Medical Device Development’s Use of 3-D Printing: Applications and Common Processes

By Nathan Müller
May 8, 2017


• Categories: 03-Manufacturing Execution, 04-Product Design & Development

From form studies to production-ready, functional components, the medtech applications for 3-D printed parts are growing rapidly. Parts created in 3-D design software can be quoted almost instantly by a myriad of 3-D printing shops. With a growing number of vendors offering online and automated quote apps, the time required to iterate on the design for manufacturability compresses significantly. Lead times for parts are being reduced—most parts can be made and delivered within 24 hours. Components can then be tested in the end-use environment, revealing possible improvements to be made, while speaking to a final manufacturing process.

The use of such parts is growing across industry, with their application to medtech products becoming more widely accepted. The human body is uniquely suited to these processes and parts; many are taking note of this and creating novel solutions to a plethora of medical needs.

Applications

The human body is a dynamic, highly variable, and organically shaped form. Integrating products with the human body has always been a challenge, as soft tissue and high range of allowable motion are difficult to accommodate. Many medical device products are created as simplified geometric shapes. This is partly due to the ease of control offered by the mathematics governing these designs. Traditional processes involved in creating these shapes—subtractive methods, such as machining and fabricating—also favor simplified geometric shapes, which are faster and more reliable within these processes to fabricate and verify through measurement.

3-D printing has shaken up these established methods of design. We are now able to create contoured surfaces to a great level of precision and repeatable accuracy. In relatively small quantities, this can be accomplished much faster than traditional methods (e.g. injection molding). Faced with how to model shapes that fit with the human body, the 3-D modeling software industry is catching up. We can employ upgrades to existing 3-D modeling software, as well as use 3-D scanning technologies to capture the human form.

Biocompatibility with living tissues, whether on or inside the body, is a major concern for medical device designers and producers. This not only relates to materials used in products but the processes used to create the shapes of parts and methods used to assemble them. Process cleanliness is a major component of medical device development. Examples of when traditional methods of fabrication cause issues include the coolant ingress into material—take the machining of parts via CNC, for example, using a mill or lathe. Since most additive processes (FDM, SLA, SLS/DMLS, jet-based) require only the virgin material and a heat source, control over the cleanliness of the final product can be finely tuned; however, the cleanliness of these processes is still under development, as their application to parts used in medical applications is only just beginning.

Wearable (on the body)

Design and fabrication processes in 3-D printing have come far in recent years, giving opportunity to many applications, especially in the medical device field. Biocompatible materials—such as acrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), and polylactic acid (PLA)—are widely used in 3-D printers. They lend themselves well to such uses as wearable monitors and prosthetics.

Wearable monitors and devices are on trend and seeing growth currently, and as a way to iterate forms, 3-D printing is key. This decreases time to market through increasing feedback from users, testing the device’s comfort and feel, with the ability to change form and implement changes on prototypes.

Prosthetics are designed to conform to and replace a body part, generally a portion of a limb. Models of the area to interface can be created through 3-D scanning methods, and a prosthetic matched to this profile using 3-D modeling software. Leveraging this relationship, the final part can conform perfectly to an individual’s physical profile. 3-D printing of the part also allows for contoured surfaces to be easily produced, matching them to the mating portion of the patient. As traditional processes favor repeatable setups, this is a key area for growth. Due to the increase in accessibility to 3-D printing technology, this application is being taken up across the globe to deal with lack of access to medical care and has provided many individuals with the opportunity to receive prosthetics at a lower cost than previously available options.

Implantable (in the body)

Bone replacements, custom splints, and sub-dermal drug delivery devices—3-D printed applications in the body are growing at an ever-increasing rate. With implantable materials such as PEEK and titanium and the process to create patient-specific conforming shapes, surgeons and clinicians are seeking companies that provide custom fabrication of these parts and systems. Bone replacement techniques can benefit from the sintering process of creating titanium using direct metal laser sintering (DMLS), and specialty ceramic blends using selective laser sintering (SLS). This is due in part to the capability of the process to produce final parts that are porous, as bones will integrate better with grow-in interfaces. This has been well documented in literature. Custom splints for specific applications is fast growing, as surgeons have many requirements for the shape and attachment requirements for different bone structures and shapes. Opposed to the traditional “pin” method, this will open many opportunities to explore integrated solutions to allow the body to repair bones and internal trauma.

