Medical uses of 3D printing

3D printing is the process in which three-dimensional (3D) objects are literally "printed out", layer by layer, until an object is formed. Using this process, 3D printing is able to achieve extreme levels of accuracy and complexity. While makers often like to talk about 3D printing (3DP) manufacturing revolution, no one can deny that 3D printers are actually being used to develop revolutionary medical applications at an amazing pace. 3DP has been applied in medicine since 2000s, when it was first used to make dental implants and custom prosthetics. <ref>Cui X.; Boland T.; D’Lima D. D.; Lotz M. K. (2012): [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3565591/ "Thermal inkjet printing in tissue engineering and regenerative medicine"], Recent Pat Drug Deliv Formul 6(2): 149–155. </ref> Since then, applications have evolved considerably due to constantly newly developed material fitting the need of patients and reducing rejection risks. Other medical fields such as surgical organ implants, replacement and reconstruction, have now also embraced the use of 3DP. General 3DP uses many material such as ABS plastic, PLA, polyamide (nylon), glass filled polyamide, stereolithography materials (epoxy resins), silver, titanium, steel, wax, photopolymers and polycarbonate. When it comes to the medical field, the chosen material depends on its applications. Cell cultures and stem cells have been used to grow “personalised material” such as blood vessels, vascular networks, tissues, and organs. The use of 3D printing is also an ethical alternative for medical research and the testing of drugs and toxicology.

Orthodonty

Dentistry was the first medical field to enthusiastically embrace 3DP and the digital technology. Before 3D technologies reached this market, dental implants and dentures were made by a milling machine that cut them out of a solid block of polymer: not always entirely accurate, and certainly not very fast. The advent of 3DP has been a revolution for dentistry. It brought absolute best accuracy, unmatched by traditional methods. Dentists are now able to print new teeth or braces on-site and on-demand, instead of sending them to labs: treatment time for patients have been greatly reduced.

Tooth and Brace printing

A patient’s mouth is digitally scanned and stored on a computer. The file is manipulated with [https://en.wikipedia.org/wiki/Computer-aided_design CAD] (Computer aided design) software to precisely measure and design the new needed tooth or brace. When ready, the file is sent to the 3D printer which creates the tooth/brace in minutes, allowing the patient to complete the intervention in a single visit. Digital files are stored onsite or hosted on the cloud. A patient's dental history is only a click away, and the thousands of plaster molds stored in offices and labs have disappeared.

What’s next?

Rapid evolution of 3DP is pushing dentistry past creating crowns and dentures: Future prints’ material will be made of ammonium salts mixed into dental resin. Ammonium salts fight bacteria which cause infections and tooth decay. This antimicrobial plastic has been developed to eventually be used to manufacture a variety of 3D-printed bacteria-zapping dental appliances, right in a dentist’s office. Tests have been successful against harmful bacteria, and the American dental association (ADA) doubts that the resin will kill every single one of the many species of bacteria present in the mouth. Most “good bacteria” necessary for oral health, shall not be harmed. Broader tests needs to be done on the ammonium salt treated teeth in order to determine their durability when exposed to saliva, toothpaste, and other factors. Once these "enhanced" dental products successfully hold up to the rigors of clinical trials, they will become the latest innovation for the industry.

Prosthetic members

One of the most common uses of 3DP in the medical and veterinary fields is the making of custom fitted prosthetics. CT scans are used to make a tomography of the patient’s anatomy, allowing the printed prosthetic to be customised with great precision, in a variety of materials.

Prosthetic limbs

3DP is used to create cheaper and better fitting prosthetics for amputees. “Bespoke Innovations” (San Francisco) uses a scan of the patient’s intact limb to design a tailored symmetric prosthetic leg, thereby, improving walking abilities thanks to a perfect balance between the two legs. Prosthetic limbs are also printed for animals who underwent amputation, or suffering from deformities. “Derby the dog” was born with deformed front limbs, functional only down to the elbow. His motion was limited and made running impossible. Derby’s owner first tried a dog wheelchair, which kept him from certain terrains and limited his interaction with other dogs. Consequently, prosthetic legs had to be made for Derby. “3D systems” used scans of Derby’s legs, to create two unique prosthetic legs personalised to his handicap and motion: a flexible loop design, printed by a multi-substance 3D printer. His new flexible legs have been allowing Derby to run without difficulty from the very moment he wore them.

Help for better planning, time and cost effective surgery

In some instances, 3D printed objects are not aimed at being used on the patient, but help health technicians to better evaluate the injury or malformation. Coupled with MRI scans, the patient’s anatomy is closely mapped through a CAD software, and reproduced by a 3D printer. This allows surgeons to appropriately plan the intervention, thereby greatly lowering the risk of complications. This use of 3DP is hoped to make reconstruction easier, quicker and cheaper.

