Tissue engineering applies principles from engineering and life sciences to develop biological substitutes that restore, maintain, or improve tissues damaged by injury or disease. This approach addresses the limitations of traditional transplants by creating living, functional constructs designed to integrate with the patient’s body.
The Building Blocks of New Tissue
Tissue engineering relies on three primary components. The first are cells, which act as the living “workers” that build the tissue by synthesizing its structural framework. Cells can be sourced directly from the patient (autologous) to avoid immune rejection, from a human donor (allogeneic), or be stem cells with the potential to develop into many different cell types.
The second component is the scaffold, a three-dimensional framework providing a structure for cells to grow on. This scaffold mimics the body’s natural tissue environment, offering physical support and guiding cells to form a specific shape. Scaffolds are made from porous materials to allow nutrient and oxygen flow and are often biodegradable, breaking down as new tissue replaces them.
Scaffold materials can be natural or synthetic. Natural polymers like collagen and hyaluronic acid are easily recognized by the body, promoting cell attachment. Synthetic polymers, such as polylactic-co-glycolic acid (PLGA), are highly customizable, allowing engineers to control properties like degradation rate and mechanical strength.
The third component is growth factors, which act as “instructions” for the cells. These signaling molecules, often proteins, regulate cellular activities like multiplication, differentiation into specialized types, and organization into a functional tissue. By adding specific growth factors, scientists can direct the formation of bone, cartilage, or other structures, ensuring the construct matures properly.
The Step-by-Step Engineering Process
The first step is cell sourcing and expansion, which involves a small biopsy to harvest cells from the patient or a donor. Because the initial sample contains few cells, they must be multiplied in a laboratory through cell culture. In a controlled environment with nutrient-rich media, the cells proliferate over several weeks until millions are available.
Next is scaffold seeding, where the cultured cells are applied onto the scaffold. The goal is to achieve a uniform distribution of cells throughout the 3D structure. Seeding methods range from simple static techniques, like pipetting a cell suspension, to dynamic methods like vacuum-assisted or rotational seeding to improve cell distribution.
After seeding, the cell-scaffold construct is placed into a bioreactor for maturation. A bioreactor is a device that mimics the conditions inside the human body, providing a constant supply of nutrients and oxygen while removing waste. In this controlled system, cells form tissue by producing their own extracellular matrix, which gives the tissue structure and strength.
For certain tissues, bioreactors also apply mechanical stimuli to aid maturation. For instance, a cartilage construct might be subjected to periodic compression, while an engineered blood vessel could be exposed to pulsatile fluid flow. These forces encourage cells to organize in a way that mirrors native tissue, enhancing the implant’s mechanical properties.
The final step is surgical implantation. Once the construct has matured in the bioreactor, a surgeon implants the new tissue at the site of injury or disease. The tissue then integrates with the host’s body, and over time, the scaffold degrades as cells remodel and strengthen the new tissue to restore function.
Examples in Modern Medicine
One of the most established uses of tissue engineering is creating skin for individuals with severe burns or chronic wounds. A patient’s own skin cells are grown on a collagen scaffold to create a construct, often a bilayered sheet mimicking the epidermis and dermis. This engineered skin is then surgically grafted onto the wound to promote healing and provide a protective barrier.
In orthopedics, tissue engineering is used for cartilage repair in joints like the knee, as articular cartilage has a limited ability to heal on its own. The process involves using a patient’s own cartilage cells (chondrocytes) seeded onto a scaffold shaped to fit the defect. The construct is then implanted into the knee, where it integrates with existing cartilage to restore a smooth, functional joint surface.
Tissue engineering has also been used to create more complex hollow organs, such as human bladders and urethras. For patients with bladder dysfunction, a small biopsy of their own bladder cells can be used. These cells are seeded onto a biodegradable, bladder-shaped scaffold, which is matured and then implanted to function as a new organ.