title: Nanopatterning titanium and PEEK for orthopaedic implants
Creators: name: Brydone, Alistair Stewart
Publisher: name: University of Glasgow
Year: 2025

description: This project was inspired by research by Dalby and Gadegaard that demonstrated nanopatterning of poly-methyl-methacrylate (PMMA) surfaces can stimulate mesenchymal stromal cells (MSCs) to differentiate into osteoblasts and produce bone mineral in vitro.[1] The motivation for this thesis was to adapt and upscale the technology for clinical application, with the aim of fabricating osteogenic imlants for orthopaedic surgery, such as intervertebral fusion cages.[2] This translation would initially involve injection mould nanopatterning poly-ether-ether-ketone (PEEK) surfaces. A further objective was to discover methods for fabricating non-planar moulds that could be used in the injection mould nanopatterning process.

Nanoimprint lithography of a novel titanium dioxide precursor sol-gel was performed using flexible polydimethylsiloxane (PDMS) stamps that could conform to non-planar contours of injection mould inlays as a demonstration of the technology. Subsequent injection moulding showed initial success, but the titanium dioxide nanopillars lacked the durability required for repeated moulding cycles.

Nanopatterned PEEK surfaces produced by injection moulding (using electroplated nickel inlays) were assessed to determine whether the nanopatterns exhibited any biological effect upon human bone marrow cells. Initial in vitro experiments by Dr Daniel Morrison and a collaborative group in Davos raised concerns regarding cell adhesion on nanopatterned PEEK surfaces and additional work was undertaken to modify PEEK using oxygen plasma treatment.[3] The use of a cell seeding device designed by Dr Paul Reynolds, led to more reliable in vitro results as it provided a more favourable environment for cell adhesion.

Due to the opacity and autofluorescence of PEEK, in vitro analysis used histological staining with reflected light microscopy and quantitative reverse transcriptase PCR. In vitro experimentation revealed that oxygen plasma treatment increased cell adhesion but reduced the bioactive effect of nanopatterning. Although bone marrow cells adhered to the PEEK nanopatterns in small numbers, the cells exhibited a more osteogenic phenotype, demonstrated by relative increased in calcium and phosphate expression.

Nanopatterned PEEK did not achieve the results required for progression to an in vivo study. Therefore, surface coating nanopatterned PEEK was considered as an alternative method to satisfy the objectives of the project. An in vivo study was undertaken in collaboration with Nijmegen to study osseointegration of titanium coated injection mould nanopatterned surfaces. Due to intellectual property negotiations, polycarbonate was used rather than PEEK and the NSQ and HEX nanopatterns were not included. The titanium coated nanopatterned implants demonstrated significantly increased bone to implant contact compared to commercially developed grit-blasted acid-etched titanium implants.

With a view to further pre-clinical studies of nanopatterned implants, improved in vivo models of osseointegration and osteogenesis in rabbits were developed. These will enable the assessment of novel implants and satisfied the UK Home Office requirements for reduction, refinement and replacement of animal models.

Although not suitable for use in high performance injection mould inlays, the titanium dioxide precursor sol-gel developed for this thesis could be used to directly nanopattern orthopaedic implant surfaces, thus promoting osteogenesis. Furthermore, as demonstrated by the in vivo study presented in this thesis, injection mould nanopatterned polymeric implants (such as PEEK) can be modified with an ultra-thin layer of titanium to improve osseointegration.

The work described herein has highlighted that nanopatterning will not necessarily provide the same results in different materials. It does, however, provide further evidence to support the hypothesis that nanopatterning directs cell behaviour by nanotopographical changes in surface chemistry and surface energy which affect cell adhesion.