Researchers at Wake Forest Baptist Medical Center in North Carolina used a 3-D printer to fabricate human baby-sized ear structures and successfully implanted them beneath the skin of mice. Within two months, blood vessels and cartilage began to grow in the ear structures, which had maintained their shape. The scientists presented their work recently in the journal Nature Biotechnology.

One of the greatest challenges for tissue engineering is producing three-dimensional vascularized tissue structures of clinically relevant size, shape and structural integrity. Current 3-D printers cannot produce human tissues and organs sturdy enough for transplantation and strong enough to survive after transplantation.

The new technology by the Wake Forest team, known as Integrated Tissue and Organ Printing (ITOP) could help overcome this obstacle.

ITOP also addresses one of the greatest challenges facing tissue engineering — diffusion limits. Within the body, cells are typically no more than 100-200 μm from the nearest capillary. This spacing provides sufficient diffusion of oxygen, nutrients and waste to support living tissue, but it poses a challenge to tissue engineering in that diffusion limits restrict construction of tissues to just a few hundred microns in thickness.

The ITOP system overcomes diffusion limits by incorporating microchannels into tissue constructs to facilitate nutrient diffusion to printed cells. The microchannels act like sponges to allow nutrients to penetrate the growing tissue.

Ten years in development, the new ITOP system allows for fabrication of stable, human-scale vascularized tissue constructs of any shape. To provide mechanical stability, ITOP prints cell-laden hydrogels together with biodegradable polymers in integrated patterns anchored onto sacrificial hydrogels.

To achieve the correct tissue construct shape, the technology uses clinical imaging data as a computer model of the anatomical defect then translates this model into a program that controls the printer nozzles dispensing the cells.

Dr. Anthony Atala, director of the Wake Forest Institute for Regenerative Medicine (WFIRM), and his team used ITOP to build baby-sized human ear structures that measured about 1.5 inches. The researchers implanted the structures beneath the skin of mice and, within two months, the structures formed cartilage tissue and blood vessels.

After implantation, the biodegradable polymers broke down, replaced by the natural structural matrix of proteins produced by the transplanted cells. Blood vessels and nerves also grew into the transplanted tissue.

The success of the study suggests the bio-ink combinations used in combination with microchannels creates the right environment for cell survival and tissue growth. The researchers had also used the ITOP system to fabricate mandible and calvarial bone, cartilage and skeletal muscle successfully.

Using the ITOP system and human cells, the scientists fabricated jawbone fragments of the correct size and shape for human facial reconstruction. These fabricated bone fragments formed blood vessels five months after implantation in rats. The scientific team printed muscle tissue and implanted it in rats. This fabricated tissue formed blood vessels and nerve formation in just two weeks while maintaining structural characteristics.

The research team is now working toward producing tissues for human applications and fabrication of solid organs and other complex tissues.

The ITOP system supports can use information from computed tomography (CT) and magnetic resonance imaging (MRI) to create structures unique to each patient. This approach would be helpful for treating the patient with a segment missing from his jawbone, for example, where doctors would use CT and MRI imaging data to drive the printer to create a jaw fragment that fits precisely into the patient's jaw.

With further development, 3-D printing systems like ITOP could potentially print living tissue and organ structures ready for immediate surgical implantation. Printed microchannels to break through diffusion limits and other advancements will someday allow surgeons to fabricate stable, human-scale tissue of any shape or size.

ITOP is an important advancement in fabricating replacement tissues and organs for transplantation.