Daniel J. Hickey

The influence of nanostructured features on bacterial adhesion and bone cell functions on severely shot peened 316L stainless steel

Substrate grain structure and topography play major roles in mediating cell and bacteria activities. Severe plastic deformation techniques, known as efficient metal-forming and grain refining processes, provide the treated material with novel mechanical properties and can be adopted to modify nanoscale surface characteristics, possibly affecting interactions with the biological environment. This in vitro study evaluates the capability of severe shot peening, based on severe plastic deformation, to modulate the interactions of nanocrystallized metallic biomaterials with cells and bacteria. The treated 316L stainless steel surfaces were first investigated in terms of surface topography, grain size, hardness, wettability and residual stresses. The effects of the induced surface modifications were then separately studied in terms of cell morphology, adhesion and proliferation of primary human osteoblasts (bone forming cells) as well as the adhesion of multiple bacteria strains, specifically Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa, and ampicillin-resistant Escherichia coli. The results indicated a significant enhancement in surface work hardening and compressive residual stresses, maintenance of osteoblast adhesion and proliferation as well as a remarkable decrease in the adhesion and growth of gram-positive bacteria (S. aureus and S. epidermidis) compared to non-treated and conventionally shot peened samples. Impressively, the decrease in bacteria adhesion and growth was achieved without the use of antibiotics, for which bacteria can develop a resistance towards anyway. By slightly grinding the surface of severe shot peened samples to remove differences in nanoscale surface roughness, the effects of varying substrate grain size were separated from those of varying surface roughness. The expression of vinculin focal adhesions from osteoblasts was found to be singularly and inversely related to grain size, whereas the attachment of gram-positive bacteria (S. aureus and S. epidermidis) decreased with increasing nanoscale surface roughness, and was not affected by grain refinement. Ultimately, this study demonstrated the advantages of the proposed shot peening treatment to produce multifunctional 316L stainless steel materials for improved implant functions without necessitating the use of drugs.

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

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 (Youngs modulus ∼1,000 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.

Effects of nanofeatures induced by severe shot peening (SSP) on mechanical, corrosion and cytocompatibility properties of magnesium alloy AZ31

Considering the sensitivity of both fatigue strength and corrosion rate to the surface characteristics, apposite surface treatments could address the related challenges for biodegradable magnesium-based materials. Herein, we treated the surface of a biocompatible magnesium alloy by a low cost and versatile severe plastic deformation technique, severe shot peening, to evaluate the potential of surface grain refinement to enhance functionality in biological environment. The evolution of surface grain structure and surface morphology were investigated using optical as well as scanning and transmission electron microscopy. Surface roughness, wettability and chemical composition, as well as in depth-microhardness and residual stress distribution, and corrosion resistance were investigated. Successive light surface grinding was used after severe shot peening to eliminate the effect of surface roughness and separately investigate the influence of grain refinement alone. Cytocompatibility tests with osteoblasts (or bone forming cells) were performed using sample extracts. Results revealed for the first time that severe shot peening can significantly enhance mechanical properties without causing adverse effects to the growth of surrounding osteoblasts. The corrosion behavior, on the other hand, was not improved by severe shot peening; nevertheless, slight grinding of the rough surface layer with a high density of crystallographic lattice defects, without removing the entire nanocrystallized layer, provided a good potential for improving corrosion characteristics after severe shot peening and thus, this method should be studied for a wide range of orthopedic applications in which biodegradable magnesium is used.The application of biodegradable magnesium-based materials in the biomedical field is highly restricted by their low fatigue strength and high corrosion rate in biological environments. Herein, we treated the surface of a biocompatible magnesium alloy AZ31 by severe shot peening in order to evaluate the potential of surface grain refinement to enhance this alloys functionality in a biological environment. The AZ31 samples were studied in terms of micro/nanostructural, mechanical, and chemical characteristics in addition to cytocompatibility properties. The evolution of surface grain structure and surface morphology were investigated using optical, scanning and transmission electron microscopy. Surface roughness, wettability, and chemical composition, as well as in depth-microhardness and residual stress distribution, fatigue behaviour and corrosion resistance were investigated. Cytocompatibility tests with osteoblasts (bone forming cells) were performed using sample extracts. The results revealed for the first time that severe shot peening can significantly enhance mechanical properties of AZ31 without causing adverse effects on the growth of surrounding osteoblasts. The corrosion behavior, on the other hand, was not improved; nevertheless, removing the rough surface layer with a high density of crystallographic lattice defects, without removing the entire nanocrystallized layer, provided a good potential for improving corrosion characteristics after severe shot peening and thus, this method should be studied for a wide range of orthopedic applications in which biodegradable magnesium is used. STATEMENT OF SIGNIFICANCE A major challenge for most commonly used metals for bio-implants is their non-biodegradability that necessitates revision surgery for implant retrieval when used as fixation plates, screws, etc. Magnesium is reported among the most biocompatible metals that resorb over time without adverse tissue reactions and is indispensable for many biochemical processes in human body. However, fast and uncontrolled degradation of magnesium alloys in the physiological environment in addition to their inadequate mechanical properties especially under repeated loading have limited their application in the biomedical field. The present study providesdata on the effect of a relatively simple surface nanocrystallziation method with high potential to tailor the mechanical and chemical behavior of magnesium based material while maintaining its cytocompatibility.

