Bio Engineered Skin Substitute for Burn Victims

Nanyang Technological University

Bioengineered Skin Substitution

Page I

Table of Contents

1.0 INTRODUCTION ......................................................................................................... 1

1.1. Background ................................................................................................................. 1

1.1.1. Theoretical Context ................................................................................................. 1

1.1.2. Historical Context .................................................................................................... 3

1.2. Purpose ........................................................................................................................ 6

1.3. Scope ........................................................................................................................... 6

2.0 DISCUSSION ................................................................................................................ 7

2.1. Availability of Material ............................................................................................... 7

2.2. Rejection of Graft ........................................................................................................ 9

2.3. Adherence.................................................................................................................. 10

2.4. Appearance ................................................................................................................ 11

2.5. Flexibility .................................................................................................................. 13

2.6. Cost Effectiveness ..................................................................................................... 14

3.0 CONCLUSION ............................................................................................................ 15

3.1. Summary ................................................................................................................... 15

3.2. Conclusion ................................................................................................................. 16

3.3. Recommendations ..................................................................................................... 16

4.0 Contact ...................................................................................................................... 16

5.0 References ................................................................................................................. 17

5.1. Sources Used ............................................................................................................. 17

Page II

List of Tables, Figures and Graphs

Figure

Page

Figure 1 – Structure of Biobrane 7

Figure 2 – Structure of Integra 8

Figure 3 – Hand Burn on Day 1 11

Figure 4 – Cosmetic results of Autograft after 6 months 11

Figure 5 – Leg Burn Day 1 12

Figure 6 – Cosmetic results of Biobrane after 21 days 12

Table

Table 1 – Comparison of Availability of material between BSG and BSS 7

Table 2 – Comparison of Rejection of graft between BSG and BSS 9

Table 3 – Comparison of Adherence between BSG and BSS 9

Table 4 – Comparison of Bacteria Count between BSG and BSS 10

Table 5 – Comparison of Appearance between BSG and BSS 11

Table 6 – Comparison of Flexibility between BSG and BSS 13

Table 7 – Comparison of Cost Effectiveness between BSG and BSS 13

Table 8 – Comparison between BSG and BSS 16

Nanyang Technological University

Bioengineered Skin Substitution

Page 1

1.0 INTRODUCTION

1.1. Background

1.1.1. Theoretical Context

Skin is the largest organ of the integumentary system in the human

body. It provides an outer covering which shields the muscles,

ligaments, bones and organs. Skin protects the body from germs and

bacteria, it also helps in regulating the body temperature, synthesizes

vitamin D obtained from sunlight and sensation. Skin has three

layers, namely the epidermis, dermis and the hypodermis.

Skin grafting or otherwise known as skin transplantation, is a

medical procedure whereby skin or substitute of skin is surgically

placed over non-healing wounds or wounds caused by burns. Skin

grafting is presently the most commonly used method in Singapore

to replace burned wounds caused by at least third degree burns.

There are six degrees of burns, the first and second degrees do not

require grafting as they involve only the epidermis and maybe the

superficial and deep dermis. Third degree burns onward, grafting is

needed. Sixth degree burns are most likely fatal.

There are four different types of skin grafting namely, Autograft,

Isograft, Allograft and Xenograft[1]. Autograft is the most

commonly used method, due to zero rejection cases, as it involves

transferring skin from an undamaged part of the patients’ body to the

wound. Isograft requires the donor to be genetically identical in order

to continue transplantation. This means that the skin donor has to be

the identical twin of the patient. Allograft, the second most

commonly used method, refers to transferring skin within the same

animal species (i.e. skin from cadaver to recipient or donor to

recipient). Xenograft is transferring skin from one animal species to

a different animal species (i.e. pig skin to human).

There are two types of skin grafts, Split-thickness grafts and Full-

thickness grafts. Split-thickness graft contains the epidermis and part

of the underlying dermis obtained from thighs and places covered by

clothing. It is for less severe burn injuries because it requires blood

supply from the patient’s wound. Full-thickness grafts defer from

split-thickness grafts, due to the massive tissue loss, as it is basically

a section of skin with muscles and blood supply. Full-thickness

grafts are obtained from the abdomen and back area.

Availability of material is an important criterion of skin grafts. If

there is not enough material for grafting, treatment cannot be carried

out.

