Elsevier

Colloids and Surfaces B: Biointerfaces

Volume 123, 1 November 2014, Pages 959-964
Colloids and Surfaces B: Biointerfaces

Nano-hydroxyapatite/polyacrylamide composite hydrogels with high mechanical strengths and cell adhesion properties

https://doi.org/10.1016/j.colsurfb.2014.10.050Get rights and content

Highlights

  • A tough nano-hydroxyapatite/polyacrylamide composite hydrogel is successfully fabricated.

  • The nanocomposite hydrogels exhibit excellent mechanical properties, which increase with the content of nHAp.

  • The possible toughening mechanism is proposed.

  • The composite hydrogels show good cell adhesion properties.

Abstract

Nano-hydroxyapatite/polyacrylamide composite hydrogels were successfully fabricated by physically mixing nano-hydroxyapatite (nHAp) particles into a peroxidized micelles initiated and cross-linked (pMIC) polyacrylamide (PAAm) hydrogel. The nanocomposite hydrogels exhibited excellent mechanical properties. The fracture tensile stresses of the gels were in the range of 0.21–0.86 MPa and the fracture tensile strains were up to 30 mm/mm, and the compressive strengths were up to 35.8 MPa. Meanwhile the introduction of nHAp endowed the composite hydrogels with good cell adhesion properties. This nHAp/PAAm nanocomposite hydrogel is expected to find potential applications in tissue engineering.

Introduction

Tissue engineering, which is the combination of materials engineering and life science, seeks to create artificial constructs for regeneration of human tissues [1], [2]. One common method used is to culture specific cells separated from the patient on a three-dimensional (3D) scaffold, and then the scaffold is delivered to the desired site of the patient's body. With the process of cell proliferation and differentiation in the scaffold which degrades gradually, new tissue is formed [2], [3], [4]. Another approach is to implant a scaffold into the human body, which stimulates and frames in situ formation of the new tissues. The employment of tissue engineering reduces the number of operations and thus the recovery time for patients is shortened [3], [5], [6].

The design and the manufacturing of the scaffold materials for tissue engineering are extremely challenging. Firstly, the biocompatibility of the scaffold materials is essential, i.e., the materials should not cause an inflammatory response, or exhibit immunogenicity or cytotoxicity. In addition, the scaffold must be mechanically strong enough to resist the physical stress and support the patient's normal activities [1]. Both natural and synthetic polymers, such as polysaccharides and poly(α-hydroxy ester) have been used as scaffolds [3], [5], [7], [8]. With structural and functional properties comparable to many of the soft human tissues, hydrogels are seen as an important candidate of scaffold materials in tissue engineering.

However, most synthetic hydrogels are mechanically very weak and fragile, which limits their practical applications in tissue engineering. In recent years, several kinds of tough hydrogels have been developed, such as slide-ring (SR) hydrogels [9], double network (DN) hydrogels [10], nanocomposite (NC) hydrogels [11], tetra-arm hydrogels [12], hydrogels based on polyfunctional initiating and cross-linking centers (PFICC) [13], [14], [15], [16], [17], etc. As one of the PFICC hydrogels, the peroxidized micelles initiated and cross-linked (pMIC) hydrogel exhibits excellent mechanical properties.

Till now, few attempts have been made to fabricate tissue engineering scaffolds by using tough hydrogels. One of the main reasons is that the smooth surface of the hydrogels cannot provide adsorption sites for the cells. To solve this problem, composite hydrogels have been developed by introducing bio-active inorganic particles into the hydrogel systems [18], [19]. It is well-known that natural bone mainly consists of collagens and apatite. Due to its chemical composition and physical structures similar to bone minerals, hydroxyapatite (HAp) particles are ideal materials for the preparation of composite hydrogels for tissue engineering [20], [21], [22].

HAp-hydrogel composites are usually fabricated through biomimetic mineralization [20], [22], [23] or simple physical mixing method [21]. For the biomimetic mineralization method, HAp particles are in situ formed in the hydrogel and their size increases with the treatment time [20], [24], [25]. But this method suffers from its inconvenient and time-consuming process, moreover the HAp particles formed are difficult to be modified [22]. On the contrary, physical mixing method is very convenient and low-cost. In addition, the size and functionalities of the HAp particles can be easily tailored [18]. Therefore, the physical mixing method is more advantageous for practical applications.

In this study, we prepared nHAp/PAAm nanocomposite hydrogels by physically mixing nano-hydroxyapatite (nHAp) particles into a peroxidized micelles initiated and cross-linked (pMIC) polyacrylamide (PAAm) hydrogel system. The microstructures, mechanical properties, swelling properties and biocompatibility of the composite hydrogels were investigated.

Section snippets

Materials

Pluronic F127 (EO106PO70EO106, PF127, Sigma–Aldrich Co., USA), acrylamide (AAm, Aladdin Chemical Co. Ltd., Shanghai, China) and nano-hydroxyapatite (nHAp, Aladdin Chemical Co. Ltd., Shanghai, China) were used as received.

Preparation of nHAp/PAAm composite hydrogel

With the bubbling of oxygen (50 mL min−1), PF127 aqueous solution (10 g L−1) was irradiated for 4 h with 60Co γ-rays at a dose rate of 10 Gy min−1. The irradiated PF127 solution was mixed with an AAm aqueous solution (CM = 3 M, 4 M, 5 M or 6 M), and the volume ratio of the PF127 solution to

Structure and morphology

SEM investigations of the PAAm hydrogel and the nHAp/PAAm composite hydrogels were carried out to understand the microstructures of the gels. The PAAm hydrogel shows an interconnected porous structure composed of quite uniform pores with a size of 1–2 μm (Fig. 1a and d). Differently, some regions composed of smaller pores are found for the composite hydrogels (Fig. 1b and e, red circle) and more such regions are found in the composite hydrogel made with a higher content of nHAp (Fig. 1c and f,

Conclusions

In summary, by introducing the nHAp particles into the peroxidized micelles initiated and cross-linked (pMIC) polyacrylamide (PAAm) hydrogel system employing the physical mixing method, we successfully fabricated nHAp/PAAm nanocomposite hydrogels with high mechanical strengths, excellent shape recoverabilities as well as good cell adhesion properties. The composite hydrogels have higher extensibilities, higher fracture tensile stresses, and higher compressive strengths with comparison to the

Acknowledgements

The authors appreciate financial support from the National Science Foundation of China (No. 21274013) and the Fundamental Research Funds for the Central Universities.

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