Research paper
Mechanical properties and shape memory effect of 3D-printed PLA-based porous scaffolds

https://doi.org/10.1016/j.jmbbm.2015.11.036Get rights and content

Highlights

  • PLA/15%HA scaffolds with recovery stress of 3.0 MPa were obtained by 3D-printing.

  • HA particles act as nucleation centers and form an additional rigid fixed phase.

  • HA particles inhibit growth of cracks during compression-heating-compression cycles.

  • SME results in “self-healing” by narrowing the cracks with shape recovery of 98%.

  • PLA/15%HA porous scaffolds may be used as self-fitting bone implants.

Abstract

In the present work polylactide (PLA)/15 wt% hydroxyapatite (HA) porous scaffolds with pre-modeled structure were obtained by 3D-printing by fused filament fabrication. Composite filament was obtained by extrusion. Mechanical properties, structural characteristics and shape memory effect (SME) were studied. Direct heating was used for activation of SME. The average pore size and porosity of the scaffolds were 700 μm and 30 vol%, respectively. Dispersed particles of HA acted as nucleation centers during the ordering of PLA molecular chains and formed an additional rigid fixed phase that reduced molecular mobility, which led to a shift of the onset of recovery stress growth from 53 to 57 °C. A more rapid development of stresses was observed for PLA/HA composites with the maximum recovery stress of 3.0 MPa at 70 °C. Ceramic particles inhibited the growth of cracks during compression-heating-compression cycles when porous PLA/HA 3D-scaffolds recovered their initial shape. Shape recovery at the last cycle was about 96%. SME during heating may have resulted in “self-healing” of scaffold by narrowing the cracks. PLA/HA 3D-scaffolds were found to withstand up to three compression-heating-compression cycles without delamination. It was shown that PLA/15%HA porous scaffolds obtained by 3D-printing with shape recovery of 98% may be used as self-fitting implant for small bone defect replacement owing to SME.

Introduction

A polymer-based scaffold for the regeneration of the damaged or missing bone must be a non-toxic, biocompatible material and in some cases biodegradable if required. It must have the high strength and have porous internal structure. In addition, the polymer should be filled with active components that stimulate osteogenesis processes and osseointegration of the implant with the surrounding tissue. Polymers with a shape memory effect (SME) are of special interest and may be used in self-fitting scaffolds for tissue engineering (Neuss et al., 2009, Bencherif et al., 2012).

SME in polymers was first described in 1953 (Flory, 1953) and has since been actively studied in different polymers (Xu and Song, 2015, Behl et al., 2010, Liu et al., 2007). Shape memory polymers (SMP) have different features and advantages over the metallic shape memory alloys due to their much higher recoverable strains (Duncheon, 2005).

The initial shape can be transformed to a temporary shape by deformation of SMP at a fixed temperature below the transition (switching) temperature, such as the glass transition temperature, Tg, or the melting temperature, Tm, when the mobility of the chain segments is limited (Kolesov, 2015, Maksimkin et al., 2014). The driving force for shape recovery in SMP is a change of polymer chain mobility and transformation from more ordered temporary configuration after deformation to a thermodynamically-favored configuration with higher entropy. Such a transformation can be activated by external stimulation through heat, electric or magnetic field, light, moisture, etc. (Lendlein and Kelch 2002a, 2002b) SMPs require coexistence of a fixed phase (crosslinks, entanglements or intermolecular interactions) and a soft phase (Liu et al., 2004, Wei et al., 1998).

The most common and convenient switching temperature for SMP in terms of practical application is the glass transition temperature Tg (Behl et al., 2010, Liu et al., 2007), which occurs in all amorphous polymers and is characterized by an increased mobility of chain segments, causing recovery of shape (Berg et al., 2014).