The failure of a skin graft can be caused by a variety of factors; some

of it are rejection rate, poor adherence, infection and induced trauma

to wound. Rejection of skin graft is the main cause of failure of skin

transplant. Reason for rejection is that certain cells of the body react

with the cells in the skin graft and the body produces antibodies to

reject the skin. Eventually these skin grafts will slough off the body.

Poor adherence occurs most commonly when hematoma is present.

Hematoma happens when blood collects at the injured tissues; it will

cause the skin graft to rise from the bed and deprives the graft from

sufficient nourishment. It is important for the wound and graft to be

sterile while transplanting as bacteria may infect the wound.

Responses to bacteria by our body will cause hindrance to fibrin

production, which is also essential for graft adherence. Diseases

might also be transferred. Trauma caused during and after the

surgery will also cause loss of skin graft. Improper handling of the

skin graft (i.e. stretching it too tight, applying excessive pressure)

will cause either semi or complete graft failure. Trauma after surgery

such as frequent movement of wound area will cause breakage of the

fibrin attachments to the graft resulting also in poor adherence.

Failure in skin grafts will require additional surgeries to replace them.

Recovery usually requires three weeks to a month if successful.

Normally the first eight hours, the graft will start to adhere to the

wound and within thirty-six hours, blood vessels will begin to grow.

Pigmentation on Full-thickness graft will generally be similar to the

normal skin colour, however in Split-thickness grafts; the skin will

be paler in comparison.

In the next section, historical aspects of skin grafts will be discussed.

1.1.2. Historical Context

This literature review will focus on the studies done on different

types of skin grafting.

In the era of ancient Greece and Roman Empire, burn wounds were

not treated properly, they are simply cleansed and applied with

animal fat or herbs and then wrapped. These methods led to various

health complications.

In the early 1800s, skin grafting was first discovered and successfully

implemented. In 1804, the first Autograft was experimented using

backs of sheep. In 1823, a man named Bunger completed the first

Autograft on humans [2].

The problem with this method was there might not be enough

uncovered skin on the patient’s body, especially when the patient is

severely burned in various places, to carry out Autograft. Even if

there is enough undamaged skin for grafting, the patient will have to

be physically healthy in order to be suitable to undergo numerous

surgeries. These surgeries are namely, excision of damage skin,

removal of skin for grafting and the actual skin grafting onto the

wounds. Thus, in the future, various methods for skin grafting came

into place.

In 1869, Allograft was first performed by using skin grafts contained

only of the epidermis. Two years later, in 1871, a better type of

allograft was discovered by using skin grafts which contains both the

epidermis and little of the underlying dermis. With this new kind of

skin graft, wounds healed faster and there was less scarring. This

new method of skin grafting paved the way for treatment of burn

wounds in the future. By the end of the century, skin grafts obtained

from cadavers were also found to be useful.

However, problems of using Allograft for treatment of burn wounds

eventually arose. One such problem is though availability for

material increased, there are not many people who would step up and

volunteer to be a skin donor. In addition, it was found out that donor

and cadaver skin grafts will eventually die within 21 days. It was not

until 1943 until it was discovered that the human immune system

would reject the skin graft. To prevent rejection, the allograft will

have to be changed every three days. Thus, allograft was only used

for Split-thickness grafts. Allograft provided a few benefits for a

temporary graft as it encourage regeneration of the dermis and

reduce the bacteria count on the wound. The search for new skin

substitutes for grafts thus continued.

Meanwhile, in the early 1900s, Xenograft was first performed using

rabbit skin. It was not until the 1960s that pig skin became popular in

the treatment of burn wounds. Pig skin became popular due to it

being readily available as compared to Autografts and Allografts.

However, the usage pig skin was found out to be problematic as well.

Due to the high risk of disease transmission from the animal to

human and also the fact that the skin will slough off, Xenografts was

also put to use as temporary Split-thickness grafts.

In the 1970s, CEA, Cultured Epithelial Autograft was developed. It

was developed to replace Allografts. CEA uses the patients’ own

keratinocytes from uninjured part of the patients’ body to cultivate

epithelial sheets.

Although this method solved the problem of availability and

rejection rate of Allograft, there are still various problems present.

