Articles Mercury Intrusion Porosimetry, Nitrogen Adsorption, and Scanning Electron Microscopy Analysis of Pores in Skin N. Nishad Fathima, Aruna Dhathathreyan,* and T. Ramasami Chemical Laboratory, Central Leather Research Institute, Adyar, Chennai 600 020, India Received March 21, 2002; Revised Manuscript Received May 20, 2002 Stability of collagenous matrixes such as skin and leather with respect to changes in their dimensions on heating has long been correlated with degree and type of cross links formed and short-range ordering in angstrom unit scales. Macroscopic dimensional changes may be expected to involve alterations in the long- range order as well as supramolecular assemblies in skin and leather. This study relates thermal shrinkage of skin matrixes with alterations observed in micro-, meso-, and macroporic structures. Changes in the pore structure of skin associated with thermal shrinkage have been studied using nitrogen adsorption and mercury intrusion porosimetry measurements. A comparison of results obtained using both techniques has been made. These results indicate that although the percentage porosity of the matrix decreases, the BET specific surface area increases on shrinkage. An insight into the changes in the pore systems of skin induced by thermal shrinkage has been gained. Introduction Skin is a composite matrix of collagen with other conjugate materials. Collagen matrix in skin is known to display a defined structural organization. 1,2 The structure of type I collagen in skin has been investigated, and its hierarchical ordering and self-assembly processes in tissues have been highlighted earlier. 3-7 Structure and connectivity of pores in skin influence the heat and mass transport processes associated with the thermoregulatory function of the organ. The fibrous collagenous network undergoes thermally in- duced structural transitionssthe shrinkage phenomenon. The extensive macroscopic shrinkage of collagen in water is considered as an outward manifestation of the helical-coil transition (melting) of collagen. 8-10 Such a change is accompanied by the absorption of heat, which is an endot- hermic process. The shrinkage of leather and collagen, when heated in an aqueous medium, has been the subject of extensive investigations. 11,12 The dimensional stability of skin matrix against heat needs to be related to the molecular organization, assembly processes, and hydration of constitu- ent fibers and fiber bundles as well as alterations in microscopic dimensions. In this regard, it is expected that the pore structure of the matrix may be used to control adsorption, diffusion phe- nomena, fluid flow, and thermal conductivity. The pore volume of a solid is expected to include the total volume of all the pores within the grains. Measurements of total porosity and distribution of pore sizes are difficult in the case of a hydrated matrix such as skin. The methods used for measur- ing pore sizes of skins are, generally, mercury intrusion porosimetry and nitrogen adsorption. These techniques assume that (a) the geometry of all pores is regular, (b) the pores are interconnected, and (c) the size distribution is not affected by the loss of water upon drying. Kanagy 13 has made studies on the pore structure in leather using a mercury porosimeter with pore radii corresponding to the macropore range and surface area measurements using nitrogen adsorp- tion. 14 Some studies have been reported earlier on pore-size distribution in the skin; 15-19 they concern mainly changes in pore dimensions in different types of leathers. In earlier studies, changes as a result of stabilization of skin through the formation of cross links have formed the focus. Changes in pore structure of the native collagen matrix resulting from thermal shrinkage have not yet been reported. In our previous study on the shrinkage phenomenon of collagen matrix, the volume changes accompanying the removal of water from skin, expressed as partial fractions of volumes of solid, liquid, and air (V solid , V liquid , V air ) was estimated using a dilatometry technique on a macroscopic scale. 20 In the present study, the effect of heat on the pore connectivity of collagen during the shrinkage phenomenon has been investigated using nitrogen adsorption and mercury intrusion porosimetry techniques. The use of nitrogen adsorption and mercury intrusion porosimetry for determi- nation of pore structures of a hydrated biological matrix has limitations resulting from changes in state of hydration during measurements. However, prior information on porosity of skin gained using these experimental techniques shows that they complement each other. 21 In the present investigation, * Author for correspondence: tel, + 91 44 443 0273; fax, + 91 44 491 1589; e-mail, adhatha@md5.vsnl.net.in. 899 Biomacromolecules 2002, 3, 899-904 10.1021/bm025541q CCC: $22.00 © 2002 American Chemical Society Published on Web 08/17/2002