Author: Jiyoon Ahn Edited by: Inês Barreiros
Three-dimensional (3D) printing is driving innovation in many industries and research fields, including engineering, manufacturing, art and, more recently, medicine. When it comes to medicine, 3D bioprinting -which might become critically important for this field- refers to the process of 3D printing organs or tissues by creating cell patterns where cell function and viability are preserved. The size of the global 3D bioprinting industry is estimated to reach £1.47 billion by 2020, as this innovation is set to transform the field of medicine. So how exactly does 3D printing work and how can this technology be applied in a biological context?
Everyday ink-jet printers are loaded with cartridges of ink which is sprayed onto a piece of paper to produce text and two-dimensional (2D) images. In a similar manner, 3D printers are loaded with cartridges of ink which is moved left and right, back and forth, and up and down in a layer-by-layer manner to produce a 3D object. 3D bioprinters work the same way but rather than cartridges of ink, they are loaded with cartridges of “bio-ink”. Bio-ink is a slurry of different types of living cells that behave much like liquid ink and can be loaded into a cartridge. A 3D bioprinter usually contains another cartridge with “bio-paper”, a dissolvable gel to support and protect the cells during printing. Natural polymers such as alginate, gelatin, collagen and fibrin, or synthetic polymers such as polyethylene glycol and polylactic acid are often used. The mixture of cells within bio-ink depends on the tissue or organ being printed. For example, blood vessels may be printed using a mix of endothelial, smooth muscle and fibroblast cells. Once these cells are ejected from the bioprinter, they naturally fuse together and rearrange themselves within the gel. The bio-paper then dissolves away, leaving behind the final bio-printed body part.
Generally, 3D bioprinters utilise the layer-by-layer method to create tissue-like structures that are later used in medical and tissue engineering fields . This means that before printing can begin, a model has to be created and appropriate materials have to be chosen. One of the first steps is to obtain a biopsy of the organ. The most common tools used are computed tomography and magnetic resonance imaging (MRI). These are used to design a digital map that models and determines where the cells and other components should be placed to create the desired 3D structure. This step should also take into account how the cells are expected to grow over time. The 3D model is subsequently subjected to tomographic reconstruction to produce 2D cross-sectional images. These 2D images are then sent to the bioprinter.
Compared to non-biological printing, bioprinting involves additional technical complexities, including choice of biomaterials, cell types, and the need for growth and differentiation factors. Biomaterials are complex combinations which require desired functional, mechanical and supportive properties. The cells used need to be well characterised and should derive from reproducible sources. The right combination of cells is required with specific functions, depending on the cell types present within the tissue to be printed. For cell growth, a direct control over cell proliferation and differentiation needs to be achieved using small molecules or growth factors. After a tissue has been printed, bioreactors are required to maintain it, during which maturation factors and nutrients need to be provided. For 3D bioprinting to realise its full potential, advances are needed in our understanding of the fundamental biology and biophysics underlying tissue regeneration.
Despite the great innovation in the field, artificial organs such as 3D printed livers and kidneys have been shown to lack crucial elements that affect the body such as working blood vessels, tubules for collecting urine, and the growth of billions of cells required for these organs. Without these components, the body has no way of getting essential nutrients and oxygen deep within the organs. Given that every tissue in the body consists of different cell types, these factors have to be adjusted to ensure stability and viability of the cells during the manufacturing process. Moreover, the 3D printed tissue needs to stay alive long enough to integrate with the body and fuse with its blood supply.
So how will 3D bioprinting revolutionise medicine? The main application for 3D printed tissues and organs is to test new medicines. Bioprinted tissues are more predictive of clinical outcomes than cell cultures, and eliminate the need for testing in lab animals or human volunteers in the early stages of drug discovery. As an example, academics at Harvard have developed a “heart on a chip” by using six printable bio-inks. One side of the chip is covered in lab-grown heart muscle cells, which contract and relax like they do in a person’s heart, giving the chip a “beat”. There is an integrated sensor which measures the “beating” of the tissue which makes it easier for the researchers to study how the tissue responds to drugs and toxins. The heart is created by a fully automated process, and there is the potential for rapid high-throughput production of customised chips to match the conditions of a disease or an individual patient. Organs placed on chips are better than animal testing for some diseases as animal models sometimes do not accurately mimic human pathophysiology.
One of the major driving forces behind 3D bioprinting is the generation of organs and tissues for transplantation, which could be the solution to the global shortage of donors. Replacement organs could be printed in a lab from a culture of the patient’s own cells, which reduces the risk of organ transplant rejection. 3D bioprinting has already been used for creating several types of human tissues, including skin, bone, vascular grafts, tracheal splints, heart and liver tissues, and cartilages. Many research teams have also successfully transplanted a bioprinted organ into an animal. For example, researchers at Wake Forest Institute for Regenerative Medicine printed ear, bone and muscle structures and implanted them in mice and rats. These implants triggered the formation of new cartilage tissues, blood vessels and nerves, which suggests that they successfully integrated with the animal’s body. Additionally, 3D Bioprinting Solutions, a Russian biotechnology firm, transplanted a 3D printed thyroid gland on a mouse, which was shown to be fully functional.
3D printed organs are not quite ready for human transplant, as, so far, they have been too weak and unstable. The parts need to be monitored for longer periods than in the current setups to ensure that they do not decay over time and that they perform well within the human body. Addressing some of these challenges will require the integration of knowledge from the fields of cell biology, medicine, physics, materials science and engineering. Although the field is at an early stage and still faces many technical challenges, it is definitely a very promising technology!