Adding MgO nanoparticles to hydroxyapatite–PLLA nanocomposites for improved bone tissue engineering applications

Abstract

Magnesium plays an important role in the body, mediating cell–extracellular matrix interactions and bone apatite structure and density. This study investigated, for the first time, the effects of adding magnesium oxide (MgO) nanoparticles to poly (l-lactic acid) (PLLA) and to hydroxyapatite (HA) nanoparticle–PLLA composites for orthopedic tissue engineering applications. Results showed that MgO nanoparticles significantly enhanced osteoblast adhesion and proliferation on HA–PLLA nanocomposites while maintaining mechanical properties (Young’s modulus ∼1000 MPa) suitable for cancellous bone applications. Additionally, osteoblasts (or bone-forming cells) cultured in the supernatant of degrading nanocomposites showed improved proliferation in the presence of magnesium, indicating that the increased alkalinity of solutions containing MgO nanocomposites had no toxic effects towards cells. These results together indicated the promise of further studying MgO nanoparticles as additive materials to polymers to enhance the integration of implanted biomaterials with bone.

Introduction

Improved biomaterials for the regeneration of bone are needed to treat the growing population of people with damaged and degrading bone [1], [2], [3], [4]. Today’s biomaterial solutions are highly invasive and lead to the insertion of permanent materials which may cause long-term problems in the body. While the regeneration of bone defects has achieved some success when using injectable cements and various scaffolding materials, there is considerable room for improvement [5], [6], [7]. Ideally, biomaterials for bone tissue regeneration should mechanically match associated tissue to reduce stress and strain imbalances, and possess suitable chemical and topographical modalities to promote bone cell adhesion, proliferation, migration and the secretion of extracellular matrix (ECM)-forming proteins; biomaterials exhibiting these properties do not currently exist.

Poly (l-lactic) acid (PLLA) is a biodegradable polymer that has been widely investigated for tissue engineering applications due to its biodegradability and versatility [8], [9]. Additionally, the mechanical properties of PLLA can be enhanced along with its viability for bone applications with the addition of a ceramic or mineral component [10], [11], [12]. Calcium phosphate-based ceramics and composites have found the most widespread applications for orthopedic applications due to their osteoconductive properties and similarity to native bone apatite minerals [13], [14]; however, few studies have investigated the bone-forming potential of calcium phosphate nanoparticles used in combination with other biologically relevant mineral components, such as magnesium (Mg).

Mg is a biocompatible, biodegradable, low-cost and environmentally friendly material that exists naturally in the human body (∼0.4 g Mg kg⁻¹ [15]). In bone, where Mg exists in its highest concentrations (up to 1% of bone ash [16]), Mg cations reside along the edges of nanostructured apatite minerals (equivalent to hydroxyapatite (HA)) to directly influence mineral size and density—important factors which contribute to the unique mechanical properties of bone [17], [18], [19], [20]. Furthermore, these Mg ions indirectly influence mineral metabolism through activation of alkaline phosphatase [21].

Beyond their cooperative role with HA in maintaining bone health, Mg ions play a key role in mediating the functions of all cells in the body, specifically through their activation of integrins. Divalent Mg²⁺ (as well as Ca²⁺) ions initiate conformational activation of integrins for ligand binding by attaching to sites on the integrin α-chain, thereby influencing cell functions such as attachment, proliferation and migration [22], [23]. Thus, integrating Mg into tissue engineering constructs could potentially improve cell–scaffold interactions.

In order to determine the role Mg can play in bone tissue engineering, here, magnesium oxide (MgO) nanoparticles were dispersed within PLLA polymer sheets, both alone and in combination with HA nanoparticles. Bulk Mg has been considered by others as a material for orthopedic applications due to its biodegradability and mechanical properties (similar in stiffness and strength to bone) [24]. However, bulk Mg has found limited clinical applications in orthopedics because of its fast degradation kinetics under physiological conditions, releasing Mg²⁺ ions, hydroxide (OH⁻) ions and hydrogen (H₂) gas into the surrounding fluid [25]. To address this problem, however, polymer coatings such as poly(lactic-co-glycolic acid) have been shown to control Mg degradation in simulated body fluid [26], [27]. Furthermore, nanostructured bulk Mg was demonstrated to support increased bone cell density compared to unmodified bulk Mg [28]. Thus, MgO nanoparticles dispersed within polymer composites have the potential to enhance bone tissue formation with limited adverse degradation reactions.

Taking advantage of these prior studies, the objective of the present in vitro study was to characterize MgO nanoparticles as additive materials for orthopedic tissue engineering applications, especially when used in combination with HA nanoparticles. The prepared nanocomposites were tested for their surface roughness, mechanical properties, degradation characteristics, and their ability to support the adhesion and proliferation of osteoblasts.