Man-made skin composed of two layers: collagen and a poly-alpha-amino acid

This invention concerns a double-layered man-made skin which is prepared by laminating a collagen sponge sheet and a poly-.alpha.-amino acid membrane that has a good affinity with tissue cells and an appropriate permeability for moisture. When the man-made skin is applied on burns, cuts or wounds, the fibroblasts proliferate in the collagen sponge sheet forming a three-dimensional structure, while the epidermal cells proliferate in the region between the poly-.alpha.-amino acid membrane and a collagen sponge sheet. The poly-.alpha.-amino acid membrane plays a role in protecting affected part and in providing an optimum condition for the proliferation of fibroblasts and epidermal cells, and then it falls off as the epidermis completely regenerates. On the other hand, the collagen sponge sheet assimilates in the living tissue after having played a general role of the dermis.

FIELD OF THE INVENTION 
This invention relates to a man-made skin intended for a use in healing 
burns, cuts or wounds. 
DESCRIPTION OF THE PRIOR ART 
Collagen has so far been used for a man-made skin because of being 
effective in helping the growth of tissue cells. For example, a Japanese 
patent publication of which laid-open number is No. sho 50-141190 
discloses the employment of a collagen-based non-woven fabric for a 
surgical material to protect wounds. However, the collagen based material 
is ineffective to stop oozing the body fluid and to hinder microorganisms 
from entering through it. Moreover, with the collagen-based material, 
epidermal cells are unable to regenerate where a third degree burn is 
caused or where a cut is made so deeply as no epidermis cell is left 
there. 
Apart from those problems, a man-made skin needs to satisfy the following 
conditions. First of all, it must finally turn into part of the living 
skin after having a role in healing. A man-made skin has to promote the 
regeneration of dermis and epidermis from the surrounding normal tissue. A 
ideal man-made skin is required to have following properties. 
1. It has to be able to control the amount of evaporating moisture at an 
optimal rate. 
2. It has to be moistened enough to help proliferation of tissue cells and 
have a good affinity with regenerating tissue. 
3. It must not produce a toxic substance at the time enzymes cause a 
decomposition reaction. 
4. It has to be able to completely shut out microorganisms from outside. 
SUMMARY OF THE INVENTION 
Under the circumstances, the present inventors made an extensive study to 
eliminate the drawbacks of conventional collagen-based surrogate skins and 
have finally succeeded in accomplishing the present invention. That is, 
the present invention pertains to a double-layered man-made skin 
characterized by laminating a sheet of collagen sponge and a membrane of a 
poly-.alpha.-amino acid which has a good affinity with tissue cells and an 
appropriate permeability for moisture.