MgO nanocomposites as new antibacterial materials for orthopedic tissue engineering applications

Regeneration of orthopedic soft and hard tissues, such as ligaments, bone, and the tendon-to-bone insertion site (TBI), is problematic due to a lack of suitable biomaterials which possess appropriate mechanical properties capable of promoting cellular functions in these tissues with limited regenerative capacity. Additionally, surgically implanted biomaterials are susceptible to bacterial infection, which can lead to implant failure, as well as further complications such as wide-spread infection. To address these issues, the current study investigated magnesium oxide (MgO) nanoparticles as novel materials to improve orthopedic tissue regeneration and reduce bacterial infection. Poly (l-lactic acid) (PLLA) was mineralized with MgO nanoparticles and tested for its mechanical properties, bactericidal efficacy, and its ability to support the growth of fibroblasts and osteoblasts. These MgO nanocomposites were compared to PLLA mineralized with nanoparticles of hydroxyapatite (HA), which have been shown to promote bone tissue growth and have been widely used as materials for bone tissue engineering. Results indicated for the first time that MgO nanoparticles increased the adhesion and proliferation of osteoblasts and fibroblasts compared to plain PLLA and PLLA-HA nanocomposites. Furthermore, MgO nanocomposites showed excellent bactericidal efficacy, killing nearly all of the bacteria seeded onto them, whereas HA nanocomposites showed increased bacterial growth compared to plain PLLA. Mechanical tensile testing revealed that the addition of a secondary nano-phase to plain PLLA increased the material elastic modulus and reduced material elasticity. Moreover, the mechanical properties could be tuned to match those of bone or ligament tissue by varying nanoparticle size and concentration within the composite.

Nanotechnology for Orthopedic Applications: From Manufacturing Processes to Clinical Applications

This chapter covers the integration of artificial materials into natural tissues of the human body, particularly bone, and what can be achieved through a couple of key nano-manufacturing techniques (such as shot peening and electrophoretic deposition). To achieve proper mechanical anchorage and integration, orthopedic implanted materials should resemble the tissues they are replacing as much as possible. Thus, provided here is an overview of the structure and function of bone tissues, as well as a review of the concepts and methods used by other researchers attempting to regenerate orthopedic tissues, with a focus on nanotechnology.

Characterizing Stem Cell Proliferation on Magnesium Nanoparticle Mineralized Poly(l-Lactic Acid) Scaffolds for Enthesis Applications

There is a dire need for an orthobiologic material that can provide a stable transition from hard bone to soft ligament tissue since reconstructed joints tend to fail at the tendon-to-bone insertion site (TBI). Here, research is being done to investigate regenerating the TBI using poly(l-lactic acid) (PLLA) mineralized with hydroxyapatite (HA) and magnesium nanoparticles. Ultimately, the goal of this research is to develop a nano-structured material fashioned in the shape of an o-ring to restore functionality to injured entheses - the graded transition of mineralized fibrocartilage connecting ligament to bone.