Different types of biotechnological wound coverages created with the application of alive human cells

Currently, the development and the implementation of the new biotechnological wound coverings (skin equivalents) designed for temporal or permanent replacement of damaged or destroyed areas of human skin remains extremely actual relevant in clinical practice. Skin equivalents or equivalents of individual skin layers which include alive cells of different types take a special place among the artifi cial wound coverings. They mostly contain two basic types of cells – fi broblasts and keratinocytes (together or separately). Such bioconstructions are usually served as temporary coverings, which supply the damaged skin by biologically active substances and stimulate the regeneration of the patient's own tissues. In this review we consider as commercially available wound coverings and those which are still studied in the laboratories. Until now ideal substitutes of natural skin have not yet created, so the efforts of many researchers are focusing on the solution of this problem.


Introduction
Autodermoplastics is traditionally used to cover vast wounds.However, this method does not allow covering the whole surface of the wound even with mesh autodermotransplantat, if the total area of the wound exceeds 30-40 % of the body area.One of the ways to solve this problem is the application of biotechnological wound coverages, which may be divided into two kinds: non-cell ones and those, containing living cells of different types and origin.There are currently no ideal bioconstructions, which would meet all the criteria, thus the research is still under way.
Let us consider the requirements to the ideal biological skin equivalent: -no antigenicity; skin compatibility, -no local or systemic toxicity, -no permeability for exogenous microorganisms, -inhibition of the activity of microorganisms in the wound, -moisture permeability, similar to that of normal skin, -suffi cient adhesion to the wound surface, -compliance with the surface irregularities, -elasticity, not disturbing the mobility of subiculums, -resistance to linear and shifting deformations, -longitudinal strength, -biodegradation ability, -extensive lifetime and simplicity of storing, -low cost, -usability for medical staff, -acceptability and minimal discomfort for a patient, -semitransparency to have the possibility of observing the process of wound healing, -shortened time of wound healing [1].Experimental data prove the relevance of the contact of wound surface and different types of cells, which may be a part of artifi cial skin substitutes.At present there are two main types of cells, which are the components of clinically available bioconstructions, -keratinocytes and fi broblasts (together or separately).The processes of proliferation and differentiation of keratinocytes are known to have better performance in the presence of fi broblasts; in turn, the functional state of fi broblasts depends on the effect of keratinocytes [2].For instance, it was demonstrated that biologically active substances, discharged by fi broblasts, affect the proliferation and migration of skin melanocytes [3].At the same time all the clinically applied skin equivalents do not contain any nervous or immune cells, sweat glands and hair follicles, therefore, they are not capable of performing all the skin functions.As a rule, these bioconstructions act as temporary biologically active wound coverages, providing the damaged tissues with the components of extracellular matrix, cyto kines and growth factors (vitronectin, tenastin, collagen, glycosaminoglycans, interleukins 1, 6, 8, interferons α and β, PDGF, FGF-1, KGF-1, PDGF, VEGF (vascular endothelial growth factor), TGF-α) [4,5].Due to this fact there is the migration of the recipient organism cells into the wound area along with their proliferation, regeneration of the dermal layer and wound re-epithelization, the wound area is «condi-tioned» for the use of skin transplantats.It promotes shortening the period of wound healing and prevents the formation of adhesions and scars compared to the cell-free bioactive wound coverages [6].
The application of autologous cells to obtain skin equivalents prevents the possibility of immune rejection and the infection of a patient with foreign microorganisms, but it hinders obtaining of the product at the required moment and increases the cost of its production.Taking these two problems into consideration, the preference is given to the connective tissue cells, fi broblasts; this fact is confi rmed by the communications about temporary immunogenic tolerance of the recipient organism to the allogeneic fibroblasts and their survival and proliferation in the donor organism for up to two months without the symptoms of the immune rejection [7,8].At the same time the allogeneic keratinocytes are highly immunodominant, they are rejected by the organism and usually perish several weeks after the application onto the wound [9].This earlier death, compared to the allogeneic fi broblasts, is likely to be the consequence of a high level of expression of human leukocyte antigen (HLA).