DETAILED DESCRIPTION OF THE INVENTION 
Copoly-.alpha.-amino acid prepared in the present work are composed of 
hydrophilic moiety and hydrophibic moiety, whose molecular weight is in 
the range of 100,000 to 200,000. These copolymer membranes have a good 
tissue compatibility and an appropriate permeability for moisture. 
(1) Copoly(.gamma.-benzyl-L-glutamyl-N.sup.5 -hydroxyalkyl-L-glutamine) 
##STR1## 
(2) Copoly(.gamma.-benzyl-L-glutamyl-N.sup.5 
-dihydroxyethylaminopropyl-L-glutamine) 
##STR2## 
In the above formulae, x stands for a molar ratio of two glutamine 
derivatives contained in the copolymer. 
Meanwhile, the .gamma.-benzyl-L-glutamate group brings about a 
hydrophobicity and the N.sup.5 -hydroxyalkyl-L-glutamine group brings 
about a hydrophilicity. The present inventors have already published a 
process for synthesizing the copolymer in Journal of Polymer Science, 
Polymer Chemistry Edition, 21, 1289-1303 (1983). The synthesizing process 
is outlined by the following reaction equations. 
##STR3## 
where R stands for 
##STR4## 
Low molecular weight substance are allowed to pass through the hydrated 
poly-.alpha.-amino acid membrane but microorganisms are blocked 
completely; therefore, the above conditions (1) and (4) can fully be 
satisfied. Moreover, fibroblasts were observed to proliferate normally on 
a poly-.alpha.-amino acid membrane having 20-40 percent of the hydrophilic 
amino acid residue, the condition (2) is also fully satisfied. 
The poly-.alpha.-amino acid is slowly biodegradated into fragments inside a 
living body by a variety of peptidases, but when applied outside as a 
component of the man-made skin, it decomposes at an extremely slow rate 
because of scantiness of enzymes. From this reason, the decomposition rate 
is as small as negligible for about one or two months until a wound 
recovers. The lamination of a poly-.alpha.-amino acid membrane on a sheet 
of collagen sponge can merely be made with an organic solvent which can 
swell or dissolve both the components together in part. 
Accordingly, the man-made skin of the present invention has a great 
advantage in the application to a living body; hence, it well satisfies 
the condition (3) at the same time. As apparent from the above, the 
poly-.alpha.-amino acid membrane of this invention satisfies all the 
conditions (1) to (4) required for a man-made skin at a time, which is 
able to be accomplished by the lamination of the poly-.alpha.-amino acid 
membrane and the collagen sponge sheet. 
EXAMPLE 
A collagen solution is stirred vigorously by a homogenizer being kept below 
20.degree. C. so as to form a cream-like viscous solution. A collagen 
sponge sheet (1) which constitutes an essential part of the man-made skin 
of this invention is prepared from the solution by lyophilization. The 
apparent density of the collagen sponge sheet is in the range 0.01 to 0.05 
g/cm.sup.3 ; it is desirable to be in the range 0.02 0.03 g/cm.sup.3. The 
mean diameter of pores left in the sheet ranges from 20 to 200 .mu.m; it 
is preferable to be in the range of 30 to 100 .mu.m. The thickness of the 
membrane can be changed at will so as to match for the depth of a wound; 
however, it commonly ranges from about 0.2 to about 0.3 mm. 
It is desirable for a collagen sponge sheet to be cross-linked in advance 
by the use of an appropriate agent since the collagen sponge sheet becomes 
less soluble by cross-linking upon being kept standing in contact with a 
wound. Glutaraldehyde, hexamethylene diisocyanate and the like can be 
enumerated as a cross-linking agent. But hexamethylene diisocyanate is 
most preferable of all. 
A poly-.alpha.-amino acid membrane (2) is prepared from a dimethyl 
formamide solution containing copoly(.gamma.-benzyl-L-glutamyl-N.sup.5 
-hydroxypropyl-L-glutamine) by the casting method. 
Subsequently, the collagen sponge sheet (1) and the copolymer membrane (2) 
are stuck together by virtue of an organic solvent which is able to swell 
or dissolve both the components together in part. FIG. 2 illustrates a 
condition in the progress of treatment in which a man-made skin thus 
obtained is applied on a wound where none of epidermis cells is found. 
The collagen sponge sheet and the poly-.alpha.-amino acid membrane both 
satisfy the conditions that they can prevent the leakage of the body fluid 
and the infection due to microorganisms from outside; in addition, they 
can keep an appropriate permeability for moisture, which is very essential 
for a man-made skin. 
Thanks to those properties, the collagen sponge sheet is able to stay moist 
to such an extent that fibroblasts are able to proliferate 
three-dimensionally. What is more, because the poly-.alpha.-amino acid is 
excellent in the affinity with tissue cells, it helps the surrounding 
normal epidermis grow and penetrate the interface (5) between the 
poly-.alpha.-amino acid membrane and a layer of fibroblasts growing 
inwardly in the collagen sponge. Like this, both the components work in 
association as an ideal man-made skin. When the surface of a wound is 
covered with the epidermis completely, the poly-.alpha.-amino acid 
membrane falls off by itself; in contrast with this, the collagen sponge 
sheet is gradually assimilated in the surrounding tissues after having 
played a general role of the dermis in the course of healing.