Collagen and Gelatin in the Solid State

Temperature dependence of reflectance and transmittance of the artery exposed to air during laser irradiation

Reflectance, transmittance, and temperature of the arterial wall exposed to air are measured during laser irradiation for different heating dynamics. Temperature dependence of the reflectance and transmittance is then deduced. Our results show a competing effect between temperature and dehydration, resulting in a distinct minimum of transmittance and maximum of reflectance. As a consequence, a direct correlation of reflectance and transmittance with temperature is only possible for a specific dynamics.

Examination of self-crosslinked gelatin as a hydrogel for controlled release

A gelatin matrix crosslinked by extensive dehydration was examined for use in controlled drug delivery in this preliminary investigation. Crosslinking is necessary to prevent gelatin dissolution and immediate drug release at body temperature. Treatment at 105 degrees C and reduced pressure induced crosslinking in discs prepared from Type B gelatin. Crosslinking was evaluated by determining changes in gelatin solubility at 37 degrees C in a USP paddle dissolution apparatus. The crosslinking treatment was reproducible and resulted in 90% of the original gelatin mass remaining after 12 h in water and in phosphate buffer solutions of pH 3 and 6.4. The treated gelatin discs remained intact for greater than 24 h at pH 6.4. Chlorpromazine.HCl (CPZ) was incorporated as a model drug by soaking the treated gelatin discs in an aqueous solution of the drug. Release of CPZ at 37 degrees C in the dissolution apparatus was fitted to an empirical equation. A coefficient of this equation was used as the initial release rate for comparison between different release profiles. The roles of drug solubility, matrix swelling and erosion, and potential drug-matrix interactions were examined by conducting release studies at pH values of 3, 4, 6.4, and 7.4. The insoluble, un-ionized form of the drug had the slowest release rate. The soluble, ionized form under conditions of maximum swelling and a possible drug-matrix repulsive interaction had the fastest release rate. General electrostatic drug-matrix interactions were noted which could influence the drug release rate depending on conditions of the study. The times for 50% release of CPZ ranged from 1.8 to 11.3 h.

Tissue regeneration by use of collagen-glycosaminoglycan copolymers

Simple chemical analogs of extracellular matrices have been synthesized by graft copolymerization of a glycosaminoglycan on to type I collagen. A few of these collagen-graft-glycosaminoglycan copolymers (CG copolymers) have diverted decisively the kinetics and mechanism of skin wound healing in animals and humans away from contraction and scar synthesis, towards the direction of skin regeneration. Detailed animal studies show that CG copolymers show maximum biological activity when the average pore diameter and the degradation rate in collagenase are controlled within critical limits. When seeded with a minimum number of cells these active copolymers induce regeneration of skin, including synthesis of a new epidermis and a new dermis in the correct anatomical relationship. Certain unseeded copolymers have also induced regeneration of peripheral nerve. Another copolymer has induced regeneration of the knee meniscus. The unusual biological activity of these copolymers has led to extensive, successful clinical testing of novel medical devices for the treatment of skin loss with severely burned patients.

Wavelength dependence of pulsed laser ablation of calcified tissue

Pulsed laser ablation of calcified biological tissue was studied at several wavelengths in the near-ultraviolet, visible, near- and mid-infrared regions of the spectrum. The primary tissue model was bovine shank bone, while human arterial calcified plaque and normal human artery wall were also studied at selected wavelengths for comparison. Laser irradiances were on the order of MW/mm2, fluences ranged up to 1000 mJ/mm2, and repetition rates varied between 0.3-10 Hz. Spot sizes on the tissue surface ranged from 150 to 850 microns. Laser craters made with wavelengths between lambda = 295 nm and lambda = 375 nm and in the lambda = 3 microns region exhibited the highest quality ablation with clean, sharp cuts following closely the spatial contour of the incident beam. Craters drilled with visible wavelengths between lambda = 450 nm and lambda = 590 nm were generally larger than the incident laser beam spot, irregular in shape and often surrounded by large flakes of tissue debris. Ablation fluence thresholds increased with wavelength through the visible wavelengths and into the mid-infrared, but dropped to their lowest values near lambda = 3 microns. Fluence thresholds obtained with the tissue under a 1 mm depth of saline were approximately twice air thresholds. Ablation yields also varied with wavelength, probably due to increased scattering in the visible region, and were the same under saline as in air.