Polylactide (PLA) is a thermoplastic SMP which is of special interest in terms of medical application because of its high elastic modulus, relatively low Tg in the range of 55–65 °C and the possibility of using it in 3D-printing. Physical entanglements of long PLA chains can act as the fixed phase, while the polymer chains between the entanglements can be stretched during deformation to a temporary shape. Shape memory properties of PLA such as recovery stress and strain may be improved by crosslinking, chemical modification, addition of co-polymers, which was studied in detail by Lendlein et al. (Lendlein and Langer, 2002, Wischke et al., 2009, Pierce et al., 2011). Another way is to fill PLA matrix with dispersed high modulus inorganic particles (Meng and Hu, 2009) which may act as an additional fixed phase. In applications for bone reconstruction, calcium-phosphate particles are of special interest.

Hydroxyapatite (HA) is most commonly used as bioactive component to increase the osseointegration properties, that allows bioresorbable polymers, including PLA, to be used as a matrix for regeneration of bone defects (Persson et al., 2014, Zheng et al., 2006). Various authors have referred to the optimum content of HA particles of 10–30%. As shown by Sadat-Shojai et al. (2013) the concentration of 15% in PHB-matrix leads to formation of bone-like apatite and maximum mineralization; it also exhibits best cell response and improves cellular activity. Introduction of HA powder into polymer matrix may be carried out in a melt. Various technologies are using to create a porous structure in the bioresorbable polymers like PLA. These include the method of supercritical fluids (Zalepugin et al., 2007) and blending with a porogen (Nam et al., 2000). 3D-printing is an actively developing method for formation of polymer products, including implants and medical devices (Ogden et al., 2014, Li et al., 2014). The fused filament fabrication (FFF) method is particularly useful because of the ease of implementation: the polymer is extruded through a nozzle and is deposited onto a substrate layer by layer (Crump, 1991, Turner et al., 2014).

Development of new biomaterials suitable for industry scale manufacturing by various techniques is critical to the success of tissue engineering. Usability of polymers with SME obtained by 3D-printing for biomedical applications has been confirmed in several studies (Shaffer et al., 2014, Kutikov et al., 2014). In this paper, FFF-based 3D-printing was used to form a porous PLA/HA scaffold. SME in PLA/HA 3D-scaffolds was studied to determine the potential use of such structures in self-fitting tissue engineering implants. Direct heating was used for activation of SME as the simplest and easiest method to perform.

Section snippets

Preparation of test specimens

Polymer pellets of polylactide (PLA) (Mw=60,000 g/mol, Aldrich) were used as a material for bioresorbable matrix. Hydroxyapatite (HA) powder with the average size of 1 μm produced by JSC "Polystom" (Russia) was used as the bioactive filler.

Drying of initial PLA and HРHA was carried out at 80 ° C for 4 h (Lai and Lan, 2013) to prevent hydrolysis, depolymerization and oxidative degradation resulting in the formation of oligomers and lactide monomer (Gupta et al., 2007). Mixing of PLA and 15 wt% HA

Structure of PLA-based porous scaffolds obtained by 3D-printing

The presence of high porosity was confirmed by scanning electron microscopy. All pores were open and interconnected with each other through a network of channels. The average pore size of the scaffolds was 700 μm as shown in Fig. 3. The measured size is less then the modeled one because of spreading and thermal expansion of PLA matrix during 3D-printing. The porous structure can support bone formation in the polymer matrix. Typically, a pore size of 50–500 μm is required for unhindered

Conclusions

PLA-based porous scaffolds with porosity of 30 vol% for bone implants were prepared by 3D-printing based on fused filament fabrication. The average pore size of the scaffolds was 700 μm. All pores were interconnected. Dispersed particles of hydroxyapatite (HA) acted as nucleation centers during the ordering of PLA molecular chains and formed an additional rigid fixed phase that decreased molecular mobility. This led to a shift in temperature of the onset of recovery stress growth from 53 to 57 °C.

Acknowledgments

This work was supported by the Federal Targeted Program “Research and Development in Priority Directions of Development of Scientific Technological Complex of Russia in 2014–2020”, with the funding from The Ministry of Education and Science of Russian Federation A: Agreement 14.575.21.0088, 21 October 2014, RFMEFI57514×0088. Further support of research into the shape-memory effect through Grant no.14.A12.31.0001 of the Russian Ministry for Education and Science is gratefully acknowledged.

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