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Polymeric Analogs to Biolubrication Systems

Periodic Reporting for period 3 - POLYBIOLUB (Polymeric Analogs to Biolubrication Systems)

Reporting period: 2018-08-01 to 2020-01-31

Articular cartilage is a tissue that covers the bone surface in the synovial joints of mammals. The structure of this outstanding natural material developed over millions of years into a specialized 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. In comparison to industrial, man-made tribological systems, the lubrication of cartilage does not mainly depend on a velocity- and force-dependent fluid film formation between contacting surfaces but 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.

Overall Objective: To fabricate polymeric analogs of cartilage

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

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

Sub-Objective 3: To generate design criteria and potentially initial prototypes of
lubricious biomaterials for temporary or permanent implantation.
"The first 30-month reporting period began with the development of the fundamental tools necessary for the overall project: in particular those connected with synthesis and characterization. Some issues that had been assumed to be well understood in the literature turned out to be less clear than previously thought, and so a certain amount of fundamental development has had to be undertaken, in synthesis and characterization-method development, which will stand us (and others) in very good stead later on. This led to a certain degree of delay, initially, as has our decision to incorporate alternative, bulk gel synthesis approaches, to act as a benchmark for our final fabricated products.

Tribometer
The overall goal of the project involves the fabrication of a cartilage-mimicking construct from polymers. The construct is expected to display very low friction, and is likely to be very soft. The measurement of very low friction values on very soft materials is highly challenging, and beyond the capabilities of our current instrumentation. Therefore we have built a new tribometer that allows us to do exactly the measurements we need. This instrument was built from the ground up, and needed sophisticated software to run the experiment and interpret the data.

In Situ synthesis
The project involves the synthesis of complex block copolymers. This involves growing chains of polymers from a surface, and periodically changing the monomer. Such a procedure is challenging, since reinitiation of the growing chain generally leads to loss of activity. In order to address this issue, which is important in areas the go far beyond our project, we have 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. The QCM is in fact a QCM-D, wehich means it also measures dissipation, i.e. viscoelastic properties of our polymers. This also has the advantage that when we crosslink polymer brushes to form gels, we can see a change in the dissipation signal as this takes place, although the mass change of the polymer is probably either minimal or negative (due to the explusion of solvent) during this process.

Nanoindentation
An important approach to the measurement of mechanical properties of soft materials, and in particular the brush-coated gels of interest to us, 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. This presents some problems when the aim is to quantify these measurements. We have now developed a two-step process that allows the initial contact point to be found, by modeling the indentation curve obtained at infinitely slow indentation, and backing it off to the contact point. This has considerably facilitated measurements of our brush-gel systems, and has great applicability outside of our project, in the characterization of contact lenses, for example.

Bulk gel synthesis, and mold effects
A ""plan B"" in this project is the synthesis of bulk gels, and the modification of their surfaces. This is less elegant and less well defined than the planned approach, but may be simpler in the end, especially when it comes to applications. 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 is still poorly understood, we have undertaken experiments to determine the precise nature of these differences, by means of infrared spectroscopy, neutron reflectivity experiments, and nanoindentation,"
All the work described above corresponds to progress beyond the state-of-the-art. It was necessary to carry out activities in synthesis, modelling (Figure 2), characterisation, and equipment construction, in order to extend the state-of-the-art to the level necessary to carry out this project. The societal and socioeconomic impact will be felt towards the end of the project. Progress so far has been of scientific importance and essential for the rest of the project.