I. Epidermal equivalents
With the possibility of cultivating human keratinocytes in vitro and multiple increase in their total number [10], they were applied in the wound treatment [11], which promoted the survival of some serious BioTissue Technologies GmbH, Freiburg, Germany Chronic venous ulcers of lower extremities [33,34] Full thickness wounds of animals [22]

CellSpray
Clinical Cell Culture (C3), Perth, Australia Partial thickness wounds; donor sites for skin transplantats [13] Different types of biotechnological wound coverages created with the application of alive human cells patients [12].The creation of epidermal equivalents involves the application of autologous keratinocytes, isolated from small (2-5 cm 2 ) skin biopsy samples and proliferating in the culture with the formation of multilayer epithelial layers.These layers may be enzymatically separated from the surfaces of cultural vials and transferred to the supporting substrate, for instance, wax-bearing gauze or other carriers.This construction is applied to the wound taking into account basal-apical orientation of cells [13].
Using the modern methods of keratinocyte expansion in vitro, it is possible during 3-4 weeks to obtain from 3 cm 2 of the tissue the epithelial autotransplantats, large enough to cover the whole body surface [14].The drawbacks of such plate-like epithelial autotransplantats are long-term cultivation, fragility, complicated application of transplantats, and, mainly, unexpected clinical results in terms of different degree of acceptability -15 to 85 % [13,15].
The enzymatic processing may change the structure of fi brils, responsible for the interaction of keratinocytes and the tissues of the recipient organism [16,17].Due to this fact there is weak adhesion of the transplantat to the wound and the formation of blisters in case of even insignifi cant shifts months after the transplantation [11].The cultivation of keratinocytes on special delivery membranes, which will be later applied to the wound surface along with the cultivated cells, allows avoiding the requirement of using enzymes [16,18] and promotes the proliferation process, especially if natural, not synthetic, material is used.In particular, if fi brin is used as scaffold [18], the rate of transplantat formation increases almost three-fold.
At present epidermal equivalents are almost absent in the burn centers of Ukraine.Common drawbacks of these skin substitutes are the impossibility of their independent application in the therapy of full thickness wounds, only in the combination with dermal transplantats [19,20] as well as their high cost.
Well-known epidermal equivalents Epicel [12], EPIBASE [21] and EpiDex [22], the earliest among similar bioconstructs, are mere sheets of keratinocytes, cultivated for 15 days.In addition to the abovementioned drawbacks, they have short (24 hours) shelf life [23].Another commercial product, MySkin [24], consists of a scaffolding silicon membrane, covered with subconfl uent layer of the cultivated keratinocytes.The keratinocytes of the product Laserskin (Vivoderm) [25] form a confl uent layer on the surface of the hyaluronic micromeshed membrane and are capable of migrating downwards, through the scaffold, to the wound bed.
Products Bioseed-S [26] and CellSpray [27] involve the use of keratinocytes, composed of the subconfl uent layer, in their most active proliferation state.The keratinocytes of the product of Bioseed-S company are evenly distributed inside the fi brin gel of allogeneic origin.Product CellSpray is merely the suspension of keratinocytes.After the application onto the wound surface the proliferation of keratinocytes continues up to the formation of the epithelial skin layer [14,26,27].

II. Dermal equivalents
Only a few of current dermal skin substitutes contain cells, since the production of large amounts of cell-free constructions is much easier and cheaper in large patches and it is much simpler to receive the license for their clinical application.It should be noted that dermal cellular coverages are almost unavailable for the application in burn centers of Ukraine.
As for well-known product TransCyte (previous name -Dermagraft-TC), the cultivated allogeneic fi broblasts are immobilized on the surface of the composite consisting of a silicon fi lm, a nylon net and dermal porcine collagen.This temporary dermal coverage for the wound bed has to be removed after 7-14 days and replaced by the autotransplantat.The scaffold of Dermagraft [35] consisting of polyglycolic acid, polylactic acid and extracellular matrix degrades on the wound surface via hydrolysis in 20-30 days.The extracelular matrix is produced by the cellular component, presented by cultivated allogeneic fi broblasts.
The bioconstruction Hyalograft 3D [36,37] is formed by the hyaluronic micromeshed membrane, which is the scaffold for cultivated autologous fibroblasts.

