Article focus:

• Many types of 3D printing are used to create prototypes of medical devices
• Scientists are working on the 3D printing of organs, on demand
• 3D printing and rapid prototyping have changed the face of medical design

The potential to build bones, organs, and tissues in only a matter of hours using 3D printing fascinates doctors, designers, and surgeons alike. Indeed, the most exciting and fastest-growing applications of these methods of rapid prototyping (RP) are currently found in the medical industry. While the initial purposes for RP were predominantly in design and product development, many doctors, surgeons, and medical teams quickly recognized the capability for 3D printing to augment the medical sphere. Recent advancements and material developments along with lowering prices continue to boost the technology’s evolution.

In 1986, 3D Systems Inc, Rock Hill, SC, made the first commercially available 3D printing machine. Unlike subtractive techniques such as CNC milling, 3D printing is an additive process because objects are built up, layer-by-layer. As each layer binds to the previous one, the cumulative thin layers transform into the three-dimensional object. Complex shapes, complete with negatives spaces and undercuts, can be "grown" in one single piece.

Over time, "3D printing" has become an umbrella term for a number of additive manufacturing techniques. Many of these methods use inkjet print-head technology to deposit layers of material or binding agent. But unlike desktop printers, which print 2D images onto a flat sheet of paper, 3D printers “print” a layer of material or binder onto the machine build bed. Depending on the machine and setup, the bed drops between 0.016 mm to 0.1, and the printer deposits a new layer on top of the preceding one. Other techniques based on the same principles use resin and UV light or lasers that sinter powdered metals to form titanium, stainless, and other metal parts.

3D prototypes for reconstructive surgery
Protoform has been working with medical teams over the last three years to supply 3D printed models. These range from prototypes of catheters and other devices for design assessment to patient-specific anatomical models used by some of South Africa’s leading surgeons. Of all the various prototypes and models we offer, nothing seems to compare to the thrill of seeing 3D printed models of bone or skull emerging from our Z Corporation 3D printer. Surgeons use the models as a source of reference to help them better plan surgeries.

A large percentage of the models we make are maxillofacial models — mandibles, maxillas, and occasionally the entire skull. Most of these are needed for cases involving dramatic reconstructive surgery to correct damage caused by tumors, gunshot wounds, motor-vehicle accidents, and so forth. Typically, a patient is scanned using either a Cone Beam or CT scanner, with an ideal slice thickness of 0.5 to 0.7 mm. The radiographer saves the cross-sectional data in the DICOM file format, which can then be processed using specialist software such as Materialise’s Mimics and 3-Matics. The software lets users perform a "virtual surgery" to isolate tumors or accurately design and plan guides and implants. The digital model is finalized and converted into an .STL file for 3D printing. Surgeons can use models to accurately create patient-specific implants, which can be built and delivered within a day in the case of emergencies.

According to Carol Spence, of the P-I Brånemark Institute in South Africa, 3D printed models can easily save surgeons, on average, two hours of operative time, whereas implants and plates would previously have had to be bent up and fitted during surgery. Naturally, operations are thus less risky to patients because they need spend less time under anesthesia. 3D printed models also ensure implants will fit precisely. Spence adds that prototyped models also permit the fitting of implants intra-operatively, leaving patients with minimal scarring. In addition, surgeons suffer less fatigue. The resulting financial savings are significant.

In a similar manner, cross-sectional data and subsequent models derived from patient scans can be used to plan accurately for incisions and drilling. Mimics software lets users identify and differentiate between veins, arteries, and bone thickness relative to the area to be operated on. 3D models make it easier to identify the best surgical approach, drilling positions, and anchor points. Using this information, a drilling guide for the surgeon, unique to the patient, can be designed and 3D printed.

The same principle applies for implants. For instance, a 3D printed model can serve as an exact reference to pre-bend a mandibular reconstruction plate. 3D prints can be used as masters from which to create a mold to cast a titanium implant. Some models can even be directly printed in titanium. The implants are accurate, with optimal fits that often have aesthetic benefits for the patient, such as ensuring symmetry.

Likewise, the technology is used in the reconstruction of the auricle, or pinna, for child burn victims, through charities such as Children of Fire (firechildren.org). For example, a burn patient’s undamaged ear is mirrored and designed in the software as a prosthetic. The model is then 3D printed and can be used either as a base for a skin graft, or alternatively, to make a mold to construct a synthetic ear.

Custom hearing aids
Siemens Hearing Instruments Inc, Piscataway, NJ, (medical.siemens.com) has become a leader in producing hearing aids by using specialized ear impression scanners and forming precise 3D models of the patient’s ear. Digitally, a custom earpiece can be designed to fit the contours of the patient’s ear, whether for use behind the ear, in the ear, or in the canal. The model of the earpiece is then 3D-printed, and the electronic internal components are inserted. The result is a comfortable and effective hearing aid. When a replacement is needed, the digital model is recalled and the model reprinted.

RP and Forensics
There is a nice irony when ancient artifacts meet state-of-the art technology. RP technology lets bones and other human remains be scanned and replicated, with minimal disturbance to the original. For example, a mummy can be CT-scanned and the brittle bone replicated using 3D-printing. The models can be handled for analysis and measured or incised. Similarly, 3D-printed models are invaluable visual tools for learning about specific case studies. They are also an ethical substitute for the use of human remains.

What’s next?
Among others, Dr. Gabor Forgacs and Organovo, San Diego, CA (organovo.com), are pioneering 3D printing of a different kind — that of human organs. Currently referred to as “bio-plotting” or “bio-printing,” the technology makes use of the same principles as most commercial 3D printers. However, rather than depositing a synthetic material, the printer deposits human cells on a scaffold gel, or biopaper, to create human tissue. The goal is to one day have the potential to print complex organs like the heart or kidneys, on demand. Because the base materials comprise cells farmed directly from the patient, in theory, transplant rejection would become a thing of the past.

Already, scientists have developmental machines that synthesize 3D-printed prototype bladder and kidney tissue.