Skin and Bones Repaired by Bioprinting

Fixing traumatic injuries to the skin and bones of the face and skull is difficult because of the many layers of different types of tissues involved, but now, researchers have repaired such defects in a rat model using bioprinting during surgery, and their work may lead to faster and better methods of healing skin and bones.

Schematic of the skin and bone bioprinting process. After scanning, the bone and then skin layers are bioprinted creating a layered repair with bone, a barrier layer, and dermis and epidermis

This work is clinically significant,” said Ibrahim T. Ozbolat, Associate Professor of Biomedical Engineering and Neurosurgery, Penn State. “Dealing with composite defects, fixing hard and soft tissues at once, is difficult. And for the craniofacial area, the results have to be esthetically pleasing.

Currently, fixing a hole in the skull involving both bone and soft tissue requires  using bone from another part of the patient’s body or a cadaver. The bone must be covered by soft tissue with blood flow, also harvested from somewhere else, or the bone will die. Then surgeons need to repair the soft tissue and skin. Ozbolat and his team used extrusion bioprinting and droplet bioprinting of mixtures of cells and carrier materials to print both bone and soft tissueThere is no surgical method for repairing soft and hard tissue at once,” said Ozbolat. “This is why we aimed to demonstrate a technology where we can reconstruct the whole defect — bone to epidermis — at once.”

The researchers attacked the problem of bone replacement first, beginning in the laboratory and moving to an animal model. They needed something that was printable and nontoxic and could repair a 5-millimeter hole in the skull. The “hard tissue ink” consisted of collagen, chitosan, nano-hydroxyapatite and other compounds and mesenchymal stem cells — multipotent cells found in bone marrow that create bone, cartilage and bone marrow fat. The hard tissue ink extrudes at room temperature but heats up to body temperature when applied. This creates physical cross-linkage of the collagen and other portions of the ink without any chemical changes or the necessity of a crosslinker additive.

The researchers used droplet printing to create the soft tissue with thinner layers than the bone. They used collagen and fibrinogen in alternating layers with crosslinking and growth enhancing compounds. Each layer of skin including the epidermis and dermis differs, so the bioprinted soft tissue layers differed in composition. Experiments repairing 6 mm holes in full thickness skin proved successful. Once the team understood skin and bone separately, they moved on to repairing both during the same surgical procedure. “This approach was an extremely challenging process and we actually spent a lot of time finding the right material for bone, skin and the right bioprinting techniques,” said Ozbolat.

The scientists have reported their results in Advanced Functional Materials.