Structure of intermediate filament assembly in hair deduced from hydration studies using small-angle neutron scattering

Intermediate filaments (IFs) are ubiquitous in biological structures including hair. Small-angle neutron scattering (SANS) data from hydrated samples were used in this study to investigate the distribution of water in hair, and model the structure of the IF assembly. A main diffraction peak at a d-spacing of ∼90 Å, and two weaker reflections show that IFs are arranged in a ∼105 Å quasi-hexagonal lattice. Changes in the diffraction peaks show that only a small fraction of the water absorbed by hair enters between the IFs, and little water diffuses into the core of the IFs. The amount of water in the IF assembly increases rapidly up to 10% relative humidity (RH), and then slowly with further increase in RH. Most of the water appears to reside outside the IF assembly, in the voids and at the interfaces, and contribute to the central diffuse scattering. The IF assembly in the decuticled hair absorbs more water and is more ordered than that the native hair. This suggests that cuticle acts as a barrier, and might constrain the structure by compressing the cortex radially. Treatments with oils that are hydrophobic, heat treatment, and reduction of the S-S linkages that opens up the matrix by disulfide bond cleavage, all affect structure and water permeability. Coconut oil was found to impede hydration more than the soybean oil because of its ability to penetrate deeper into hair. A new model for the IF assembly that is sterically more favorable than the previous models is proposed.

Multiscale analysis of water uptake and erosion in biodegradable polyarylates

The role of hydration in degradation and erosion of materials, especially biomaterials used in scaffolds and implants, was investigated by studying the distribution of water at length scales from 0.1 nm to 0.1 mm using Raman spectroscopy, small-angle neutron scattering (SANS), Raman confocal imaging, and scanning electron microscopy (SEM). The measurements were demonstrated using L-tyrosine derived polyarylates. Bound- and free- water were characterized using their respective signatures in the Raman spectra. In the presence of deuterium oxide (D(2)O), H-D exchange occurred at the amide carbonyl but was not detected at the ester carbonyl. Water appeared to be present in the polymer even in regions where there was little evidence for N-H to N-D exchange. SANS showed that water is not uniformly dispersed in the polymer matrix. The distribution of water can be described as mass fractals in polymers with low water content (~5 wt%), and surface fractals in polymers with larger water content (15 to 60 wt%). These fluctuations in the density of water distribution are presumed to be the precursors of the ~ 20 μm water pockets seen by Raman confocal imaging, and also give rise to 10-50 μm porous network seen in SEM. The surfaces of these polymers appeared to resist erosion while the core of the films continued to erode to form a porous structure. This could be due to differences in either the density of the polymer or the solvent environment in the bulk vs. the surface, or a combination of these two factors. There was no correlation between the rate of degradation and the amount of water uptake in these polymers, and this suggests that it is the bound-water and not the total amount of water that contributes to hydrolytic degradation.

Microphase separation in copolymers of hydrophilic PEG blocks and hydrophobic tyrosine-derived segments using simultaneous SAXS/WAXS/DSC

Hydration- and temperature-induced microphase separations were investigated by simultaneous small- and wide-angle X-ray scattering (SAXS and WAXS) and differential scanning calorimetry (DSC) in a family of copolymers in which hydrophilic poly(ethylene glycol) (PEG) blocks are inserted randomly into a hydrophobic polymer made of either desaminotyrosyl-tyrosine ethyl ester (DTE) or iodinated I(2)DTE segments. Iodination of the tyrosine rings in I(2)DTE increased the X-ray contrast between the hydrophobic and hydrophilic segments in addition to facilitating the study of the effect of iodination on microphase separation. The formation of phase-separated, hydrated PEG domains is of considerable significance as it profoundly affects the polymer properties. The copolymers of DTE (or I(2)DTE) and PEG are a useful model system and the findings presented here may be applicable to other PEG-containing random copolymers as well. In copolymers of PEG and DTE and I(2)DTE, the presence of PEG depressed the glass transition temperature (T(g)) of the copolymer relative to the homopolymer, poly(DTE carbonate), and the DTE/ I(2)DTE segments hindered the crystallization of the PEG segments. In the dry state, at large PEG fractions (> 70 vol%), the PEG domains self-assembled into an ordered structure with 14-18 nm distance between the domains. These domains gave rise to a SAXS peak at all temperatures in the iodinated polymers, but only above the T(g) in non-iodinated polymers, due to the unexpected contrast- match between the crystalline PEG domains and the glassy DTE segments. Irrespective of whether PEG was crystalline or not, immersion of these copolymers in water resulted in the formation of hydrated PEG domains that were 10-20 nm apart. Since both water and the polymer chains must be mobile for the phase separation to occur, the PEG domains disappeared when the water froze, and reappeared as the ice began to melt. This transformation was reversible, and showed hysteresis as did the melting of ice and freezing of the water incorporated into the polymer. PEG-water complexes and PEG-water eutectics were observed in WAXS and DSC scans, respectively.

The hydrolytic degradation of polydioxanone (PDSII) sutures. Part I: Morphological aspects

The morphological changes in polydioxanone sutures (PDSII), and the resulting changes in properties are explored as a function of in vitro degradation time with the use of small- and wide-angle x-ray scattering, optical microscopy, differential scanning calorimetry, dynamic mechanical thermal analysis, and measurements of tensile properties and mass change. There are significant changes to the properties of the suture on initial hydration. In the time range of 0-80 days investigated, two subsequent stages of degradation are apparent. Chain scission is thought to occur in the first stage, beginning as soon as the chains become hydrated. However, in this stage, the effects of scission on the physical properties of the suture are not apparent. The transition from the first (dormant) stage to the second (active) stage, in which changes to the physical structure and mechanical properties of the fiber become measurable, is gradual, with the first change occurring after about 15 days. The relationships between the changes in the microstructure and the properties are discussed and a general model for the degradation process proposed.