CEA takes a couple of weeks to be cultivated. In addition, this timing

may be prolonged if blistering of the CEA happens during cultivation.

This might threaten lives as the burn patients will have to wait for the

cultivated skin and increase the risk of infection due to the open

wounds.

Bioengineered skin became available in the late 1990s. Over the

years to the present, more types of Bioengineered skin have been

appearing. Some include temporary skin substitutes used for Split-

thickness grafts such as Biobrane® and Transcyte®. And permanent

skin substitutes used for Full-thickness grafts such as Integra®,

Apligraf® and Alloderm®. In this report we will only focus on the

two most widely used skin substitute which is Biobrane and Integra.

Biobrane was first created in 1979[3]. Biobrane is a temporary skin

substitute made up of a silicon outer layer and the inner layer of

Biobrane contains nylon filament. Biobrane is used for burn wounds

involving the epidermis to some of the underlying dermis.

Biobrane rejection rate is lowered as it is adhesive, impermeable to

bacteria and such. Due to low rejection rate, changing of Biobrane

dressing is unnecessary which in turns lower the number of

operations and also the total cost. Biobrane have been used from

1990s to the present by three countries namely California, Boston

and Ohio.

Integra was first approved by the U.S Food and Drug Administration

in 1996[4]. Integra is also the only treatment currently that

regenerates the dermal layer of the skin. Integra does not contain any

living cells but it has two layers, a silicon outer layer and an inner

layer which contains a porous matrix of bovine collagen and sugars.

The inner layer provides the framework for the regeneration of the

dermis. After a few days, the matrix will degrade and the dermis will

grow through the inner layer. After which, the silicon layer is

removed and replaced either with a thin layer CEA or Autograft,

which will heal in a week, placed over the regenerated dermis.

Advantages of Integra include higher flexibility, impermeable to

bacteria, controls fluid loss, better cosmetic result and such. In

addition, its availability will also surpass any other biological method

mentioned above. The downside to Integra is that it is more

expensive than any other method. Integra is currently approved for

use in 30 countries and all are satisfied with the results. This shows

that Integra can gradually replace the usage of Biological Skin Grafts.

Nonetheless, BSS may still be a concern to the Singapore

government regarding its safety as it is a new process yet to be

practiced in the country. This report will prove that BSS is indeed

feasible to replace biological methods of skin grafting.

1.2. Purpose

This document proposes to carry out a research on the use of skin

transplantation/grafting in comparison to using bioengineered skin substitute

for burn victims in order to find out whether it is feasible for implementation

in Singapore.

1.3. Scope

Our project will focus on the properties of BSS and BSG in terms of their

availability of material, rejection of graft, adherence, appearance, flexibility

and cost effectiveness. Aspects such as social and political issues will not be

covered under our project.

2.0 DISCUSSION

2.1. Availability of Material

Characteristics Biological Skin

Grafting

Bioengineering Skin

Substitute

Availability of material Lower Higher

Table 1 – Comparison of Availability of material between BSG and BSS

BSG or Biological Skin Grafts are skin grafts are excised from humans.

BSG can be obtained from the patients’ own body or from cadavers or skin

donors. For skin grafts obtained from the patient’s body, if more than 30% of

the body surface area is burnt, it is difficult to obtain enough BSG for

transplant. For skin grafts obtained from cadavers or skin donors, the

Singapore General Hospital found out that, over the past few years, there had

not been enough skin donors for massive burn cases [5]. Treatment of these

patients was thus delayed. Patients suffering serious burns do not have the time

to wait for the skin grafts which is requested to be sent from US and Australia.

This shows the seriousness of the problem of availability.

Due to the fact that Bioengineered Skin is being made by non-living materials,

it is readily available. A BSS called Biobrane is made up of a silicon outer

layer and the inner layer of Biobrane contains nylon filament.

Figure 1 – Structure of Biobrane

Another BSS called Integra is a combination of a silicon outer layer and an

inner layer which contains a porous matrix of bovine collagen and sugars.

Figure 2 – Structure of Integra

2.2. Rejection of Graft


Grafting

Bioengineering Skin

Substitute

Rejection of graft Higher Lower

Table 2 – Comparison of Rejection of graft between BSG and BSS

Rejection occurs when the body’s immune system sees the skin graft as a

foreign object, or otherwise known as antigenic, and it produces antibodies to

reject the skin graft.

