Developing customized materials and processes is key to extending 3-D printing to the manufacture of biocompatible nanotechnologies.

Bob Michaels

Medical device manufacturers have their sights on 3-D printing in order to fabricate components, medical instruments, and such devices as orthopedic implantables. However, to create implantable devices, manufacturers must have access to a range of biocompatible materials. David Mills, professor of biological sciences and biomedical engineering at Louisiana Tech University, will explore this theme at MD&M East on June 11 in a presentation titled "Developing Biocompatible Nanotechnologies via 3-D Printing."

Customizing 3-D Printing Materials

"While procedures that use 3-D printing to fabricate custom medical implants are still in their infancy, they have gained traction in recent years," Mills notes. "We are beginning to see the gradual introduction of customized 3-D printed implants for orofacial, cranial, dental, and orthopedic repair applications."

To get the ball rolling, FDA held a public workshop in October 2014 on "Additive Manufacturing of Medical Devices: An Interactive Discussion on the Technical Considerations of 3D Printing." As the regulation of 3-D printed devices becomes clearer, more patient-specific 3-D printed implants will likely follow. However, as researchers seek to customize 3-D printing processes, a wider array of materials and the ability to specially tailor material properties will be required, according to Mills.

Developments over the past five years have shown that the impact of 3-D printing technology is no longer restricted to simple materials and hobbyists, according to Mills. Thus, nontoxic biocompatible and bioresorbable bioplastics such as polylactic acid (PLA) and polycaprolactone (PCL) are coming to replace such commonly used resins and plastics as acrylonitrile butadiene styrene, polyamide (nylon), glass-filled polyamide, silver, titanium, steel, wax, photopolymers, polycarbonate, and stereolithography materials such as epoxy resins.

"Most basic additive manufacturing techniques use readily moldable materials," Mills notes. "Thus, 3-D printing and additive manufacturing process are primarily performed using polymers. While it is also possible to additively manufacture metals and ceramics, fabrication methods must evolve to enable the printing of solid-, liquid-, and powder-based materials interchangeably."

Solid additive manufacturing, Mills explains, is primarily restricted to laminated objects. To fabricate such objects, sheets of material are fused through pressure and heat and then cut to the desired shape using a carbon dioxide laser. Although this method is applicable to metals and ceramics, it generates large amounts of waste. Liquid-based additive manufacturing, in contrast, primarily involves melting a substance and laying it down in sequential layers to build the desired object. Alternatively, it can involve polymerization, whereby a layer of liquid material is polymerized to retain the shape of the object and then stacked upon another layer. Powder-based additive manufacturing, finally, consists of sequentially layering powdered materials. In this technique, the materials are either melted or blended before the next layer is laid down.

Extruding Filaments

"One challenge in extruding plastic filaments with unique material properties is consistently dispersing an additive into the host filament without inhibiting the abilities of the 3-D printer," Mills remarks. To meet this challenge, customized filaments are being created that are compatible with a manufacturing method known as fused deposition modeling.

Fused deposition modeling printers normally use a plastic or polymer filament to layer a 3-D construct. The most common types of plastic used are PLA and ABS. Most printers move a plastic filament measuring 1.75 to 3 mm in diameter through a heated print head with a narrow nozzle measuring approximately 0.4 mm in diameter, melting the plastic and passing it through the nozzle as the print head continues moving along the print path. Printed at temperatures normally ranging from 220 to 230°C, PLA and ABS cool rapidly, enabling subsequent layers to be built sequentially without loss of resolution. This capability results in highly customizable material properties. Because the print-head temperature, percentage fill, and resolution can be modified easily, constructs with highly variable designs can be obtained.

The fabrication process can be customized further using designer filaments with tailored material properties. Used for cosmetic purposes, one such filament combines PLA and sawdust, enabling the fabricator to print constructs that appear to be made of wood. Temperature can be controlled to change the wood tones and coloring. Another filament can be used to print low-cost sensors using plastic mixed with conductive carbon. In this application, a 3-D printer with multiple heads can be used to fabricate a construct that contains both conductive and nonconductive segments.

"At Louisiana Tech, we have developed a method using silicone oil acting as a suspension medium for powdered additives on the surface of typical polymer pellets," Mills says. "This method allows for minimal loss of the additive through the filament-extrusion process." On a tabletop scale, the team has also extruded plastic filaments with dopant percentages up to 25% by weight of metals, ceramics, and bioactive compounds, including antibiotics and chemotherapeutics. This technique allows for complete customization of the dopants without inhibiting 3-D printer functionality. "Intense research efforts, Mills adds, "are aimed at creating novel bioplastic and metal polymers for use in filament extrusion."

3-D Printed Drug-Delivery Vehicles

In the area of biocompatible nanotechnologies, 3-D printers are now being used to create customized delivery systems for transporting therapeutic drugs directly to the targeted area, after which they degrade safely or are expelled from the body, Mills comments. Medical-grade PLA and PCL beads, disks, and filaments loaded with antibiotics or chemotherapy drugs have been developed by a Louisiana Tech team to perform such focused drug delivery--a breakthrough that could generate improved drug-delivery devices, implants, and catheters.

The inspiration behind the creation of this novel technique was to use a standard 3-D printer to freely produce beads and filaments to reduce infection and deliver chemotherapeutic drugs, Mills states. "The device emphasized controlled drug release in terms of how much and when and was designed to support whatever drugs are needed, including antibiotics, antifungals, chemotherapeutics, and more. Now, we have extended this technology to drug-impregnated stents and IUDs." With this in mind, 3-D printing clearly has a bright future ahead in the medical device industry.
 

Bob Michaels is senior technical editor at UBM Canon. Reach him at [email protected].