Supercritical fluid technologies – basis for creating innovative implants
CHOICE IN FAVOR OF XENOPLASTY
In modern medical practice, various approaches are used to replace damaged tissue in reconstructive operations. Use of own tissues (autoplasty) is the safest technique, since in this case the probability of an immune response is minimal with the optimal biocompatibility of the graft. However, autoplasty is associated with additional trauma to the patient, which increases the duration of rehabilitation and the time of surgery. Another option is the use of cadaveric tissues (alloplasty), the disadvantages of which are the risk of viral contamination, a higher probability of rejection, a limited supply of raw materials, as well as the complexity of documenting the quality of the biomaterial. Implants can be made of synthetic materials based on relatively cheap and affordable raw materials, but such products are not always fully involved in metabolism and can not have biologically active properties, which limits the scope of implants.
FOR LARGE TASKS
In 2010, the first serial product of Cardioplant was a xenopericardial plate, which is made from a pericardium bag of pigs and bulls and is used to correct a whole group of surgical defects. Different processing modes allow to vary the properties of the material for different applications. The material is created to restore, strengthen and reconstruct affected parts of organs and tissues, in particular, heart and vessel defects, tendon plasty in traumatology and orthopedics, denture prosthetics in neurosurgery, urinary tract plasty, strengthening of the pelvic floor muscles in urogynecology, for the reconstruction of soft tissues in abdominal surgery.
Another innovative serial product of Cardioplant is an extracellular collagen matrix made from the submucosal layer of the small intestine of the pig. The material has the highest potential for colonization by its own cells of surrounding tissues, the ability to rebuild without scar formation and the possibility of healing without completely closing the recipient's own tissues. It has already been successfully used to treat burns of varying severity, trophic ulcers and injuries received by patients with oncology during long-term irradiation courses.
At the moment, Cardioplant is developing an osteoplastic matrix using the bones of young bull-calves. Such material is used to replace defects in human bone tissue or to increase its volume. For example, in dentistry, the osteoplastic matrix can be used for dental implantation.
All serially produced materials passed a series of technical, preclinical and clinical tests. Leading specialized research institutes of the country including N.N. Priorov Central Research Institute of Traumatology and Orthopedics, Saratov Research Institute of Traumatology and Orthopedics, federal centers of cardiovascular diseases, A.N. Bakulev National Medical Research Center of Cardiovascular Surgery (Moscow) and others were involved in clinical research and in studying the needs of practitioners.
"By changing the methods and regimes of processing, it is possible to regulate the rate of resorption and bio-integration, elastic-deformative indices and properties of the surface of the material, getting products intended for different areas of medicine", says Dmitry Smolentsev, head of the development of osteoplastic biomaterials. "But there are general principles that are mandatory for all materials. Since our products are made from biological tissue, the main task is to remove all antigens and foreign structures that can cause rejection of the implant. Such are the cells, fragments of their membranes, non-collagen proteins, and in the case of working with bone tissue – also lipids, which prevent complete purification of the material. At the same time, it is required to preserve the natural architectonics of the tissue".
Bone matrix is filled mainly with lipids and other lipophilic substances, which are difficult to get rid of. Polysaccharides, proteins and other substances that can cause the body to react and from which it is necessary to clean the material are not only on the surface, but also lie deep enough, under a layer of lipids. That is, satisfactory cleaning of the material is possible only with complete disposal of fats.
Traditional methods of removing these elements using organic solvents or temperature influence negatively affect the properties of the material. For example, when eliminating lipids with an organic solvent, it is very problematic to get rid of the solvent itself afterwards. Therefore, after several years of experiments, the specialists of Cardioplant have chosen the technology of supercritical fluid extraction (SFE), which allows achieving the maximum technically achievable purity of the material without damaging it.
SFE is an extraction process using a supercritical substance as a solvent. In the region above the critical temperature and pressure, the difference between the liquid and the gas disappears, and a single fluid, relatively dense, easily compressible medium appears. In this form, the substance, on the one hand, still has a significant dissolving power, like liquids, and on the other hand, has the transport ability more characteristic of gases, low viscosity, easily variable density, high diffusion coefficients.
The density and solubility of supercritical fluid is easily controlled by changing the pressure or temperature. Therefore, when it is used as a solvent, it becomes possible to fine-tune the dissolving power for a specific task, in particular, to obtain fractions enriched in various components. And due to the absence of interfacial tension, supercritical fluids easily penetrate into porous materials.