Rejection occurs in BSG if the skin grafts are obtained from cadavers and skin

donors. In the average human, BSG have to be changed every three days in

order to prevent rejection. However, in an immunosuppressed human, BSG

can be changed every five days. Otherwise the BSG will degenerate and

slough off the body.

As for BSS, rejection does not occur [6]. This is due to both of the materials

not being created from living cells.

2.3. Adherence

Hours Biological Skin

Grafting

Bioengineering Skin

Substitute

24hrs Later 56 g/cm2 87 g/cm2

72hrs Later 214 g/cm2 153 g/cm2

Table 3 – Comparison of Adherence between BSG and BSS

All the different types of skin grafts will have different levels of adherence.

Production of Fibrin on the skin will bind the skin graft onto the wound bed.

Unless there are complications such as hematoma, the pooling of blood

beneath the graft, it will cause the skin graft to be undernourished.

BSS contains collagen in their inner layer. Collagen is a fibrous protein which

is essential in connective tissues. In addition Collagen is an important

component for the regeneration of tissue development. As can be seen from

the Table above, BSS has low adherence in the first 24 hours, but has the

highest adherence at the end of 72 hours. This is an important criterion for the

success of skin grafting. Good adherence will reduce the need for replacing

new skin grafts and the bacteria count on the wound.


Grafting

Bioengineering Skin

Substitute

Bacteria Count 103 105

Table 4 – Comparison of Bacteria Count between BSG and BSS

As can be seen from Table 4, bacteria count was lowest in BSS. This indicates

that BSS is a good bacterial barrier which will reduce the risk of infections or

other complications. The concept is similar to the level of adherence. The

criteria that reduce the bacteria count on the surface of BSS is the silicon

outer layer. Silicon is a good barrier to bacteria. In addition, it also reduces

fluid loss.

2.4. Appearance


Grafting

Bioengineering Skin

Substitute

Appearance Poorer Better

Table 5 – Comparison of Appearance between BSG and BSS

People undergoing skin grafting will consider cosmetic effects to make a

decision of the type of skin grafting to use on themselves in order to avoid

scarring and the difference of pigmentation of skin colour. It was found out

that using BSG for skin grafting, the colour at the region of the skin graft will

be paler than normal skin. This can be seen from the figures below.

Figure 3 – Hand Burn on Day 1

Figure 4 – Appearance of grafted site after 6 months

However for BSS, cosmetic results proved to be better than BSG. As the

colour of the skin will appear natural and blends in with the normal skin.

Figure 5 – Leg Burn on Day 1

Figure 6 – Appearance of grafted site after 21 days

2.5. Flexibility


Grafting

Bioengineering Skin

Substitute

Flexibility Lesser More

Table 6 – Comparison of Flexibility between BSG and BSS

BSG are found to be much more fragile as compared to BSS. In BSG, after the

skin graft is placed onto the wound, the patient will have to immobilise the

part of the body for 5 days for adherence to take place. This is required

because such BSG disintegrate easily and has low tolerance for stress. Even

light trauma can cause the BSG to disintegrate and repeated grafting is

necessary to replace it.

BSS only require one to two day of immobilisation. The materials that are

used to create BSS are flexible. This allows BSS to cover uneven surfaces and

increase flexibility and elasticity to allow motion of the grafted area. This is

what Biological Skin grafting methods cannot mimic.

2.6. Cost Effectiveness


Grafting

Bioengineering Skin

Substitute

Cost effectiveness

*USD $3000 - $7000 $500 - $4000

Table 7 – Comparison of Cost Effectiveness between BSG and BSS

BSG are not considered to be cost effective as in certain cases due to repeated

grafting, the number of surgeries increased; duration hospital stays also

increase resulting in high cost. This is especially the case when Autograft is

implemented, as surgeries will have to double up due to the excision of the

patients’ own undamaged skin.

Depending on the type of BSS we are using, cost can be reduced to very low

or approximately half of the cost of BSG. The low cost of the material, used to

generate BSS, and good results of grafting decrease the overall cost by a

significant amount. In normal cases, for BSS to completely heal, it takes

approximately one to two weeks. This is compared to BSG which uses 3

weeks to 1 month for successful skin grafting with no complications.

