394 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the non-helical "tails" of this protein probably play a vital part in the physical interactions between neighboring microfibrils and the matrix in which they are embedded. The high sulfur protein extract is confined to the matrix, and with the high glycine-tyrosine protein found in varying quantities in o•-keratin fibers forms the non-crystalline protein material of the matrix. The high sulphur and high glycine- tyrosine proteins contain fewer hydrophyllic sites, and the former protein is more crosslinked than the low sulphur protein of the microfibrils. This fact has raised the question, why does the matrix absorb more water than the microfibrils (4). Aside from the crystalline and hence less labile state of the microfibrils, the proteins of the matrix do not have to absorb water within their own structure. The matrix must be considered to consist of water plus the high sulphur and high glycine tyrosine proteins (47). The proteins themselves can form globules, and the water form an enveloping, continuous network of hydrogen bonds with the proteins acting as a filler within this network, interacting with the network, at the globule surface. The existence of protein globules has been indicated by X-ray diffraction measurements (48). The mechanical properties in water of different o•-keratins show a progressive stiffening with increase of the high sulphur plus high glycine-tyrosine content of the fiber, an increase which has been associated with the matrix (49). Further, the dry to wet change in fiber diameter is progressively reduced. This can be understood clearly if we recognize that the matrix consists of water plus these two proteins. Because of the physical limitation of intermicrofibrillar spacing (50,51) set during formation of the fiber in the follicle, an increase of these proteins results in a decrease of water content, and a resultant decrease of mechanical mobility of the matrix structure for the wet fiber. A similar effect can be obtained by introducing into the matrix of a keratin fiber large dye molecules (52). It has been shown that with increase of dye uptake the dry to wet diametral swelling of the fiber is reduced and its mechanical stiffness for small strains, particularly in torsion (51), is increased. With suitable dye molecules this procedure is quite reversible and indicates that the main physical effect of these molecules is to displace water in the matrix structure. On the basis of the two-phase model regarding the structure as consisting of microfibrils containing the organized o•-helical structure, and the matrix, broadly considered as corresponding to the rest of the structure, labile and weakened in water, estimates have been made of the mechanical contribution of each phase to the equilibrium Young's modulus of 1.4 x 109 pascals for o•-keratin fibers in water. These measurements (53,54) are based on dynamic measurements at about 102herz and at various humidities, together with conventional mechanical tests at different tempera- tures in water. All tests show the basic agreement that the contribution of the microfibrils to the equilibrium Young's modulus is close to 1.2 x 109 pascals, and the matrix contribution is of the order of 0.2 x 109 pascals. This latter contribution of the matrix is, as expected, small and is of the order expected from the elastomeric stiffness of an amorphous chains. SWELLING IN FORMIC ACID, ALCOHOLS, AND UREA SOLUTION X-ray diffraction evidence shows clearly for both formic acid (33) and the simple alcohols (55) (methanol, ethanol, n-propanol, and n-butanol) that all these solvents swell the crystalline regions of the microfibrils in o•-keratin fibers. In the case of the alcohols the range of swelling is 9-11% in the distance between the helices with no

PHYSICAL PROPERTIES OF ALPHA-KERATIN FIBERS 395 apparent loss of the helical content of the fiber. In the case of the alcohols (56), Speakman obtained a progressive increase of mechanical stiffness of the fibers with increase of molecular size. However, if the tests are carried out to mechanical equilibrium so that the dynamic processes, which are slower for a fiber in the alcohols compared with water, have time to relax, then the fibers indicate a considerable reduction in their stiffness in the "Hookean" region (57). This reduction is the expected consequence of the swelling of the crystalline regions. Bendit (33) has shown that in 99% formic acid the distance between o•-helices in the crystalline region has increased by about 17% above the value in water. For an aqueous solution of formic acid up to 70% (V/V) optical birefringence measurements show no change in the crystallinity of the fibers. However, the "Hookean" Young's Modulus of the fibers up to this formic acid concentration decreases continously to a value of 108 pascals, a factor of 20 down on their stiffness in water. This indicates a progressive penetration of the microfibrils by the formic acid with the o•-helices intact but certainly weakened by the penetration. The result is in contrast to the effect of immersion of fibres in increasingly higher concentration of lithium bromide solutions, as indicated earlier in this review (33). In the latter case the rapid mechanical weakening of the fiber between 6 and 7 Molar concentrations of aqueous lithium bromide solution results from a randomization of the o•-helical structure. Up to just below 6 Molar concentra- tion the mechanical weakening of the fiber was confined to the matrix. In concentrated urea for aqueous solutions as high as 10 Molar the stiffness of the fiber suggests no weakening of the microfibrils (33), with all the change being confined to the matrix. Measurements of optical birefringence also indicate that the crystallinity of the fiber is little changed. Summarizing the effect of swelling on the physical properties of an o•-keratin fiber, we see that three distinct categories of events may occur with three different effects on the mechanical properties of the fiber: (a) The matrix is the main structure swollen, as in the case of water and urea solutions which results in only a weakening and increased mobility of the matrix with the ordered microfibrils intact. (b) The matrix and microfibrils swollen with the o•-helices intact as in the case of the alcohols and formic acid up to 70% concentration in water (V/V), results in a progressive mechanical weakening of the matrix and micro fibrils but has little effect on the optical birefringence, with the X-ray diffraction o•-pattern still present but indicating the swelling. (c) The matrix, microfibrils swollen and the o•-helices randomized as for fibers in concentrated aqueous solutions of lithium bromide at molarities greater than 6.6M. The fiber's mechanical stiffness is drastically reduced and its value corresponds to that of a purely elastomeric solid. The high angle X-ray diffraction o•-pattern and the optical birefringence have both disappeared in agreement with the structure now consisting of randomized polypeptide chains. THE YIELD REGION MECHANICAL When an cr-keratin fiber is extended at a constant rate of straining beyond 2% strain, especially in water, the value of the stress on the fiber does not increase markedly until