A food polymer science approach to structure-property relationships in aqueous food systems: non-equilibrium behavior of carbohydrate-water systems

Descriptions of the functional significance of carbohydrates based on the familiar equilibrium thermodynamics of very dilute solutions fail for pragmatical time scales and conditions, which are far from equilibrium. This is not too surprising, since limiting partial-molar properties reflect the independent behavior of solute in the limit of infinite dilution where free volume is maximum at a given temperature, while Tg'-Wg' properties reflect the cooperative behavior of solute-plasticizer blends at the limiting minimum value of free volume to observe relaxation within experimental time scales. Carbohydrate-water systems, with well-characterized structure and MW above and below the entanglement limit, provide a unique framework for the investigation of non-equilibrium behavior. Thermal analysis by DSC reveals the central role of water as a plasticizer for carbohydrates and of the glass transition as a physicochemical parameter that governs their properties, processing, and stability. A classical polymer science approach is used to study structure-property relationships of carbohydrates as water-compatible food polymers, which are treated as homologous systems of polymers, oligomers, and monomers with their plasticizers and solvents. Mechanical relaxation behavior is described by a "transformation map" of the critical variables of moisture content, temperature, and time. The glass curve is a reference contour, which represents the limiting isogram for free volume, local viscosity, relaxation rates, and rotational and translational mobility. Map domains are discussed as aspects of "water dynamics," to dispel the myth of "bound water," and "glass dynamics," to relate to macroscopic structure and collapse phenomena. A particular glass with invariant composition and Tg (prepared by freeze-concentration) is identified as a pivotal and practical reference state. The Tg observed during DSC analysis is often an effective Tg, resulting from instantaneous relative relaxation rates and non-uniform distribution of total sample moisture. Non-equilibrium melting, annealing, and gelation/recrystallization of kinetically metastable, partially crystalline carbohydrate systems exhibit non-Arrhenius kinetics which depend on the magnitude of delta T above the appropriate Tg, as defined by WLF relaxation transformations. Thermally reversible aqueous gels (crystallized from an under-cooled, rubbery melt) are described by a "fringed micelle" structural model for a three-dimensional polymer network, composed of microcrystalline junction zones crosslinking plasticized amorphous regions of flexible-coiled, entangled chain segments.

Beyond water activity: recent advances based on an alternative approach to the assessment of food quality and safety

Water, the most abundant constituent of natural foods, is a ubiquitous plasticizer of most natural and fabricated food ingredients and products. Many of the new concepts and developments in modern food science and technology revolve around the role of water, and its manipulation, in food manufacturing, processing, and preservation. This article reviews the effects of water, as a near-universal solvent and plasticizer, on the behavior of polymeric (as well as oligomeric and monomeric) food materials and systems, with emphasis on the impact of water content (in terms of increasing system mobility and eventual water "availability") on food quality, safety, stability, and technological performance. This review describes a new perspective on moisture management, an old and established discipline now evolving to a theoretical basis of fundamental structure-property principles from the field of synthetic polymer science, including the innovative concepts of "water dynamics" and "glass dynamics". These integrated concepts focus on the non-equilibrium nature of all "real world" food products and processes, and stress the importance to successful moisture management of the maintenance of food systems in kinetically metastable, dynamically constrained glassy states rather than equilibrium thermodynamic phases. The understanding derived from this "food polymer science" approach to water relationships in foods has led to new insights and advances beyond the limited applicability of traditional concepts involving water activity. This article is neither a conventional nor comprehensive review of water activity, but rather a critical overview that presents and discusses current, usable information on moisture management theory, research, and practice applicable to food systems covering the broadest ranges of moisture content and processing/storage temperature conditions.

Nonlinear viscoelastic behaviour of the flexor tendon of the human hand

The mechanical behaviour of the flexor tendon of the human hand is here investigated from the point of view of its nonlinear viscoelasticity. The samples are subjected to several single and multiple step loading histories. A quasilinear viscoelastic constitutive relationship between strain and stress history is assumed. Its characteristic material functions are determined with the aid of simple creep results, and model predictions are compared with the experimental results of complex loading histories. The validity of the quasilinear approach to tendon behaviour is discussed in connection with the deformation mechanism suggested by it.