III. Dermo-epidermal (composite) skin substitutes
The dermo-epidermal equivalents contain cultivated fi broblasts and keratinocytes, located in different layers of the bioconstruction.
The bioengineered construction Apligraf (Apli graf Organogenesis Inc., USA) is currently the most successful clinical dermo-epidermal equivalent [9].The cells of allogeneic origin form a confl uent layer in the gel matrix of bovine type I collagen.The product is a temporary transplantat, resorbed on the wound surface, its drawback being short shelf life (up to 5 days).
The allogeneic fi broblasts of OrCell (OrCel Ortec International, Inc., USA) are immobilized inside the sponge-like gel of bovine type I collagen, one side of which has a formed confl uent layer of allogeneic keratinocytes.The product is resorbed on the wound surface 7-14 days after the application.
The scaffolding porous matrix of PolyActive [44,45] (PolyActive HC Implants BV, the Netherlands) consists of a soft component -poly(ethylene oxide) (PEO) -and a solid component -poly(butylene te rephthalate) (PBT); it contains autologous cultivated cells.Notably, this bioconstruction is mainly used for bone reconstruction rather than for skin repair.
The Product TissueTech Autograft System (Fidia Advanced Biopolymers, Italy) [46]) is not a «pure» two-layer skin substitute, but rather the combination of dermis equivalent Hyalograft 3D and epidermis equivalent Laserskin.

The elaboration of matrices using gelatin
Gelatin, a natural product, is formed due to partial denaturation and degradation of collagen fi brils and like collagen is the attractant for fi broblasts in the wound healing processes [49,50].Because of low cost and the possibility of in vivo degradation, gelatin is widely used as a coverage in wound treatment [51,52,53].Other useful properties of gelatin are the absence of antigen activity, contrary to collagen [54], and strong hemostatic effect [54,55].As gelatin is water-soluble, the molecules of gelatin-containing structures should be cross-linked for the application in long-living bioconstructions.The stimulation of dermal-epidermal maturity [36,37].Ulcers of lower extremities (in combination with Laserskin) [43].S. B. Lee et al. [56] described that the scaffold for human fi broblasts and keratinocytes was the surface of a porous matrix, consisting of gelatin and (1-3),(1-6)-β-glucan.The latter is the carbohydrate of the microorganism Schizophyllum commune, which demonstrates the antibacterial, antiviral, anticoagulant and wound-healing activity [57].
Noteworthy is the work of H. M. Powell and S. T. Boy ce [58] who used a fi brous matrix, formed by the electrospinning method.The composition of the construction included human fi broblasts and keratinocytes, forming confl uent cellular layers between the fi brils.It should be noted that electrospinning is the high voltage process of creating long fi brils with the diameter ranging from nanometers to fractions of micrometer.Such e-spun fi brils are remarkable for high poro sity and strong interrelation between the pores [59].
In contrast to the abovementioned works, Sh.Huang et al. [60] obtained so called gelatin microspheres of dozens to hundreds of micrometers, and viewed gelatin as a potential depot for the biologically active substances.Then the surfaces of microspheres were used to cultivate multipotent mesenchymal stromal cells (MMSC) of murine bone marrow, after which the microspheres were mixed with the solution of type I collagen.After gelatin jellying these bioconstructions were tested for treating burn wounds of mice with positive results.
Other authors, R. Quan et al. [61] used lyophilization while creating the cellular scaffold of the following composition -gelatin + chondroitin-6-sulfate + + hyaluronic acid.The stem cells of hair follicles, rarely used in similar research, served as a cellular component.The potential of these bioengineered products in the therapy of skin damage of laboratory rats was demonstrated.R. V. Shevchenko et al. [62] used a bilayer cellular scaffold.The porous layer of gelatin contained human primary fi broblasts, and the silicon layer with smaller pores was cultivated with human keratinocytes.From the authors' standpoint, the results of the application of this dermal substitute in treating porcine skin damage allowed considering it as one of the biomedical constructions which will be used in future.

