance their interface bonding in bone scaffold manufactured by selective laser sintering (SLS). The results indicated that PDA acted as an interfacial molecular bridge between PCL and nano-HAP. On one hand, the amino groups of PDA formed hydrogen bonding with the hydroxyl groups of nano-HAP, and on the other hand, the catechol groups of PDA formed hydrogen bonding with the ester groups of PCL. Compared with the HAP/PCL scaffolds, the tensile and compressive strength of the P-HAP/PCL scaffolds loading 12 wt% P-HAP were increased by 10% and 16%, respectively. Meanwhile, the scaffold possessed great bioactivity and cytocompatibility that could accelerate the formation of apatite layers and promote the cell adhesion, proliferation and differentiation.

Introduction

Polycaprolactone (PCL), a synthetic polyester-based biopolymer, has been widely served as an artificial bone scaffold material due to its acceptable cytocompatibility, bioabsorbability and processability [1], [2]. While scaffold made of single PCL material can not meet the requirements of bone defect repair due to the lack of necessary bioactivity and osteoconductive ability [3]. Hydroxylapatite (HAP) is a dominant inorganic constituent of natural bone and exhibits excellent biocompatibility, bioactivity, and osteoconduction, which is usually used as a bioactive component to endow a biopolymer with bioactivity and osteoconductive ability [4], [5]. Therefore, the combination of HAP with PCL might exhibit the great potential for bone tissue engineering applications [1], [6]. However, the interfacial bonding [7] between HAP and PCL is very weak because of the immiscible thermodynamic properties [8], resulting in the deterioration of mechanical properties [9].

In recent years, various strategies such as chemical grafting [10] and coupling agent modification [11] have been used to increase the interfacial bonding between polymer with HAP. For example, Liu et al. grafted poly bisphenol A glycidyl methacrylate (poly(Bis-GMA)) onto pre-silanized HAP crystals and found that poly(Bis-GMA) acted as a transition layer to improve the interfacial affinity between whisker and resin [12]. Qi et al. introduced silane coupling agent ethenyltrimethoxysilane (YDH171) onto the surface of HAP nanoparticles and found that YDH171 played a role of bridging and increased the interfacial interaction between between HAP and poly(lactide-co-glycolide acid)/poly(trimethylene carbonate) polymer matrix [13]. The above methods can enhance the interface bonding between HAP and polymer, while chemical grafting usually requires a complicated procedure [14], and coupling agent modification may introduce toxicity from chemical reagents [15].

Polydopamine (PDA), an adhesive material produced by the self-polymerization of dopamine (DA) [16], possesses strong adhesive properties due to its rich active amine and catechol groups [17]. Some researchers have used PDA coating to modify inorganic materials for enhancing interface bonding with polymer [18], [19]. Chung et al. used PDA to modify boehmite nanoparticles (BNPS) and then added them into epoxy resin. It was found that the amino groups in PDA could covalently react with the epoxy groups in epoxy resin matrix, thus improving the interfacial compatibility between BNPS and matrix [20]. Phua et al. introduced PDA onto the surface of clays and then added them into polyether polyurethane (PU). The results indicated that the introduced PDA could form hydrogen bonding with PU, thus improving the interfacial bonding between PU and clay [21]. In addition, PDA has been proved to be beneficial for cell adhesion and proliferation, and it can induce osteogenic differentiation [22], [23] by adsorbing extracellular matrix and related proteins [24]. Therefore, the coating of PDA on HAP might be an appropriate approach to increase the interfacial bonding with polymer matrix in composite scaffold for bone tissue engineering application.

In the present work, PDA was introduced onto HAP nanoparticles (nano-HAP) through self-polymerization of DA [25], and the modified nano-HAP were added to PCL matrix for fabricating bone scaffold via selective laser sintering (SLS). The microstructure and chemical composition of nano-HAP before and after PDA coating were studied. Tensile and compressive properties of the scaffold were investigated, the interfacial bonding mechanism was discussed. Meanwhile, the bioactivity, cytocompatibility and osteogenic ability were analyzed by mineralization, cell adhesion and alkaline phosphatase (ALP) expression experiments, respectively.