Collagen banded fibril structure and the collagen-platelet reaction

Bovine hide collagen dispersions were swollen in the pH range 1.6-7.0, treated with glutaraldehyde, and dialyzed to neutral pH. The intensity with which these collagens reacted with human platelets in plasma was studied by aggregometry and scanning electron microscopy. Collagen swollen at a pH below 4.25 +/- 0.30 and treated with glutaraldehyde exhibited greatly reduced platelet aggregating ability after restoration of neutral pH. In addition, the state of supramolecular order in these collagens was investigated by transmission electron microscopy and infrared spectroscopy. Native, insoluble collagen fibrils were found to lose their banded structure, as observed by transmission electron microscopy, reversibly when exposed to low ionic strength aqueous solutions below pH 4.25 +/- 0.30. During the disorder transition, which occurred by time dependent swelling of fibrils, but without their disaggregation, the packing order in the fibrils was largely abolished while the triple helical structure of individual collagen molecules was retained. Chemical modification of collagen by glutaraldehyde treatment was found to prevent recrystallization of collagen during dialysis to neutral pH but did not otherwise affect the collagen-platelet reaction. The results of altering collagen mass dose (concentration) demonstrated the critical importance of traces of banded fibrils which resisted disordering below pH 4.25. The data suggest that collagen preparations which are free of significant traces of banded fibrils, but which are made up of collagen molecules possessing triple helical structure do not induce platelet aggregation, irrespective of dose.

Infrared laser bone ablation

The bone ablation characteristics of five infrared lasers, including three pulsed lasers (Nd:YAG, lambda = 1,064 micron; Hol:YSGG, lambda = 2.10 micron; and Erb:YAG, lambda = 2.94 micron) and two continuous-wave lasers (Nd:YAG, lambda = 1.064 micron; and CO2, lambda = 10.6 micron), were studied. All laser ablations were performed in vitro, using moist, freshly dissected calvarium of guinea pig skulls. Quantitative etch rates of the three pulsed lasers were calculated. Light microscopy of histologic sections of ablated bone revealed a zone of tissue damage of 10 to 15 micron adjacent to the lesion edge in the case of the pulsed Nd:YAG and the Erb:YAG lasers, from 20 to 90 micron zone of tissue damage for bone ablated by the Hol:YSGG laser, and 60 to 135 micron zone of tissue damage in the case of the two continuous-wave lasers. Possible mechanisms of bone ablation and tissue damage are discussed.

Swelling studies of gelatin. II: Effect of additives

Interactions between gelatin and six cationic, anionic, and nonionic drugs or excipients were investigated through their effects on initial swelling rate and equilibrium swelling of gelatin. Short rectangular strips of Type B gelatin containing the additives were immersed in buffer solutions of pH 7.0 at 20 degrees C. Their weight gain due to uptake of buffer solution and their weight loss due to leaching of the additive and of gelatin were determined as a function of time. During preparation of the strips, methyprylon and dicloxacillin sodium crystallized, while octoxynol 9 separated as small droplets in the gelatin matrix. Up to 7% of gelatin leached into the buffer solution during 96 h of immersion from strips of plain gelatin and strips containing five additives. The sixth additive, cetylpyridinium chloride, tripled the amount of gelatin leached while most of this additive remained in the gelatin strip. The other five additives were largely or completely extracted by the buffer solution. Potassium chloride underwent the fastest leaching, being completely dissolved within the first half hour. Octoxynol 9 was extracted most slowly because the swelling solution formed a viscous liquid crystalline phase inside the gelatin. Swelling followed second-order kinetics. Initial swelling rates and equilibrium swelling were calculated with a linearized function. Cetylpyridinium chloride, dodecylammonium chloride, and methyprylon reduced the initial swelling rate of gelatin while dicloxacillin sodium increased it. Octoxynol 9 and potassium chloride left it unchanged. Cetylpyridinium chloride and dodecylammonium chloride reduced the equilibrium swelling of gelatin substantially. Dicloxacillin sodium and octoxynol 9 increased it substantially, while potassium chloride and methyprylon increased it slightly. The extensive interaction of the cetylpyridinium ion with gelatin may result in reduced bioavailability.