The elaboration of matrices using collagen
As shown by many authors, collagen is a good substrate for adhesion and proliferation of the cultivated cells.For instance, in the work of J. George et al. [63] the collagen porous matrix was applied on the layer of proliferating fi broblasts, which entered the pores and prolonged the proliferation for 60 days.As a result, the formed structure was morphologically similar to the dermal skin layer.
Different groups of researchers, for instance, A. Kinster et al. [64], S. H. Ahn et al. [65] also created porous collagen matrices.In these constructions fi broblasts were located inside the porous layer, whereas keratinocytes were on its surface, forming a well-developed multilayer epithelium [64] and even the external stratum corneum [65] similar to natural skin.A. El Ghalbzouri et al. [66] obtained the bioconstruction with a similar structure and cell distribution, but they created the matrix using human procollagen-I instead of commonly applied murine or bovine collagen.
In contrast to the abovementioned works, Kuikui Hu et al. [67] used as a scaffold the compressed collagen gel, obtained from the polymerized collagen with incorporated murine fi broblasts under pressure the compressed gel lost water and shrank while drying out, its mechanical properties were improved and the concentration of cells was increased.The murine keratinocytes formed a multilayer analogue of epidermis while being cultivated on the gel surface.The compression of collagen hydrogel, containing fi broblasts, was also used by C. Helary et al. [68].The hydrogel became more stable and provided more effective proliferation of inside cells under favorable storage conditions.
The group of authors, H. M. Powell et al., used a sponge, made of collagen and chondroitin-6-sulfate, as a scaffold for human fi broblasts and keratinocytes [69].In another work of the same group [70] a fibrous collagen matrix for cells was made by electrospinning.A similar fi brous scaffold for fi broblasts and keratinocytes was made by M. Kempf et al. [71] using bovine collagen-I.
The group of scientists from Great Britain and Taiwan [72,73,74] synthesized a copolymer fi lm using the combination of collagen and poly-ε-caprolactone (PCL) in 1:20 ratio.The latter is a biocompatible polymer [75], capable of enzymatic biodegradation [76].At cultivation, one side of the fi lm was covered with fi broblasts and other -with keratinocytes.J. Venugopal and S. Ramakrishna [77] obtained the copolymer nanofi brils using the combination of collagen and poly-ε-caprolactone via the electrospinning.Human fi broblasts were cultivated on the spun matrices.In [78] Ma et al. for the electrospinning of fibrils used collagen and polymer poly-(D,L)-lacticco-glycolic acid (PLGA), consisting of residues of lactic and glycolic acids at a ratio of 75:25, which is capable of biodegradation via hydrolysis.Human MMSC of bone marrow were used as a cellular component.As shown by the experiments on rats, the product obtained promoted the collagen synthesis and epithelium maturation in the wound bed.
M. Ananta et al. [79] used the combination of collagen with the same polymer; the latter was described as poly-(lactide-co-glycolide) (PLGA) and had commercial name Vicryl.The collagen layer with fi broblasts, evenly distributed within the volume, contained a PLGA net, which served as a frame to strengthen the construction.One of the surfaces of the collagen sheet carried keratinocytes.The cells were obtained using the skin of laboratory rabbits; further on the rabbits were used for the experiments, aimed at the study of the effi ciency of applying this skin equivalent in burn therapy.The authors concluded that the bioconstructions obtained enhanced wound healing.
Noteworthy is the experimental work, in which P. Liu et al. [80] obtained MMSC out of bone marrow and cultivated them on the surface of the membrane, containing collagen and glycosaminoglycans.These bioconstructions were used in skin burn therapy.Yorkshire swines were used as laboratory animals to obtain MMSC and to simulate burns.The results were the improvement of wound vascularization, the stimulation of wound healing and the formation of epidermal skin layer.
In contrast to the abovementioned work, M. Keck et al. [81] cultivated human keratinocytes and preadipocytes on the previously constructed bioconstruction Matriderm®, which was a sheet of bovine collagen, covered with dried hydrolysate of connective α-elastin.Keratinocytes formed a confl uent layer on the surface and preadipocytes penetrated into the samples.
Finally, R. K. Chan et al. [82] tried to create all three layers of skin coverages.The basis of the cellular component was adipose-derived stem cells (ASCs).The matrices for cellular masses were formed out of collagen, polyethylene glycol and fi brin.The incubation of separate constructions with cells in cultural media of different composition was used to achieve the transformation of ASCs into epithelial and adipose cells.During the creation of a dermal equivalent the cells did not undergo directed differentiation.The preclinical trials of the bioconstructions obtained are not described in the article, however, it can be understood («Further demonstration of this methodology in a preclinical model is under way») that to the moment of the article submission they have already been started.
The work of M. Itoh et al. [83] is noteworthy for the fact that they used human induced pluripotent stem cells (iPSC) as a source of cells.Previously these authors have informed about the possibility of artifi cial activation of iPSC differentiation into keratinocytes.In the mentioned work they developed a protocol of differentiating the same human iPSC into dermal fi broblasts.During the experiments they made samples of skin equivalents, containing the same cells, as those formed at the directed differentiation of iPSC.A collagen layer was applied on the surface of the polyethylene-terephthalate membrane, and after polymerization the fi broblasts and keratinocytes were cultivated consecutively, with the interval of 5-7 days.The authors did not conduct any preclinical trials of the bioconstructions obtained, but they informed that the histological structure was compatible with that of the equivalents, obtained using a similar method but with the application of normal human fi broblasts and keratinocytes.

