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Polymeric Analogs to Biolubrication Systems

Periodic Reporting for period 4 - POLYBIOLUB (Polymeric Analogs to Biolubrication Systems)

Reporting period: 2020-02-01 to 2020-10-31

The structure of cartilage shows a unique architecture that not only supports the body load acting on the joints but also protects opposing bone surfaces from direct contact along with lubricating the movement in synovial joints. The lubrication of cartilage is driven largely by polymer chain interactions surrounded by a water-based lubricant. Cartilage failure is a major health and societal issue, and is often related to osteoarthritis. A better understanding of cartilage structure and function is necessary to advance the field of cartilage replacement.

The purpose of the project is to mimic, by means of polymer-synthetic approaches, the structure and function of cartilage. A prime motivation is to be able to study the mechanical and tribological properties of structures that are known to be present in cartilage, but are more easily studied in isolation, i.e. via synthetic analogues. A further motivation is to provide insights that may be useful in the future construction of artificial cartilage for implantation. Expected useful side products of the project are a) novel lubricious materials with potential industrial applications, b) a better understanding of the fundamentals of layered polymer gel/brush synthesis, and c) development of novel polymer synthetic techniques for the fabrication of complex, multilayer polymer systems.

Sub-Objective 1: To investigate the relative mechanistic roles of individual
components of cartilage

Conclusions: The polymeric analogs that were fabricated during the project showed mechanical and tribological behavior that indeed mimicked cartilage, but the highly controlled systems investigated allowed various aspects of the tribological system to be monitored in detail. In particular, it was found that contact geometry plays a key role, with brushy gels showing better lubrication when in a configuration that allows the brush to take up water during sliding.

Another observation was that a fixed, brushy slider cannot hold the water within the brush, if continuously in contact, i.e it gets squeezed out. On the other hand a surface in intermittent contact, such as the disk in the same experiment, remains slippery. This reflects an important characteristic of natural joints and has consequences for the design of brush-based implants.

Sub-Objective 2: To fabricate biomimetic, polymeric, highly lubricious, highly wear-resistant
materials that function in an aqueous environment

Conclusions: Many such systems were fabricated, using methods that either relied on the mold material during gel formation to impart the slipperiness (via a mechanism that we have now established), or a variety of brushes could be subsequently grafted onto gels using a novel synthetic approach.

Sub-Objective 3: To generate design criteria and potentially initial prototypes of
lubricious biomaterials for temporary or permanent implantation.

Conclusions: Useful insights were obtained concerning the necessary contact geometry in implanted materials, that will lead to the best tribological performance. Also, a new, non-toxic approach to growing the brushes was developed.

The measurement of very low friction values on very soft materials is highly challenging, and beyond the capabilities of standard instrumentation. Therefore we built a new tribometer that allowed us to do exactly the measurements we needed. It proved to be a cornerstone of our tribological and microindentation measurements.

In Situ synthesis
The project involved the synthesis of complex block copolymers. In order to do this efficiently we developed an in situ flow system, where we can rapidly switch monomers during synthesis, and, in order to monitor exactly how much polymer we have synthesized, the entire process is monitored by a quartz crystal microbalance (QCM). Thus we have control over the composition of our polymer as we synthesize it.

An important approach to the measurement of mechanical properties of soft materials is nanoindentation. This gives us the all-important mechanical characteristics of the outer layer of the material. The outermost layer is so soft (and lubricious) that during the nanoindentation process it is, in fact, extremely difficult to detect by nanoindentation in an atomic force microscope. We developed a two-step process that allows the initial contact point to be found, by modeling the indentation curve obtained at infinitely slow indentation. This considerably facilitated measurements of our brush-gel systems

Bulk gel synthesis, and mold effects
By means of a combination of x-ray and light-scatteriing methods, new insights were gained into the hierarchical structure of acrylamide hydrogels—an important system that was previously thought to be characterizable by a single ""mesh size"". A particularly interesting phenomenon of some bulk gel syntheses is that the nature of the surface of the gel appears to depend on the chemical nature of the surface against which they are synthesized. While this process was poorly understood, we have undertaken experiments to determine the precise nature of these differences, by means of infrared spectroscopy, neutron reflectivity experiments, and nanoindentation.

Synthetic methods
A host of novel polymer brush synthetic methods have been developed, mostly based on ATRP, but significantly extending the technique to render it either biocompatible, more suitable for carrying out under less stringent conditions than usual, or for coating large surface areas of a variety of materials with polymer brushes. Also several new methods have been developed for the grafting of almost any polymer-brush chemistry onto gels.

Lubrication insights
It has been found that while brushy gels are generally lubricious, the precise nature of the lubricated contact can have a significant effect. For instance, a convergent contact (e.g. ball on disk) is significantly favored, since the convergence promotes the brush hydration.

This has been mostly through many articles in academic journals, and conference talks throughout the project given by all members of the team."
All the work described above corresponds to progress beyond the state-of-the-art. It was necessary to carry out activities in synthesis, modelling, characterisation, and equipment construction, in order to extend the state-of-the-art to the level necessary to carry out this project. The synthetic approaches are all beyond state of the art, and the insights gained into the mechanisms of gel-surface modification by molds are a major paradigm shift, and potentially of technological interest. IP is currently being filed at the European Patent Office.