RESEARCH
Sols Allow Rebuilding Biological Materials West German chemists rebuild cow's aorta from sols of its three main layers; gel spheres similar to pearls also formed Colloid chemists at the University of Kiel (West Germany) have succeeded in rebuilding the aorta of a cow from sols of its three main structural layers. They re-formed the tissue by infusing cadmium ions into the sols, causing partial discharge and dehydration of the particles and ordered gelation. The work, led by Dr. Heinrich Thiele of Kiel's department of colloid chemistry, opens the door to many farreaching and intriguing possibilities. Among the first of such possibilities is making surgical implants. Dr. Thiele feels his system can be used to make artificial parts for the human body. These parts would be superior to plas tic parts now being used, he believes, and pose fewer complications. Reconstruction of other, more com plex biological materials also looks promising. Co-workers of Dr. Thiele at the Institut Pasteur, in Paris, have dissolved the five layers of a cornea (from a cow) and reconstructed three of them. They have also dissolved
and re-formed a lens, although they are not sure whether they have re formed the fibers into their original helical shape. Attempts at rebuilding bone, teeth, and skin will also be made. Ulti mately, such material may be synthe sized from its chemical elements. But Dr. Thiele stresses that he is a colloid chemist mainly interested in struc tures. However, he feels that his work will prove valuable in developing im proved therapeutic techniques based on improved knowledge of structure. Tissue. Basically, Dr. Thiele and his co-workers have dismantled, then reassembled biological tissue. "We treat biological tissue as a gel con structed of ordered particles," Dr. Thiele says. Normally, in artificial gels, the molecules are not ordered. But in most natural gels, such as starch, collagen, and the like, the molecules are ordered and show bire fringence, swelling, and ion exchange. Using a frozen aorta, the Kiel chem-
TISSUE. Dr. Heinrich Thiele of the University of Kiel studies structure of tissue. Re-forming of tissue by the technique that the German chemists have developed may be useful for making surgical implants, Dr. Kiel says 40
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ists first remove fat and connective tissue. Then they separate the aorta into its three main layers: the intima (innermost layer), media, and ex terna (outer layer). Preserving the aorta under water with a few drops of chloroform at 40° C. causes the mate rial to swell and discharge mucopoly saccharide ( MPS ) acids—hyaluron and chondroitin sulfuric acids, for ex ample—and other soluble substances. The swelling makes it easier to sepa rate the layers. The three layers are then treated separately and dissolved into three collagen-elastin sols. To form the sols, it's first necessary to completely remove fat, MPS acids, and other soluble material. To do this, the Kiel group chops the separate layers into small pieces. To extract fat, they stir the pieces with acetone or methylal-methanol in a vibrator.1 The extraction must be completed in less than three hours to avoid denatur ing the proteins. Next, the chemists remove MPS acids and soluble materials with a salt solution or dilute alkalies. They pre serve the three layers separately in 0ΛΝ lithium hydroxide at 25° ± 1 ° C. for five days, renewing the lithium hydroxide solution eight times. Following this treatment, the layers swell considerably and are transparent and glassy. The material is diluted with an equal amount of 0.4N lithium hydrox ide in a homogenizer, then dialyzed for six days. All three layers yield highly viscous, 1% collagen-elastin sols having a pH of 7 to 7.5 and strong birefringence. Infusion. In preliminary experi ments aimed at re-forming the sols into gels, the German chemists found that infusion of the sols with copper, cadmium, lead, mercury, and zinc ions (as nitrates) forms ionotropous gels. In all cases, these sols contain no MPS acids. Except for cadmium, these ions form gels with poor mechanical prop erties and little birefringence. Lan thanum ion forms gels comparable to
AORTA. A real aorta (left) can be transformed into a sol, from which an artificial aorta (right) can be made. One potential use for the technique—developed by West Germany's Dr. Heinrich Thiele and co-workers—is to make surgical implants that would pose fewer complications than do some plastic implants
the ones formed with cadmium ion. Sols containing 1 or 2% MPS acids, however, give gels that are comparable to tissue, with good mechanical properties and strong birefringence. At the same time, concentration of the electrolyte can be reduced. Cadmium or copper nitrate solutions (as dilute as 0.0IN) produce gelling, for example. Thus, Dr. Thiele concludes, satisfactory gelling can come about only under physiological conditions— in the presence of the MPS acids. In their studies, the German chemists have found that the threadlike molecules of collagen-elastin gels are interconnected in a network through the MPS acids. MPS acids and protein form a symplex—a slightly soluble salt of two colloidal ions. In living bone and tissue, MPS acids decrease with age. Realizing that they would not be able to rebuild the aorta in a single chemical reaction (since this is too complex a system to duplicate), Dr. Thiele and his co-workers decided to build up the three layers of the aorta separately. To do this, they hung a porous clay tube, closed at one end, in a wide test tube filled with sol of the intima. The sol contained MPS acids. Then they injected I N cadmium nitrate into the clay tube. The electroyte diffused through the clay tube into the sol, forming a cadmium gel around the tube; this system was analogous to the original intima. Next, the clay tube, coated with the reconstructed intima, was dipped into the sol of the media. Again, the cadmium ions diffused and caused a gel analogous to the media to grow on top of the intima. The outer layer or externa was then grown the same way.