The cost range for BSS is from five hundred to four thousand USD depending

on the type of BSS used. Meanwhile, if many complications are present for

BSG, cost can go up to as high as seven thousand USD.

3.0 CONCLUSION

3.1. Summary

This report is an investigation on the comparison between the Allograft and

Biobrane in the temporary skin substitution group; as well as Autograft and

Integra in the group of permanent skin substitute; to find out whether the latter

methods in the respective group are more suitable to be implemented for burnt

victims in local hospitals. The purpose was achieved through the comparison

between the methods stated above in terms of their availability of material,

rejection of graft, adherence, appearance, flexibility and cost effectiveness.

The most significant finding was that rejection did not occur in BSS owing to

the reason that the materials were not created by living cells which would not

cause the production of antibodies in human body. Besides, adherence of BSS

at the end of 72 hours appeared to be higher that in turn reduced the number of

bacteria, which was one-hundredth times of the case of skin grafting.

Moreover, skin grafting showed not to be cost effective due to the repeated

surgeries needed for having poorer flexibility of skin which caused the failure

of grafting.

A list of summary for comparisons between the 2 methods is shown below:


Grafting

Bioengineering Skin

Substitute

Availability of material Lower Higher

Rejection of graft Higher Lower

Adherence 24Hrs later 56 g/cm2 87 g/cm2

72Hrs later 214 g/cm2 153 g/cm2

Appearance Poorer Better

Flexibility Less More

Cost effectiveness

*USD $3000 - $7000 $500 - $4000

Table 8 – Comparison of BSG and BSS

3.2. Conclusion

Our findings imply that there would be potential for BSS to be used in

Singapore since it is already in use in countries such as Boston for years.

Besides, owing to the 6 characteristics discussed above, BSS is preferred for

its advantages against skin grafting.

3.3. Recommendations

Future studies may include the satisfaction of patients which would improve

the accuracy in determining the suitability of BSS against skin grafting. They

may also include improvements to be made to BSS without replacing the

silicon layer with Autografts. Education of the medical personnel in Singapore

can be carried out to increase awareness towards BSS and the effectiveness of

treatment using these methods.

4.0 Contact

For any further information or enquiry, please contact Andy Wong at 98333956; or

email to [email protected].

mailto:[email protected]

5.0 References

5.1. Sources Used

[1] Emedicine “Skin, Grafts”, Feb 2006 [Online]. Available :

http://www.emedicine.com/plastic/topic392.htm [Accessed:Sep.6, 2008]

[2] Journal of Drugs in Dermatology “History of Skin Grafts”, Dec 2002

[Online]. Available:

http://findarticles.com/p/articles/mi_m0PDG/is_3_1/ai_110220336

[Accessed: Sep.6,2008]

[3] Burn Surgery Organisation “Skin Substitutes” Burnsurgery.org, 2000.

[Online]. Available:

http://www.burnsurgery.org/Betaweb/Modules/skinsubstitutes/sec1.htm

[Accessed: Sep. 4, 2008]

[4] Integra® DRT “Intergra Dermal Regeneration Template” [Online].

Available: http://www.integra-ls.com/products/?product=46 [Accessed:

Aug. 4, 2008]

[5] Singapore General Hospital “The Skin Bank” Singapore General

Hospital, SingHealth, November 2002. [Online]. Available:

http://www.sgh.com.sg/clinical_specialties/SkinBank/index.html

[Accessed: Sep. 4, 2008]

[6] HS Wang, Dec 2005, “The application of new biosynthetic artificial skin

for long-term temporary wound coverage”, Burns (03054179); Vol. 31

Issue 8, p991-997, 7p. [Journal Article]. [Accessed: Sep. 4, 2008]

http://www.emedicine.com/plastic/topic392.htm

http://findarticles.com/p/articles/mi_m0PDG

http://findarticles.com/p/articles/mi_m0PDG/is_3_1

http://findarticles.com/p/articles/mi_m0PDG/is_3_1/ai_110220336

http://www.burnsurgery.org/Betaweb/Modules/skinsubstitutes/sec1.htm

http://www.integra-ls.com/products/?product=46

http://www.sgh.com.sg/clinical_specialties/SkinBank/index.html

Bio Engineered Skin Substitute for Burn Victims

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