Bone Fracture repair or congenital bone deformation

From 3D views of the computed MRI scan, An acrylic model of the injured area is reproduced by a 3D printer. the reproduced bones allows the proper diagnostic on the possible fracture of inherited malformation and allow the surgeon to choose the right equipment prior to surgery. Metal plates, for instance, are adapted (bent) the the 3D model so it perfectly fits the patient’s bone shape and size. screw zones are also carefully picked without visibility restrictions of the surgical area. Once the material is perfectly adapted the surgical approach is made easier, the intervention is quicker and more precise, contributing to a successful bone repair. In some malformation such as radius curvus, the combined use of angled rotation centers, MRI and acrylic 3D model acrylic give an accurate assessment of bone deformities that could not otherwise be obtained. With these new methods, a precise pre-surgical strategy can be planned to deal with very large bone defects that can be fixed in a single intervention. 􀀁

Cartilage/ bone replacement

In these kinds of interventions, the injured or missing organs need to be replaced by similar tissue taken from another body part. Needless to say that the material supply is limited and precious. In the example of ear replacement surgery, cartilage is harvested from the rib cage, from which the new ear is immediately carved. The time and error allowance is therefore very low. Before the appearance of 3DP, surgeons have been practicing carving of the needed shape on materials such as soap, fruits, pig cartilage, etc. Training material rarely had the size consistency or characteristics for a true preparation. A better solutiom came with a low cost 3D print cartilage model close to the look and feel of human cartilage, which enables surgeons to practice on the procedure with a superior training efficiency.

Organ implants

Tissue or organ failure, diseases, accidents, and birth defects are critical medical problems. Current treatments rely mostly on organ transplants from living or deceased organ donors, who need to be a “match” (3). While the global population is aging and unseen diseases appear, 3DP brings an alternative to (the lack of) organ donations or appropriate medication. It is a revolutionary step in the world of medicine. 3DP now allows the use of biocompatible materials, cells, and other biological components to print complex structures such as tissues and organs, destined for organ transplantation. Scientists have so far been able to create several kinds of tissues including multi-layered skin, bone, vascular tissue, tracheal splints, heart tissue, and cartilaginous structures. An innovative solution involves using a cell sample from the patient, to grow a replacement organ which can minimise risks of tissue rejection, and lifelong need of immunosuppressants (6). 3DP is already hard at work in the implant industry, taking over the business with custom-made and unique 3D printed implants. However, there are no magic solutions: though they provide a much better fit, 3DP implants are still followed by months or even years of painful hobbling around and rehabilitation.

Soft tissue implants

In the first stages of soft tissue 3DP trials, the tissue was collapsing on itself while being printed. This has eventually been solved by using a slurry of gelatin to support the tissue or organ as it is being printed. Once the printing is complete the gelatin is melted away in room-temperature water, and the finished tissue is able to support itself.

New material

A vastly used 3DP technique is the "Two-photon polymerization". It creates small detailed objects from several types of photoreactive liquid precursors, which contain light-reactive chemicals that turn the liquid into a solid polymer. Unfortunately, most of these chemicals are toxic, and might cause complications when used in a medical implant. An alternative has been however found in riboflavin (vitamin B2): both nontoxic and biocompatible, it is mixed with a precursor material to make it photoreactive, and create medical implants from non-toxic polymers (17). This discovery brought up many more biocompatible implant materials, which can be used for 3D implants printing.

Printing Organs

In March 2011, Anthony Atala, M.D. (Director of the Wake Forest Institute for Regenerative Medicine) demonstrated the 3DP of a kidney from a solution of living cells, though it was lacking the necessary tubules and blood vessels to make it functional. Researchers had tried printing the kidney, leaving holes for the vessels and tubules, but this led to a lack in the structural stability needed to withstand blood pressure (28). This has been the main issue with creating functional organs: living cells need to be within 150-200 microns of a capillary to survive. Ongoing research is working on solving the problem, and create the incredibly small spaces needed for the blood supply within the 3D printed organ, along with the structural stability necessary to withstand normal blood pressure. In march 2015, a thyroid gland was successfully transplanted on a mouse (18). This specific organ was chosen due to its relative simplicity, with hope that later on, other organ transplants would be possible as technology allows it. The mouse’s own stem cells (from fat tissue) were used as primary material in order to prevent rejection, and the gland mature cells were grown thanks to a 3 directions robot, which dripped fabric layers of living cells through an automated syringe. Before printing, the cells were transformed into ‘spheroids’ (layered cells), then placed in hydrogel, to enable the printing process. After printing, the organ was placed in a bioreactor which dissolved the gel to leave the thyroid organ was left to mature on its own before the surgical implant(18).

Dura Mater

The brain is protected by dura mater. it is divided into a superficial and meningeal layer, which need to be cut open in case of brain surgery. Upon completion of the surgery, The Dura mater must be closed up by a suture and onlay procedure. “Maipu Regenerative Medical Technology” developed 3D printing of a substance they called “Redura”. Printed onto the surgical site “Redura” provides a suitable environment for the cells and tissues to regenerate. Within 2 months, the meningeal tissue grows back and the “Redura” begins to dissolve into 2 non-harmful natural components: water and carbon dioxide. The “Redura” is used in hospitals globally and has been successfully used in over 10 000 patients without any adverse effects.

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