Finally, the gel tube—the rebuilt aorta —was stripped off the clay tube and placed in 0.0IN hydrochloric acid, where hydrogen ions replaced the cadmium ions. The construction and decomposition are reversible. Addition of an alkali dissolves the gel back into a sol. But the gel can be re-formed if desired. Nature. The techniques used in rebuilding the aorta stem from years of research into the nature of biological tissue and gels. Previous work has shown that biological tissue is a gel that consists of well-ordered, threadlike molecules. In his research, Dr. Thiele has found that certain ions diffusing into sols of some fibrous colloids order and arrange the particles to form gels similar to the natural substance. For example, when a copper nitrate solution is added to a sol made from sodium alginate in water, each drop of the nitrate is immediately encased by a membrane and becomes a small sphere. These spheres are biréfringent. The copper ion exchanges with the sodium ion to form the relatively insoluble copper alginate (the gel) and sodium nitrate. Pectate, cellulose glycolate, hyaluronic acid, and deoxyribonucleic acid, among others, behave similarly. Synthetic polyanions, organic or inorganic, also behave the same way. Sodium poly aery late, for example, and I N silver nitrate react to form gel balls of silver polyacrylate that are highly biréfringent. Using such materials, both natural and synthetic, Dr. Thiele has made a number of models. Some of these models closely match biological structure. It was this work that led to rebuilding of the aorta.
Only colloids having ionic groups— that is, polyions or poly electrolytescan be ordered by ions, Dr. Thiele says. Colloids without ionic groups or with undissociated ionic groups give amorphous, isotropic gels. In an analogy that Dr. Thiele likes to use, the polyelectrolyte is the framework, the ionic groups are the hand-holds, and the diffusing counterions are the workers who place the steel. The counterions (they must be polyvalent) that give the best results vary with the polyion. Radius, valence, and activity are important factors. Copper ( I I ) , for example, works well with alginate, pectinate, and cellulose glycolate, but silver (II) is better suited for polyacrylates. The degree of order of the threadlike molecules depends on the chain length of the polyion, concentration of the sol, and type, number, and dissociation of the ionic groups in the molecule. After construction of the framework is completed, counterions are no longer needed and can be replaced by other ions, such as hydrogen ions. This does not affect the order. A key factor in forming an ordered gel from a sol is that the colloidal particles in the sol be only partially discharged and dehydrated. Complete discharge and dehydration result in coagulation with an amorphous coagulum. Pearls. Dr. Thiele has come across several sidelights in his works. Using a metal salt of alginic acid and colloidal gold, he has made gel spheres that are similar to natural pearls. He has also found that five layers form between sol and electrolyte during gelation. In the zone next to the electrolyte, a smooth, dense membrane of ordered molecules forms. This membrane can be used as an ultrafilter for colloids, germs, and viruses. The next zone, or layer, is a capillary zone formed by microscopic drops of liquid being forced through the sol by the gel. The capillaries are equal in length (20 to 30 mm.), and have diameters of 1 to 200 microns; they number from several thousand to a few million per square centimeter. The number, diameter, and length of the capillaries can be changed at random by changing the chain length of the fibrous molecules and their concentration, and by adjusting the activity, diameter, and valence of the counterions. The capillaries can be stabilized mechanically and chemically to form a useful filter. JUNE
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