The elaboration of matrices using chitin/chitosan and other materials
Chitin is a straight chain polysaccharide, whose mono mer unit, N-acetylglucosamine, is a part of hyaluronic acid, capable of stimulating dermis regenera-tion.Therefore chitin promotes the wound healing.Chitosan is a product of partial deacetylation of chitin and its separate monomer links.It should be noted that chitin and chitosan are biocompatible, i.e. they do not cause infl ammatory or allergic reactions after local application or implantation into a human body [84], may be resorbed under the lysozyme action [85], and have bacteriostatic and fungistatic activity [86].
The example may be found in the work of G. M. Mykhailov et al. [87] who synthesized elastic chitin fi brils and wove a fi lm of them.This fi lm was later used to cultivate human fi broblasts, attached to the fi brils and fl attened against them.A. B. Hilmi et al. [88] also used only two components -chitosan and primary fi broblasts of human origin, but the chitosan scaffold was made in the form of porous sheets, with the cells located inside them.
Two groups of researchers [89,90] created a porous matrix using the mixture of collagen and chitosan solutions.The latter increases matrix biostability, which was demonstrated in the tests with collagenase [89] and lysozyme [90].Human fi broblasts [90] or murine fi brosarcoma L929 cells [89] were used as a cellular component.
A noteworthy experimental work was conducted by I. N. Bolshakov et al. [91] who used as a cellular scaffold the collagen-chitosan complex, containing ascorbic, chondroitin-serine and hyaluronic acid, heparin and a medical preparation Adhelon (based on the bovine serum growth factor).They obtained fi lms or porous sheets, on the surface of which the fetal murine fi broblasts were cultivated.The experiments with rats demonstrated that the application of these bioconstructions promotes complete restoration of the epidermal-dermal complex to the state, corresponding to that of healthy skin.
A. V. Yeremeyev et al. [92] observed good proliferative and metabolic activity of fetal murine fi broblasts and MMSC of adipose murine tissue while cultivating them on wound collagen-chitosan coverage Collakhit-bol (Patent of the Russian Federation N 2252787, 27.05.2005;Patent of the Russian Fe deration N 2254145, 20.06.2005).During cultivation the stromal cells started to differentiate into epithelial ones.J. Mao et al. [93] experimented with the cellular scaffold, consisting of chitosan and gelatin layers.Human fi broblasts and keratinocytes were used as a cellular component.The latter were introduced into the bioconstruction some time after the cultivation of fi broblasts on the matrix.Contrary to the abovementioned works, C. M. Han et al. [94] created a collagen-chitosan porous membrane.After a week-long cultivation its surface was covered with a fi brin layer, on which a layer of human keratinocytes was applied.
An original work was performed by Sh.Ikemoto et al. [95], where the authors succeeded in cultivating human keratinocytes on the membrane, consisting of chitosan, conjugated with protein AG73.This is one of representatives of the family of lamininsover 30 large glycoproteins, located on the basal membrane.Among them, AG73 peptide is the most effi cient stimulator of cell migration, adhesion and stimulation of axon germination.The authors deem their product to be a good vehicle to deliver keratinocytes to the wound bed.

