61st Annual Report on Research 2016

The overall aim of the project is to explore energy dissipation and transport of highly hydrated, phosphocholinated macromolecular vectors; within this overall aim there were several sub-aims, including the preparation of the different phosphocholinated vectors and their examination using several techniques, the main one being the surface-force balance which can measure frictional energy dissipation with unique sensitivity and resolution.

The main results and achievements of the project were published in several papers as appear in the relevant online report. Here we summarize some of the main outcomes. These derive from our studies on several aspects of energy dissipation when nanovectors of highly hydrated species (primarily phosphphocholinated nanoparticles and phophocholine-exposing lipid vesicles). Related studies were also carried out which could shed further light on these basic phenomena.

The basic underlying dynamics, and thus energy dissipation, of hydrated nanovectors lie in the shear and dissipation within the hydration layers themselves, which are a sensitive function of the shear environment. We examined therefore also the nature of the interactions at different surface potentials (see Tivony et al. 2015 in the report on publications from this PRF project), and in particular carried out a sensitive study of the nature of the dissipation as a function of different hydrated ions (Gaisinskaya-Kipnis et al. 2016). These revealed that to interpret the dissipative frictional interaction one must carefully consider also the nature of the shearing surfaces, and their topography.

A broad survey of what is currently known of frictional dissipation with hydrated macromolecular species was carried out (Jahn and Klein 2016). Further studies on the nature of thin-film shear revealed that finite dissipation could arise both from friction between the surface-attached vectors themselves (Sorkin et al. 2016) and also in particular cases due to slip between the surface-attached species and the confining walls themselves (Goldian et al. 2015).

Two main types of nanovectors were studied. The phosphocholine groups (as in the head-groups of the most common phosphatidylcholine (PC) lipids in our body) are known to be especially highly hydrated, with some 12-15 water molecules in the primary hydration shell, and vesicles formed of such lipids, or liposomes, have been shown previously to form very efficient lubrication elements via the hydration lubrication mechanism. In the present project to elucidate frictional dissipation mechanisms we extended this to examine polymeric (polystyrene) nanoparticles (NPs) that had been functionalized to expose phosphocholine groups at their surface, and measured how such nanovectors dissipated energy upon being sheared between two confining surfaces (Lin et al., 2016). Our results revealed that the extent of such dissipation depended strongly on the nature of the NP/confining-surface interaction. For the particular NPs used, we found they attached simultaneously to both surfaces (bridging the gap between them), so that the shearing motion caused substantial drag across the surface and energy loss on sliding.

To elucidate further the nature of the dissipation with the other type of nano-vector used, namely PC-vesicles or liposomes, particularly as a function of the lipid tail-lengths, we investigated systematically the effect of changing chain length of the PC-lipids on the energy dissipation when the respective PC-liposome surfaces were sheared (Sorkin et al. 2016). To our surprise we discovered a marked effect of self-healing of the lipid bilayer surfaces on shear, but only for the case of the more fluid lipids, those with gel-phase-to-liquid-phase transition temperatures lower than the temperature of the measurements (25C), while lipids with longer chains did not exhibit this. We attribute the behavior to the greater ease with which the more fluid lipids are able to re-arrange and restructure in the presence of an available lipid reservoir (as was the case in these experiments).

We also emulated the energy dissipation expected when such nanovectors are in a biological environment, such as at the surface of articular cartilage (described in Jahn et al. 2016). To do this we created a surface layer of hyaluronic acid (HA), a negatively-charged polysaccharide, which attracted PC-vesicles via charge –dipole interaction with the exposed phosphocholine groups (as it has been proposed that such HA-lipid complexes reside at the surface of articular cartilage as a boundary lubricating layer). These then formed surface complexes of HA-lipids on our model surfaces, once again exposing the highly hydrated phosphocholine groups. Upon shear the energy dissipation was found to be greater by about an order of magnitude relative to smooth liposome surfaces. The difference between the two was attributed to the much rougher surface topography of the HA-lipid complex, which enables other frictional dissipation pathways – involving viscoelastic losses, and irreversible hopping processes (releasing phonons). Nonetheless the overall low friction was indeed characteristic of the extremely well lubricated cartilage surface in vivo.

Further work on the nature of liposome surface interactions using fluorescence methods is being summarized and will be published in due course. These studies reveal that in parallel with shear and dissipation of PC-liposomes they undergo rupture to reform as extended bilayers, with a higher energy dissipation arising from the availability of the additional dissipation pathways.

Overall therefore we have achieved to date the greater part of the objectives of this project.