For burn victims and patients with wounds that pierce through all three layers of skin, treatment options are limited. Wake Forest University surgeon Anthony Atala has witnessed the effects of this first hand: “There’s nothing more devastating as a surgeon [than] to be in the operating room and not have the tissue that you need for reconstruction,” he said.

Now, in a recent study published in Science Translational Medicine, Atala and his collaborators 3D printed fully functioning, multi-layered skin (1). The team transplanted their bioprinted skin into animal models, improved wound closure, and reduced scarring, providing a potential alternative for treating major wounds in patients.

Human skin is made up of the epidermis, the dermis, and the hypodermis. Atala’s team had previously managed to construct single layers of skin by hand. But they soon found that using the same method for full-thickness skin was too cumbersome and time consuming to generalize to all clinics. “We realized that we needed to really start to automate the process,” said Atala. “That’s when we started exploring 3D printing as a potential way to do that.”

The team’s first and main challenge was to obtain and cultivate the many different cell types that make up the skin. “[It was] a lot of work having all these different cell types growing and expanding at the same time and making sure that you could combine them and then print them in a way that you could maximize [the] viability of the cells,” said Atala. 

Through years of trial and error, they came up with a method that involved cultivating individual skin cell lines separately in vitro and then mixing them in specific ratios into one of three bioink cartridges, each corresponding to one of the three skin layers. After printing, all of the cell types remained localized to their corresponding skin layers, emulating the structure of normal skin.

“This is already a big achievement that they are able to print these three layers,” said Akhilesh Gaharwar, a biomedical engineer at Texas A&M University who was not involved in the study. “When [the different cell types] talk to each other, [it] results in paracrine signaling, which is crucial for wound healing.” Before the new study, researchers in the field had not been able to print multiple cell types, Gaharwar said.

Atala’s team next ran a series of tests to see whether the skin they printed functioned like normal skin. They confirmed that the bioprinted skin underwent layer-specific cell death, which is a crucial process for skin to mature. They then transplanted their bioprinted skin to mice models, where they found that wounds treated with bioprinted skin took half the time to close compared to wounds treated with plain hydrogel or untreated wounds. Their bioprinted transplants were also able to form small vascular structures and ridges that were absent in hydrogel-treated and untreated wounds, confirming that the wound healing was based on skin as opposed to scar tissue. “You need that initial vascularity to make sure that tissue grows [at a] normal [rate] and in a normal manner,” said Atala. “That is very, very important.”

I wish I could say there was just one eureka moment, but it really was just a lot of work. 
- Anthony Atala, Wake Forest University

The structure of their bioprinted skin also accelerated and improved wound healing. In healthy human skin, collagen fibers overlap and intertwine in a kind of basketweave formation. After 90 days of healing, collagen fibers in their bioprinted skin transplants mirrored this basketweave structure, whereas collagen fibers in hydrogel and untreated wounds stayed parallel to each other. Immunofluorescent staining later showed that host skin cells moved into wounds treated with bioprinted skin but stayed outside of hydrogel and untreated wounds, which meant that only the bioprinted transplant successfully integrated with normal skin. 

“It was not an overnight thing,” said Atala. “I wish I could say there was just one eureka moment, but it really was just a lot of work.”

Finally, Atala’s team bioprinted skin grafts for porcine models using cells from each individual animal. “A lot of [the motivation for this work] has to do with using the patient's own cells to create tissues that fit back into the same patients, so [that] you don't have a lot of the challenges of using foreign tissues [or] using devices that really don't belong there,” said Atala. The autologous transplant showed the same biomarkers for skin integration and accelerated healing, offering a permanent wound healing treatment developed from the subjects’ own cells.

“What [Atala and his team] have done is amazing, it's pathbreaking,”said Gaharwar. “The next step will be to integrate either hair follicles, nerve endings, or sweat glands.”

Atala and his team are currently working to expand their 3D printing repertoire to other tissue types, moving from flat structures like the skin to more tubular structures like the urethra, and looking towards eventually reconstructing solid organs. In the meantime, they are actively working to meet regulatory guidelines with plans to one day get their bioprinted skin into the clinic. 

“The goal is to really keep working on these technologies so that patients can benefit,” said Atala.

Reference

1. Jorgensen A.M. et al. Multicellular bioprinted skin facilitates human-like skin architecture in vivo. Sci Transl Med  15, eadf7547 (2023).