At the beginning of the project, we focused on the firefighters from the real case above. As an undoubtedly respectful but also dangerous career, firefighters devote themselves to protecting people's lives and property, which is worthy of the public's love and esteem. It is universally acknowledged that there are about 200,000 to 300,000 fires broke out in China every year, and fire brigades would send 34,400 firefighters to rescue every day. More than 300 firefighters are burned or even disabled in the process of rescue annually.

Besides gratitude, we also desired to do something for them with our meager strength, so we got into contact with two firefighters (For more details: Integrated human practice). During the interview, we were touched by the danger they experienced personally. In addition, they also talked about the high cost of treating deep burns currently and their expectation for a more cost-effective alternative, which inspired us to consider the possibility to explore the field of dermal injury repair. That’s how the BCAID was born.

We have tried to build a Bacterial Cellulose (BC) composite scaffold with living cells to assist in the healing of large skin wounds, which is creative and challenging, for the fact that it is unprecedented in the iGEM competitions before and is even prospective in the entire related research field.

Wound healing is accompanied by the whole process of human occurrence and development. As the largest organ of the human body, skin is the first barrier against external environmental damage. Full-thickness skin defect wounds caused by various injury factors are very common in clinic. On the one hand, accidental injuries in daily life such as burns and mechanical injuries may lead to serious wounds of the skin. On the other hand, the number of patients suffering from chronic wounds (mainly including venous ulcers, compressive ulcers and diabetes ulcers¹) has also increased yearly with the arrival of an aging society. Such diseases have placed a huge burden on human health, family harmony and the development of societies.

In the worldwide, there are about 100 million patients suffer from surgical wounds every year, and about 300 million patients have chronic wound treatment needs. In China, there are 3.2 million patients who need treatment for burns, mechanical wounds or chronic skin ulcers every year. According to WHO, about 11 million people suffer from severe burns every year, of which 180,000 die from burns². In 2014, there were 4.22 million patients with diabetes in the world, of whom about 15.0% had lower limb ulcers³.

The heavy economic burden, physical and psychological barriers caused by severe wounds and chronic wounds severely affect the quality of lives and social participation of patients. In addition, with population aging and the increase of metabolic diseases such as obesity and diabetes, the related problems become more and more serious. Therefore, accelerating wound healing and improving the quality of wound healing have become one of the most challenging and marginal disciplines in modern medicine. Therefore, the research on wound healing and tissue regeneration has become an increasingly concerned field for medical researchers.

Despite there is a great demand, there are still many limitations in the treatment of severe wounds and chronic wounds. Unlike some small wounds that can heal quickly, such serious wounds are difficult to complete re-epithelialization and tissue filling by relying on the epithelium and basal tissue of their own wound margins during the healing process. The period for healing is very long with high risk of infection. In clinical practice, skin grafting and skin flap transplantation are used to accelerate the repair. Autologous skin transplantation, which is commonly used in clinic, has a good effect in repairing wounds, but it is inevitable to face problems such as limited donor area and new wounds. The larger the skin area and depth are, the more serious the related problems are. Therefore, the corresponding treatment methods need to be optimized.

The core of promoting the regeneration of whole-thickness skin tissue defects is the regeneration of dermal tissue. The degree of dermal tissue defect affects the wound healing process. Once the dermis regeneration is successful, the thickness of the skin sheet required to seal the wound will be greatly reduced (even no need to take the autologous skin), and the healing quality of the wound will be further improved. Therefore, the scaffolds that assist the dermal regeneration have received more and more attention in recent years.

We paid attention to a kind of material that is very suitable for dermal regeneration scaffold. This material is Bacterial Cellulose (BC). It is a natural nano scale cellulose with high purity, high water holding capacity and strong tensile resistance synthesized by microorganisms. BC can be produced by different kinds of bacteria⁴. The most widely studied of the bacteria is Acetobacter, especially Acetobacter xylinum.

In the process of dermal regeneration, BC scaffolds can provide a carrier for cells to grow and proliferate so that cells can climb and grow. This helps us better control the direction and trend of cell growth and provides a stable environment for wound tissue regeneration. BC materials have the following advantages as dermal regeneration scaffolds. First of all, BC has unique nanofiber network structure to support cell penetration and proliferation. Besides, BC is highly biocompatible and has no cytotoxicity to cells involved in wound healing, such as fibroblasts and keratinocytes. In addition, BC has the potential to regulate cell adhesion. Vitro evaluation has shown that BC has excellent effects in promoting cell adhesion, proliferation and cell transfer⁵.

At present, the relevant research of BC in the field of wound medicine has been relatively mature. Its excellent scaffold performance has been affirmed by an enormous amount of research, and compared with the scaffold materials that have been applied, mostly collagen scaffolds, its costs of production and application have been greatly reduced. However, BC has not been actually applied as a dermal regeneration scaffold. The main limiting factor is that BC is non-degradable in human body. As a biological scaffold material, biodegradability is very important. The lack of cellulase in the human body will cause the BC scaffold that has completed the task of assisting dermal regeneration to stay in the skin for a long time, which may cause unnecessary immune reaction and have certain safety problems.

This year, BNUZH-China 2022 iGEM team aims to use the synthetic biology method, breaking through the non-degradable limit of BC in vivo. We are the first team to try to culture live cells on BC membranes to form a composite scaffold to repair severe wounds in the skin in iGEM. Our project provides new solutions for BC's application in skin regeneration and creates a skin regeneration scaffold system with multiple functional modules of antibacterial and healing promotion, exploring new treatment path and application scenarios.

To achieve our objectives, four modules are designed for this project.

The first one is the basic BC production module. We selected Acetobacter xylinum, a strain with excellent BC production performance, to produce BC, and made preliminary modification on this basis.

Then there is the healing promoting module, which aims to improve the antibacterial capability of the system and promote the wound healing function. In terms of antibacterial properties, we chose to use the antimicrobial peptide LL-37, and to promote wound healing, we chose to express the growth factors bFGF and EGF to perform this function.

Then the third one is the blue light-activated BC degradation module, which is designed to initiate the cellulase secretion and expression in the engineered fibroblast ATCC CRL-2522(BJ) under blue light⁶, so as to degrade the subcutaneous unfunctional cellulose scaffold and prevent scarring and possible immune response.

Finally, there is red light-activated suicide module. When BC is completely degraded, in order to prevent engineered BJ from continuing to secrete cellulase and causing an unnecessary immune response, this project increases the red light regulatory system REDMAP⁷ to control the expression of the downstream toxin protein MazF.