In vitro and in vivo characterization of synthetic polymer/biopolymer composites

Collagen, extracted from rat tail tendons using dilute acetic acid, was fabricated into films for subsequent characterization and biocompatibility testing. The reconstituted collagen was characterized with infrared spectroscopy, solution viscosity, contact angle, and tensile testing techniques and was found to be pure with molecular and physical properties consistent with findings of previous researchers. Composites composed of collagen coated on urethane and Silastic Rubber films were fabricated to give improved tear resistance. The biocompatibility of the composites and individual polymers was evaluated by discs implanted in the paravertebral muscle of rabbits. After four weeks none of the materials induced any gross changes in the muscle. Histopathological evaluation revealed a fibrous capsule around all of the materials. Collagen and collagen composites exhibited a stronger reaction as evidenced by a larger fibroblast layer and a variety of inflammatory cells, lymphocytes, eosinophils, and macrophages. The urethane was rated with a response index of 1.5 versus 3.25 for the urethane/collagen composite; Silastic Rubber rated a response index of 1.67 versus 3.12 for the Silastic Rubber/collagen composite; collagen rated a response index of 3.3. The polyester sutures also induced a reaction with a larger fibrous capsule but fewer inflammatory cells as compared to collagen and collagen composites.

Swelling studies of gelatin. I: Gelatin without additives

The swelling rate and the equilibrium swelling of gelatin (type B) were studied by casting warm gelatin solutions into films, cutting them into short rectangular strips after gelation, drying them, and measuring the weight gain on immersion in buffer solutions as a function of time. The process variables investigated included concentration of the gelatin casting solutions, the thickness, drying conditions, age and residual moisture content of the film strips, the chemical nature and concentration of the buffers in the swelling solutions, and the temperature of these solutions at a constant pH of 7.0 (1.9 pH units above the isoionic point). The swelling kinetics followed a second-order equation. The initial swelling rate and the equilibrium swelling of the amorphous portion of the gelatin strips (which was somewhat smaller than the total observed swelling) were calculated from a linearized form of the rate equation. Of the factors investigated, the equilibrium swelling was increased most strongly when the temperature of the swelling solution was raised from 20 to 25 degrees C. Strip thickness was the predominant factor governing the rate of swelling, which was inversely proportional to the thickness. Conditions leading to slower drying and longer storage times promoted more extensive crystallization, thereby increasing the density of the gelatin strips and reducing their swelling rate.

Intermolecular interactions in collagen self-assembly as revealed by Fourier transform infrared spectroscopy

When a solution of collagen molecules, at neutral pH and moderate ionic strength, is warmed from 4 degrees to 30 degrees C, a spontaneous self-assembly process takes place in which native-type collagen fibers are produced. Events occurring during thermally induced fibrillogenesis process can be monitored, in aqueous media and in real time, by Fourier transform infrared spectroscopic techniques. Tentative assignments of observed spectral bands are given.

Tensile properties of antler bone

The tensile deformation characteristics of compact bone from deer antler were measured in both the "dry" and "wet" states and compared with published values for bovine compact bone. The tensile strength in the wet state (108 +/- 5.1 MN/m2) was comparable to the value for bovine compact bone tested at the same strain rate. The modulus values was very low: 7.5 +/- 0.9 GN/m2. The work to fracture was comparatively high, about 3 times that for bovine compact bone. Fractographic examination revealed fibrillar and osteonal shear for samples fractured in the dry state. In the samples tested in the wet state, some regions exhibited pullout of lamellar segments from within a Haversian system. The results are explained in terms of the higher collagen content and lesser degree of mineralization in the antler.

Design of an artificial skin. II. Control of chemical composition

Detailed methodology is described for the reproducible preparation of collagen--glycosaminoglycan (GAG) membranes with known chemical composition. These membranes have been used to cover satisfactorily large experimental full-thickness skin wounds in guinea pigs over the past few years. Such membranes have effectively protected these wounds from infection and fluid loss for over 25 days without rejection and without requiring change or other invasive manipulation. When appropriately designed for the purpose, the membranes have also strongly retarded wound contraction and have become replaced by newly synthesized, stable connective tissue. In our work, purified, fully native collagen from two mammalian sources is precipitated from acid dispersion by addition of chondroitin 6-sulfate. The relative amount of GAG in the coprecipitate varies with the amount of GAG added and with the pH. Since coprecipitated GAG is generally eluted from collagen fibers by physiological fluids, control of the chemical composition of membranes is arrived at by crosslinking the collagen--GAG ionic complex with glutaraldehyde, or, alternately, by use of high-temperature vacuum dehydration. Appropriate use of the crosslinking treatment allows separate study of changes in membrane composition due to elution of GAG by extracellular fluid in animal studies from changes in composition due to enzymatic degradation of the grafted or implanted membrane in these studies. Exhaustive in vitro elution studies extending up to 20 days showed that these crosslinking treatments insolubilize in an apparently permanent manner a fraction of the ionically complexed GAG, although it could not be directly confirmed that glutaraldehyde treatment covalently crosslinks GAG to collagen. By contrast, the available evidence suggests strongly that high-temperature vacuum dehydration leads to formation of chemical bonds between collagen and GAG. Procedures are described for control of insolubilized and "free" GAG in these membranes as well as for control of the molecular weight between crosslinks (Mc). The insolubilized GAG can be controlled in the range 0.5--10 wt. % while "free" GAG can be independently controlled up to at least 25 wt. %; Mc can be controlled in the range 2500--40,000. Studies by infrared spectroscopy have shown that treatment of collagen--GAG membranes by glutaraldehyde or under high-temperature vacuum does not alter the configuration of the collagen triple helix in the membranes. Neither do these treatments modify the native banding pattern of collagen as viewed by electron microscopy. Collagen--GAG membranes appear to be useful as chemically well-characterized, solid macromolecular probes of biomaterial--tissue interactions.