The elaboration of matrices using hyaluronic acid
Hyaluronic acid is a polymer, consisting of the residues of D-glucuronic acid and D-N-acetylglu co sa mine, successively connected by β-1,4-and β-1,3-gly cosidic linkage.Natural molecules of hyaluronic acid may contain up to 25.000 disaccharide links.This substance , being one of the main components of a extracellular matrix, is a part of many biological fl uids, participates in the processes of adhesion, migration and proliferation of fi broblasts and keratinocytes as well as in fetal non-scar wound healing [98].
Let us consider the series of experimental works involving the application of hyaluronic acid.T. W. Wang et al. [99] prepared a composite matrix via polymerization of the solution, containing gelatin, hyalu ronic acid and chondroitin-6-sulfate in the concentrations of 5 %, 0.2 % and 0.05 % respectively.H. Liu et al. [100,101] synthesized the matrix using the solution of gelatin, chitosan acetate and hyaluronic acid in the concentrations of 0.7 %, 0.4 % and 0.001 % respectively.During the process of combined cultivation on the composite biopolymers the obtained human fi broblasts and keratinocytes demonstrated high viability and proliferation ability.The works [100,101] showed that after two weeks keratinocytes gradually self-organized into a layer-like structure, which in particular included the layer of cubic cells and the surface layer of compressed cells.H. M. Wang et al. [102] also created a three-component matrix, consisting of collagen, hyaluronic acid and gelatin, whereas the cellular component included three types of cells instead of two: keratinocytes, melanocytes and fi broblasts.The skin equivalent obtained promoted wound healing in the in vivo experiments.
Noteworthy is the work, in which N. Scuderi et al. [103] described a clinical case of treating a surgical wound using the bioconstruction, based on the matrix of polymerized hyaluronic acid.The fi broblasts, keratinocytes and melanocytes, obtained during skin biopsy prior to the surgery, were jointly cultivated on this matrix.J. Enrione et al. [104] studied mechanical and physical properties of the same matrix and checked its cytotoxicity using cultivated human fi broblasts.
In the following work Sh.Kondo et al. [105] prepared composite porous matrices using hyaluronic acid, cross-linked collagen and epidermal growth factor (EGF).The latter considerably stimulated the fi broblasts proliferation on the matrix surface along with the formation of granular tissue in the wound, angiogenesis and epithelization of the wound in the experiments with rats.

The elaboration of matrices using different synthetic polymers
The fi lm of the abovementioned poly-ε-caprolactone (PCL) was used in the work of K. W. Ng et al. [75] as a scaffold for human fi broblasts.Arun Richard Chandrasekaran et al. [106] used the electrospinning method to create fi brils, containing poly-L-lacticacid-co-poly-ε-caprolactone and gelatin.Human fibroblasts served as a cellular component of the bioconstruction.
In the works of two groups, K. A. Blackwood et al. [107], S. G. Kumbar et al. [108], the combination of solutions of lactic and glycolic acids was used to elaborate copolymer fi brils of the poly(lactide-coglycolide) composition using the electrospinning method.This polymer has been mentioned above, but the authors of these works did not combine it with other materials.The fi brils obtained were used to spin biodegradable scaffolds for human fi broblasts and keratinocytes [107] or for human fi broblasts only [108].In [107] the most effi cient ratio of polylactic and polyglycolic acids was estimated to be 85:15 or 75:25.