SFE is used for the extraction, separation and concentration of nonpolar products of vegetable and animal origin, such as fatty and essential oils, steroids, waxes, medicines from natural substrates, low-molecular compounds of polymeric and composite materials. Supercritical CO2 is capable of extracting many nonpolar substances: terpene compounds, waxes, pigments, high molecular weight saturated and unsaturated fatty acids, alkaloids, fat-soluble vitamins and phytosterols.
To date, supercritical fluid extraction can already be attributed to technologies that are firmly embedded in the complex of methods of "green" chemistry. This technology is intensively used in the food, cosmetic and pharmaceutical industries for the isolation and purification of active ingredients from natural and synthetic raw materials, both at the laboratory and industrial levels. Decaffeination of coffee, cleaning of optical cables from test oil solvents, extraction of hops for brewing, extraction of essential and fatty oils from plant raw materials, sample preparation in chemical analysis, cleaning of cork material, regeneration of adsorbents and catalysts have become classic examples of successful introduction of SFE.
SFE FOR DEGREASING
OF XENOGENIC MATERIALS
Most often, supercritical CO2 is used as a solvent in SFE, the advantages of which are high solvency, cheapness, availability, nontoxicity and low critical parameters (critical temperature is 31°C, critical pressure is 74 atm). The Cardioplant's technologists also chose this material. "Carbon dioxide does not need to be removed from the finished product, because after the treatment it simply evaporates, passing into the gaseous phase, and leaves no traces behind it", Smolentsev explains. "In addition to the automatic removal of the solvent, a higher degree of recovery of fat and other soluble components is achieved in the SFE. Carbon dioxide penetrates deeply throughout the thickness of the bone for qualitative cleaning of the pores. In this case, the integrity of the collagen is not impaired. In addition, the purified extracts do not contain residual amounts of solvent, and the solvent itself can be easily regenerated. Thus, the use of SFE allowed to reduce production time and costs, as well as to achieve high quality of products".
To conduct the SFE in the production of Cardioplant, equipment of Waters Corp. (USA) is used, which was supplied and put into operation by SCHAG company. The SCHAG specializes in the field of supercritical fluid (SCF) technologies since 2006. It is a supplier of equipment for SCF technologies of domestic and foreign manufacturers. In addition to delivering serial solutions, the company's specialists carry out the finalization of the equipment for the specific clients' tasks and methodological support. This service became possible thanks to the creation of a joint laboratory with the Kurnakov Institute of General and Inorganic Chemistry of RAS, in which it is possible to consult on the possible application of SCF technologies for solving specific tasks, to conduct trial tests or to develop a methodology.
Supercritical equipment produced by Waters Corp. distinguishes high reliability and automation, the ability to implement various modes of processing. D. Smolentsev: "The most important requirements are the uninterrupted operation and controllability of the equipment, since the slightest deviation of temperature or pressure can lead to a change in the properties of the final product. Waters' SFE system is characterized by functional and convenient program control, which allows monitoring the technological process 24 hours a day. If the parameters deviate from the set values, the system immediately informs us about this, and we can quickly take the necessary measures".
The technique of degreasing of animal tissues developed by Cardioplant with the help of SFE provides the best extraction of lipids and fatty substances with minimum consumption of extractant and processing time. The application of this environmentally friendly method makes it possible to significantly reduce costs and facilitate the production of medical devices. The technique is implemented in the production line of Cardioplant with use of supercritical equipment in a clean room.
The development of innovative medical products is a task that is complex not only technologically, but also in terms of compliance with a variety of formal procedures. "All of our main problems are probably connected with a huge amount of time", D. Smolentsev states. "In addition to the safety check, the materials must undergo an efficiency test when implantation is performed to correct the defect modeled in the experiment, and then it takes from two to four months to collect the information. If the results do not meet expectations, then another cycle of improvement and testing of product is performed".
Nevertheless, Cardioplant shows an impressive dynamics of the development of innovative medical products, and the prospects look even more exciting. And the wide possibilities of SCF technologies, which go far beyond the tasks associated with degreasing materials, will help in this movement. D. Smolentsev: "Virtually all currently applied implantable products are inactive, that is, they serve only as a frame for human tissues, or perform some kind of mechanical function. I believe that in the future implants will acquire their own activity, for example, antibacterial. To do this, it is necessary to impregnate antibiotics in them with a given release rate. And SCF technologies will help to solve this problem". ■