Design of an artificial skin. I. Basic design principles

Individuals who suffer extensive loss of skin, commonly in fires, are acutely ill, in danger of succumbing either to massive infection of to severe fluid loss. Patients who survive these early threats must often cope with problems of rehabilitation arising from deep, disfiguring scars and crippling contractures. In this report we describe the physiocochemical, biochemical, and mechanical considerations that form the basis for two-stage design of a membrane useful as an experimental wound closure. Stage I of the design, applicable to short-term acute use, calls for a membrane which displaces efficiently air pockets from a carefully prepared woundbed, free of weak boundary layers, and maintains the moisture flux through the wound at an optimal level. Optimization of the surface energy, modulus of elasticity, energy to fracture and moisture permeability of the membrane are among the essential attributes of Stage I design. Stage 2 of the design, applicable to long-term, chronic use, focuses on a nonantigenic membrane which performs as a biodegradable template for synthesis of neodermal tissue. A survey of candidate materials suggests reasons for selection of a porous, crosslinked collagen-glycosaminoglycan coprecipitate as the chemical basis of an evolving design which was initiated 10 years ago. Over the past several years a set of membranes has been iteratively designed on this basis and has been used to cover satisfactorily large experimental full-thickness skin wounds in guinea pigs. Such membranes have effectively protected these wounds from infection and fluid loss for over 25 days without rejection and without requiring change or other invasive manipulation. When appropriately designed for the purpose, the membranes have also strongly retarded wound contraction and have become replaced by newly synthesized, stable connective tissue. Several rules relating the molecular structure and morphology of these membranes to cellular response of adjacent tissue have also been derived. This report is the first in a series which details the methodology of preparation and the record of performance.

In vitro blood compatibility of glycosaminoglycan-precipitated collagens

Precipitation of bovine hide collagen by chondroitin 6-sulfate at low pH and subsequent crosslinking enhances the blood compatibility of native collagen. Both dehydrothermal crosslinking and complexation with chrondroitin 6-sulfate separately decrease the platelet-aggregating activity of collagen. Crosslinking also decreases the number of free acidic and free basic residues on collagen, which suggests that crosslinking involves these residues in condensation reactions with formation of intrachain and interchain synthetic peptide bonds. Clotting times for collagen precipitated with chondroitin 6-sulfate indicate that this surface does not activate or interfere with coagulation via either the intrinsic or extrinsic pathway. These findings support further consideration of collagen modified by chondroitin 6-sulfate as a blood compatible material.

The multicomposite structure of tendon

A revised morphological model for the crimp structure of tendon is presented. The 300-500 mu diameter tendons of the mature rat tail are comprised of from one to more than ten substructures, called fascicles, of 80-320 mu diameter. Fascicles each possess a "crimp structure" demonstrable in the polarizing microscope and neighboring fascicles within a tendon usually exhibit crimp registry. The fascicle itself is shown to be a cylindrical array of planar-zig-zag crimped 500-5000 A diameter collagen fibrils. The approximate cylindrical symmetry of the fascicle is domonstrated by SEM not equal to and polarizing optical microscopy. A method of replacing native water with other liquids of refractive index near to that of collagen is utilized to reduce or eliminate light diffusion and therby greatly improve OM observations. Small bunches of collagen fibrils removed from the tendon are shown to exhibit the simple planar zig-zag morphology described in previous literature. The planar crimping of collagen fibrils and their assemblage into cylindrically symmetric fascicles is verified by small angle X-ray diffraction.