Small angle neutron scattering (SANS) and dynamic light scattering (DLS) measurements were made on near physiological solutions of a bottlebrush shape polyelectrolyte, aggrecan. Aggrecan is a biologically important molecule whose complexes with hyaluronic acid (HA) provide the osmotic resistance of cartilage. We have investigated the effect of complexation of aggrecan with HA on the structure and dynamic properties of aggrecan solutions. SANS reveals that the supramolecular structure of aggrecan assemblies is only marginally affected by the HA. DLS indicates that the dynamic response of the aggrecan-HA complex is slower than that of the corresponding aggrecan solution. However, addition of calcium ions slightly increases the relaxation rate of the autocorrelation function of the aggrecan solution.

The effect of ions on the structure and dynamics of aggracan solutions was investigated using a variety of scattering techniques (neutron spin echo, small angle neutron scattering, static and dynamic light scattering). The results show that aggrecan forms microgel-like assemblies which remain stable in the presence of counter-ions. As opposed to many synthetic polyelectrolytes which exhibit a great sensitivity to ion concentration and charge valency, it was found that aggrecan solutions are remarkable insensitive to charge valence. This property of aggrecan is consistent with its biological role as an ion reservoir mediating calcium metabolism and maintaining dimensional stability in cartilage and bone.

Cartilage is a load bearing tissue that has multiple biological functions. Themajor proteoglycan in cartilage is the bottlebrush shaped aggrecan whosecomplexes with hyaluronic acid provide the compressive resistance of cartilage.The negatively charged aggrecan-hyaluronic acid complexes generate an osmoticswelling pressure within the tissue, which is balanced by the collagen network.To better understand the function of cartilage at the tissue level, we studyaggrecan assemblies using an array of microscopic and macroscopic techniques.The organization of aggrecan assemblies at the supramolecular level is probed bylight scattering, small-angle neutron scattering and small-angle X-rayscattering. Osmotic and rheological measurements are used to investigate themacroscopic physical properties.

It is biologically and clinically important to understand and explain the functional properties of cartilage, such as its load bearing and lubricating ability, in terms of the structure, organization, components and their interactions. Our approach tries to explain functional material properties of these tissues as arising from polymeric interactions between and among the different molecular constituents within the tissues at different hierarchical lengthscales. We treat the tissue effectively as a complex molecular composite containing highly charged polysaccharide microgels trapped within a fine collagen meshwork. We have been developing a multi-scale experimental and theoretical framework to explain key material properties of cartilage by studying those of its constituents and the interactions among them at a variety of length and time scales. We use this approach to address important biological questions. One novel application we highlight here is the use of non-invasive magnetic resonance imaging (MRI) methods to characterize different components and compartments within cartilage and the different water environments associated with each one, in an attempt to provide a comprehensive picture of the mechanical/chemical state of cartilage.

Cartilage is a complex biological tissue that exhibits gel-like behavior. Its primary biological function is providing compressive resistance to external loading and nearly frictionless lubrication of joints. In this study, we model cartilage extracellular matrix using a biomimetic system. We demonstrate that poly(vinyl) alcohol (PVA) hydrogels are robust biomaterials exhibiting mechanical and swelling properties similar to that of cartilage extracellular matrix. A comparison is made between the macroscopic behavior of PVA gels and literature data reported for cartilage.

Articular cartilage is a low-friction, load-bearing tissue located at joint surfaces. It experiences static and dynamic forces including shear, compression and tension. We investigate the relationship between structure and function by measuring the osmotic and mechanical properties in cartilage layers as a function of the distance from the articular surface. Atomic force microscopy is used to probe the mechanical properties at high spatial resolution. The mechanical measurements are complemented by osmotic swelling pressure observations made on the same samples using a novel tissue osmometer. The results show that the osmotic modulus significantly depends on the distance from the articular surface. Its value is highest in the deep zone and lowest in the middle zone.

A quartz crystal microbalance (QCM) has been used to investigate polymer samples. The vapor sorption of three different polymer samples (poly(vinyl acetate), polybutadiene and polydimethylsiloxane) was studied. The change in resonance frequency of the quartz sensor uniformly coated with polymer films was measured as a function of the film thickness and water absorption at different temperatures. The range of linear frequency vs. mass response was determined in the absence of absorbed water. The glass transition temperature of thin poly(vinyl acetate) films (10 nm <thickness< 1000 nm) was found to be in reasonable agreement with published values for macroscopic samples.