The idea of scientists growing body parts in the lab usually conjures up images of popular science fiction, too farfetched to ever be considered outside of a mad scientist stereotype. In reality, the fields of medical and tissue engineering have made great strides in utilizing 3D printing technology in combination with biological materials to possibly do just that. This installment of “What’s it all about?” aims to make the concepts of 3D bioprinting a little less spooky and a little more comfortable to talk about for the average biologist.
What is 3D printing?
3D printing is an umbrella term for the creation of a 3D object using various additive manufacturing approaches. The most common material now, and the one you’ve likely seen before, is liquid plastic being squirted out of an inkjet-like printing head layer after layer into a pattern that then solidifies, creating a 3D object (see Yoda in Figure 1). It took several important discoveries to get to this point. 3D printing similar to the form we see today was first patented in 1988 as “Fused Deposition Modeling” (FDM). FDM takes a coil of thermoreactive material such as metal or plastic and feeds the wires from that coil to a printing head. Here the wire material is melted and extruded through the printing machinery so it can be added to in a specific pattern like ink. As the material cools, the object takes a solid form. The idea of 3D printing hasn’t changed much since this original patent, but the term 3D printing didn’t take hold until 1993 when the printer heads themselves were optimized at MIT. There they started using the powder bed process employed by most inkjet printers, where a layer of powder is deposited in-between layers of “ink” (in this case liquid plastic) to help the layers stick together. Since 1993, new companies have vastly expanded 3D printing to make it available for commercial use or to have in your home or lab. There is also a push for 3D printing to be a sustainable form of building, using recycled plastic pellets as materials.
Central to 3D printing is the design of the 3D object, usually created with computer-aided design (CAD). There now are a number of both open source and paid software packages for CAD, specifically for 3D printing. These range from software for hobbyists, like TinkerCAD or Thingiverse, to those specifically aimed at bioprinting applications, like Within Medical. Interestingly, traditional 3D printed plastic objects are increasingly being used in biology. Many researchers have started 3D printing custom tube racks, equipment parts, and I even 3D printed a special coverslip holder! Check out “Using 3D printing or bioprinting in your own research” for more information.
How do you do this with biological materials?
3D bioprinting involves the actual 3D printing of two materials: 1) the biomaterial or non-living scaffold that will support and provide cues to the living materials and 2) the “bioink” or living parts of the structure, which will cellularize the structure. Both the biomaterial and the bioink vary largely based on application, but generalities do exist. A biomaterial is defined as any material that is engineered to interact with a living system. These are usually some form of hydrogel that serves as the extracellular matrix and contains chemical and biophysical cues specific for instructing the bioink. Bioink is generally a cell slurry mixture with one or more cell types depending on the specific application. Both components present two major optimization concerns: functionalization and extrusion. 3D bioprinting still largely relies on FDM techniques to build objects as opposed to inkjet printing mechanisms. This is due to the nature of the materials and the ability to get them through a narrow nozzle as a liquid with their functional capacity intact. For a full review of the process, check out the review, “Three-dimensional printing of biological matters.”
What are the applications of 3D bioprinting?
There are three main focuses in the field of 3D bioprinting.
Biomimicry – This is the true attempt at making functional body parts in lab. As the name suggests, these applications are trying to completely mimic a biological system in the hopes of providing a whole or partial replacement of that biological system when necessary. Much of this work started with decellularized extracellular matrix scaffolds or whole organs to which cells or bioinks were reintroduced. Studying how cells organized within these precise microenvironments has now allowed researchers to better engineer those microenvironments. Given the huge gap between the vast need for organ transplants and the number of organs available, this is an aspirational goal.
Self-assembly – This area of research combines the principles behind organoids with that of 3D printing. The biomaterial and bioink are usually premixed and printed together as a type of spheroid, allowing for the cells to expand on their own into a 3D environment. Stem cells are often used as the bioink, giving the whole object the capacity to further differentiate. When used with individual induced pluripotent stem cells, this field has the potential to revolutionize personalized medicine.
Mini tissues – Here, 3D bioprinting again combines with another trending technique, microfluidics. Underlying this focus is the idea that each organ or tissue is formed from fundamental building blocks. You can then recreate those building blocks in miniature for ease of manipulation and study. Often this involves the flow of liquid through the 3D printed object to emulate blood flow through microvasculature. Mini tissues or organs-on-a-chip might not seem to directly impact patients, but they have become the norm for understanding the fundamental biology of tissue-scale events.
What’s happening now and where is this field going?
3D bioprinting may be the newest trending technique covered to date by “What’s it all about?” with PubMed results first appearing in 2007 and a significant rise in publications beginning in 2013 (see figure). Though new, the field has grabbed the attention of scientists and the general public alike. 3D bioprinting has been utilized for either the study of or mimic of every organ system of the human body. Also, the scale of 3D printing for use in humans has been proven possible, but the complete functionality and durability for a transplantable organ has not been shown. It is likely that you will first see technologies emerge that focus on 3D printed organoids or mini-tissues for laboratory testing of compounds, such as cosmetics or individualized drug sensitivity screens. Though no functional artificial organs have been created yet, it’s safe to say this is on the horizon as a groundbreaking advancement in medical technology. With this technology being so new, the ethical implications are far from worked out. A recent review on the socioethical views of bioprinting found that it’s not a topic currently being discussed outside of the biomedical sciences. For 3D bioprinting to realize its full potential, the ethical implications and the practices to address them will need to evolve just as fast as the technology itself.
Using 3D printing or bioprinting in your own research
As mentioned above, scientists are finding use in traditional 3D printing by creating their own plastic lab equipment. This is seen as beneficial for both the low cost and the customization possibilities. Several open sources exist for getting you off the ground with pretested designs, including Open-Labware and Public Lab. Additionally, there are many guides for beginners looking to purchase their first personal 3D printer, including “The Ultimate Beginners Guide” and “How to Choose the Best 3D Printer for Your Lab or Classroom.
For true bioprinting innovation, it’s best to start with the limited commercial products available. A recent article summarized the “Top Ten Companies in Medical 3D Printing.” Notably left off this list is CELLINK, which sells both custom bioprinters and bioink. Thus far, 3D bioprinting is probably not quite ready for those who want to just buy some stuff and try it out, but most of these companies do invest in collaborations with academia for those who want to get in on the ground floor.