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Electrical Discharge Machining for enhanced osseointegration and antibacterial capabilities of β-phase Titanium Implant

Periodic Reporting for period 1 - EDiMplant (Electrical Discharge Machining for enhanced osseointegration and antibacterial capabilities of β-phase Titanium Implant)

Reporting period: 2019-07-05 to 2021-07-04

Over the last 30 years, orthopaedic surgery has immeasurably improved the lives of millions of people, restoring their mobility, bringing pain relief and ultimately giving them a better quality of life. However, orthopedic implants, especially hip and knee joint replacements, have a limited lifetime because of implant failure and the need for revision surgeries within certain time periods in patients who might require frequent, complicated, and expensive surgeries. For this reason, lengthening the life span of implants for several decades would prevent considerable patient suffering and save health care costs. Long-term survival and favorable outcome of orthopedic implant use are mainly determined by bone–implant osseointegration and absence of infection near the implants.

Most commonly recorded indications for these revisions are aseptic loosening and infection while other indications include pain, dislocation, fracture, etc. An ideal orthopaedic implant material is thus expected to promote osseointegration and inhibit bacterial attachment, enabling a better treatment procedure with less risk of infection while still accelerating the bone healing process. Development of novel surface modification techniques to endow these capabilitiescto biomaterials has recently surged and is of great clinical promise; it is viewed as one of the most promising future directions in orthopaedics. However, currently available surface modification techniques do not satisfactorily combines ease of processing complex shapes with inhibition of bacteria and support of bone matrix production. Based on this context, the overall aim of EDiMplant project was set to develop a competent surface modification approach suitable for enhanced osseointegration and antibacterial capabilities of biomaterials. In this project, electrical discharge machining (EDM) based surface modification approach was proposed for fabrication of surfaces exhibiting anti-bacterial and osteogenic characteristics through surface alloying, metal oxide globule formation, and controlled surface texturing mechanism of EDM. Following objectives were set for the project:
(i) To characterise the properties of hydroxyapatite (HA) powder mixed dielectric and their influence on surface modifications during powder mixed electrical discharge machining (PM-EDM) of Ti alloys:
(ii) To fabricate novel Zn-Ag electrodes and characterise their properties and analyse their appropriateness for EDM applications in biomaterial treatment (WP2):
(iii) To evaluate the influence of Zn-Ag electrodes in PM-EDM on osseointegration and antibacterial resistance of Ti alloys (WP3)
(iv) Demonstrate the competence of EDM to process implants quickly, accurately, to achieve the treatment of complex shapes, to a high quality.
Following work packages (WP) were formulated to achieve the aim and objectives of the project:

WP1: Modifications of acquired EDM machine and characterisation and experimentation with powder mixed dielectric fluid.
•It was proposed to purchase a new or second-hand EDM machine at UoS, but later we decided to carry-out work the EDM related experimentation work at University of Manchester and Erodatools Ltd. (Industry in Sheffield). The fellow designed powder mixed tank setup for PM-EDM operation and successfully used it for experimentation purpose. As a part of work package, extensive experimental work was carried out to evaluation HA mixed EDM performance in terms of material removal rate, coating thickness, and surface roughness. The treated surface were characterised using Scanning electron microscopy (SEM), optical profilometry and X-ray diffraction analysis (XRD). It was found that PM-EDM coated a HA containing porous layer over surface of suitable for promoting initial cell attachment and improving osteogenic response of alloy.

WP2: Fabrication and Characterisation of Zn-Ag tool electrodes.
•Three different combination of Zn-Ag tools viz. Zn90%-Ag10%, Zn80%-Ag20%, and Zn70%-Ag30% were successfully fabricated using FAST technique. It was proposed to evaluate the mechanical, thermal and electrical characteristics of these tool electrodes, but, the fellow could not avail the required training for these testing on various equipment due to COVID-19 restrictions at UoS. However, the fabricated tools were characterised using SEM and XRD. These tools were used for EDM based coating of Ti6Al4V alloy. The coatings were tested and characterised SEM, XRD, surface profilometer and contact angle tester. Finally, tests for bioactivity, cell proliferation, and antimicrobial efficacy were performed to assess coatings performance. The coated layer with Zinc and Silver in

WP3: Experimentation with Zn-Ag electrodes in HA mixed EDM and evaluation of osseointegration and bacterial resistance.
•The PM-EDM setup with HA powder mixed dielectric along with fabricated tool electrodes in WP2 was used for experimentation part in WP3. Coatings containing HA along with Zn and Ag were successfully generated on Ti6Al4V surface and their osstogenic and antibacterial characteristics were assessed. The coatings mechanical properties could not have been assessed since the fellow was not trained on required facility due to COVID-19 restrictions at UoS.

The findings of this project have are being prepared for publication in two significant research papers. Results were also presented to scientific community at two conferences including BioMedEng 2021 and ESB 2021 (31st Annual Conference of the European Society for Biomaterials).
Further exploration and exploitation of this novel mechanism will have important impacts on European society, by increasing the knowledge base of the European scientific community in the field of surface treatment of orthopaedic implant materials.
Fellow had delivered four Invited Talks. Further, the fellow has been actively promoting projects to both members of the public and fellow scientists on social media (Twitter, ResearchGate). This online presence will be further leveraged to increase the impact of the publication of the fellowship findings.
Through this project, a novel surface modification techniques suitable for fabrication of Ti6Al4V surfaces exhibiting anti-bacterial and osteogenic characteristics through electrical discharge machining (EDM) was successfully demonstrated. We found that the EDM treated surface were hydrophobic in the nature and to induce hydrophilic nature, the surface were further treated by plasma treatment. To induce a better osseointegration and antibacterial characteristics, a hydroxyapatite mixed dielectric and novel Zn-Ag tool electrodes were used for the first time. The modified surface had coated layer of 50-150 µm thickness with HA, Zn and Ag incorporated in it. The modified surface improved the osteogenic response as well as antibacterial properties of the Ti alloys. The proposed approach of PM-EDM based treatment of biomaterials is suitable for processing complex shapes of implant quickly and accurately with better osseointegration, antibacterial capability and mechanical properties. Further, the demonstrated approach has potential improve the implant life reducing costs, in terms of days of hospitalisation and cost of implant revisions.
Project group, methodology and current publications