Other materials, used to elaborate matrices
Complex matrices were made by A. L. Mazlyzam et al. [109]: for instance, they initiated the polymerization of human fi brin in the suspension of human fibroblasts.The abovementioned gel layer, carrying keratinocytes, was formed in a similar way.A layer of silk was applied for stabilization during the transportation and transplantation.
Human amniotic membrane was used as a cell scaffold in two works.S. S. Kim et al. [110] immobilized rabbit mesenchymal stromal cells (MSC) on its surface.In the work of Lujun Yang et al. [111] at fi rst the membrane surface was covered with fi broblasts after the removal of the epithelial layer.Five days later, when the fi broblasts penetrated the membrane stroma, the surface was seeded with keratinocytes.
The remarkable work was performed by C. Castagnoli et al. [112], in which the cell-free dermal layer of dead donor skin was used to create a skin equivalent.Then they cultivated keratinocytes on its surface for 21 days.
A work to be noted is the research of Chung Eun Yeum et al. [113] who cultivated MMSC, obtained from the bone marrow of laboratory mice, on the surface of small intestinal submucosa (SIS), one of the layers of intestinal walls, mechanically stable and formed by porous connective tissue.It was demonstrated that cells migrated from the surface of this scaffold into the wound bed, evolving growth factors.Here the number of cells, migrating to the wound surface, reached some maximal value (depending on the wound size) and further on remained constant, even if the number of MMSC, brought to the wound bed in the composition of transplantats, was increased.

V. Our studies on the basis of Human Genetics Department of IMBG NAS of Ukraine.
Since 2001 we have conducted biotechnological research in developing dermal skin equivalents, containing living human cells.Human multipotent mesenchymal stromal cells (MMSC) of different origin were studied as a cellular component.Primary skin cells and the cells of 4BL line were used [114].Currently MMSC, obtained from human placenta, became a new source of cells [115,116].The works in obtaining dermal skin equivalents using MMSC are being conducted in the collaboration with the Cell Therapy Institute, Kyiv, which has a license (AG 579626) for the elaboration and maintenance of low temperature bank of human cells and tissues.
Different materials of synthetic and natural origin are used as a scaffold.Previously there have been some experiments with hydrogel membranes, synthesized in the F. D. Ovcharenko Institute of Biocolloid Chemistry NASU (Kyiv) using combined polymerization of acrylamide and acrylonitrile [117,118].There have been joint clinical studies of different dermal coverages on the limited number of patients with massive and deep burns in the collaboration with the Center of Thermal Injuries and Plastic Surgery of the Kyiv City Hospital No. 2. The samples of hydrogels, based on the synthetic polyacrylamide matrix and natural collagen matrix with immobilized MMSC, were applied on the wounds, previously cleansed from necrotic tissues.Twenty-four hours later the temporary dermal coverages were removed with the subsequent autotransplantation of skin.In the control cases the autotransplantation of skin was conducted without previous application of hydrogel dermal coverages on wounds.It was demonstrated that both synthetic and natural biomatrix/ cell constructions were non-toxic, solid, elastic, transparent or semitransparent, with good fi xation on the wound and partial adsorption of excessive tissue exudate, and convenient in the application.The

Conclusion
At the current stage of biotechnology development the available epidermal, dermal and dermo-epidermal skin equivalents are not capable of replacing skin tissue completely.Usually they serve as a temporary biologically active coverage, providing the damaged part of skin with cytokines, growth factors and other biologically active substances, which promotes wound healing due to the stimulation of regeneration of the patient's own tissues.Therefore, the skin substitutes play a incomplete, yet relevant role in the complex treatment of the deep and/or extensive skin damage, facilitating the survival of patients and improving the quality of their life after the trauma.Further studies in this fi eld are rather urgent for the elaboration of new cellular technologies in biology and medicine.
use of temporary skin substitutes made subsequent fi xation of meshed autotransplantats more reliable, accelerated their transfer to the vascular type of nutrition and increased the rate of epithelization of meshed spaces almost twice.The positive results of clinical trials allowed receiving two patents of Ukraine Patent of Ukraine (N 20134, 15.01.2007;Patent of Ukraine N 82583, 10.01.2008), the third patent is currently being considered.