Ion Polymer Interactions in DNA Solutions and Gels

Hydrogel composite mimics biological tissues

A novel composite hydrogel was developed that shows remarkable similarities to load bearing biological tissues. The composite gel consisting of a poly(vinyl alcohol (PVA) matrix filled with poly(acrylic acid) (PAA) microgel particles exhibits osmotic and mechanical properties that are qualitatively different from regular gels. In the PVA/PAA system the swollen PAA particles "inflate" the PVA network. The swelling of the PAA is limited by the tensile stress P_el developing in the PVA matrix. P_el increases with increasing swelling degree, which is opposite to the decrease of the elastic pressure observed in regular gels. The maximum tensile stress Pmaxel can be identified as a quantity that defines the load bearing ability of the composite gel. Systematic osmotic swelling pressure measurements have been made on PVA/PAA gels to determine the effects of PVA stiffness, PAA crosslink density, and Ca²⁺ ion concentration on Pmaxel. It is found that Pmaxel increases with the stiffness of the PVA matrix, and decreases with (i) increasing crosslink density of the PAA and (ii) increasing Ca²⁺ ion concentration. Small angle neutron scattering (SANS) measurements indicate only a weak interaction between the PVA and PAA gels. It is demonstrated that the osmotic swelling pressure of PVA/PAA composite gels reproduces the osmotic behavior of healthy and osteoarthritic cartilage.

Soft matter: rubber and networks

Rubber networks are important and form the basis for materials with properties ranging from rubber tires to super absorbents and contact lenses. The development of the entropy ideas of rubber deformation thermodynamics provides a powerful framework from which to understand and to use these materials. In addition, swelling of the rubber in the presence of small molecule liquids or solvents leads to materials that are very soft and 'gel' like in nature. The review covers the thermodynamics of polymer networks and gels from the perspective of the thermodynamics and mechanics of the strain energy density function. Important relationships are presented and experimental results show that the continuum ideas contained in the phenomenological thermodynamics are valid, but that the molecular bases for some of them remain to be fully elucidated. This is particularly so in the case of the entropic gels or swollen networks. The review is concluded with some perspectives on other networks, ranging from entropic polymer networks such as thermoplastic elastomers to physical gels in which cross-link points are formed by glassy or crystalline domains. A discussion is provided for other physical gels in which the network forms a spinodal-like decomposition, both in thermoplastic polymers that form a glassy network upon phase separation and for colloidal gels that seem to have a similar behavior.

Osmotic Properties of Cartilage

Understanding the interactions and organization of various molecular constituents of cartilage is an essential prerequisite to designing and developing effective nuclear magnetic resonance and magnetic resonance imaging (MRI) strategies to characterize the state of this tissue in normal and abnormal development, aging, health and disease, and trauma. In this chapter the osmotic properties of cartilage are discussed. An attempt is made to relate the tissue’s macroscopic behavior to its hierarchical organization and the physical–chemical interactions among its main macromolecular constituents. It is shown how to separate the osmotic contribution of the proteoglycan assemblies from that of the collagen network. The interactions between the main macromolecular components of cartilage extracellular matrix (ECM) are determined using an array of complementary experimental techniques (osmotic pressure measurements, small-angle X-ray scattering, small-angle neutron scattering, dynamic light scattering, and atomic force microscopy) probing the ECM structure and dynamics over a broad range of length and timescales. This knowledge is also important to construct magnetic resonance phantoms for quantitative MRI that exhibit osmotic, mechanical, and relaxation properties similar to that of cartilage. Such biomimetic phantoms with well-characterized physical and imaging properties are critically important to validate MRI characteristics.