As 3-D printing can employ several jetted materials simultaneously, multi-substance prints can combine to create a composite final product. This could contain multiple components, such as a drug within a polymeric matrix. This is especially useful when considering the timed release of drugs in patients, as the dosage can be finely tuned due to the ingress of fluid or biodegradation of the material. Patient compliance to taking drugs at set times is therefore not a factor, as with traditional dosage methods like pills.

The design solutions created by 3-D printing are numerous and diverse. The capabilities of customized printed parts with fast design cycles lends itself to medical devices greatly. Whether on or in the body, many advantages can be gleaned from these processes. The potential of 3-D printing in medical device development are limitless –future years will provide even more exciting applications for these and novel technologies to come.

Common Processes

What 3-D printing processes are available and what are their relative merits? Several techniques have been commercialized on large scales. With a blossoming of material choices and refining of processes in recent years, 3-D printing has spread its potential and is now used across the entire design process in medical device development. From concept modeling in early stage development to production, 3-D printed parts are becoming indispensable. The flexibility of 3-D printing in medtech comes down to several aspects: speed of the design cycle, cleanliness of the process, resolution of the finished part, and the variety of materials and processes available.

Continue to page 2 below to read about each manufacturing method.

FDM/FFF (Fused Deposition Modeling/Fused Filament Fabrication)

FDM uses thermoplastic filament that is fed through a nozzle to create fine micro-scale strands, deposited side-by-side and layer-by-layer. This eventually builds a finished part. The part’s walls can be solid, but more commonly have variable in-fill density, which can make for lighter parts. This can be anything from parallel layers of material to stronger, honeycomb-like internal structures, and is managed through software with input on density and dimensions. Since engineering-grade plastics can be used, the parts can exhibit strength properties approaching injection molded or machined plastic parts. Also, because thermoplastics are reflowable, heated inserts can be used for fastening. This increases part application potential due to the strength of both the parts and fastening points. However, surface quality is generally an issue due to the cosmetic lines/layers on surfaces and support witness points created during fabrication. Certain materials, such as ABS, can be manually polished with abrasives; however, other materials, like PLA, cannot. It is important to consider the appearance requirements of the final state of the 3-D print and any post-processing techniques you wish to employ.

The build direction for the print is also important to consider, as it affects both function and appearance. Certain orientations will produce a more pronounced layered finish, such as contours. The build direction can also influence the strength of the part (tension, compression, bending, etc.), the amount of additional support material used, and the level of detail or amount of functionality in a given feature. Another parameter to consider is shrinkage of parts. Certain materials with higher processing temperatures, like ABS, will exhibit shrinkage, especially when used for larger parts. With these materials, this creates design constraints such as a required lower in-fill (reduced density) of material in larger parts.

Depending on the capabilities of an FDM printer and the functional material used, water soluble support structures exist. This can help when complex shapes and cosmetic supported surfaces are required in the final part. The soluble material is used to support the part being built and, after the build is complete, the composite part can be soaked until the support structures simply melt away. Some manual application with tools to scrape away remaining support structures is necessary; however, soluble support material removal can be automated with the use an ultrasonic or circulation tank.

SLA/SL (Stereolithography)

SLA employs a photo-polymer resin bath, platform and ultraviolet (UV) laser beam to create parts on a micro-scale, layer-by-layer. These parts can be completely solid or hollow. Solid parts have the benefit of strength and rigidity but end up costing more in materials. Hollow parts reduce the required material and lower the part cost; however, they require drainage features to allow for entrained resin to escape, which must be considered in the initial stages of design. SLA resins form thermoset plastics when cured, which cannot be reflowed and therefore heated inserts cannot be used for fastening. However, alternative insert methods can be employed, such as helical or press fit inserts.

High resolution builds using SLA can yield finished parts that show little evidence internally or externally of layering, unlike FDM. Standard post-processing by vendors (blasting) creates a matte finish on the parts, which is suitable for most uses. However, when optical clarity is required, vendors can provide additional processing to produce surfaces that emulate polycarbonate or acrylic, allowing for optical applications such as lenses or parts used to visualize internal flow of fluids. This can also be achieved by considering build direction and the flatness of a surface, as side walls and top surfaces without contours or support structures will be optically clear (of course, without blasting specified when ordering).

There are a wide range of SLA materials that can emulate the strength of traditionally manufactured materials, from polypropylene to die-cast aluminum. Metal-emulating parts built with SLA printers require significant post processing that increases cost of parts, but results in high detail with mechanical properties approaching that of metals. Finally, because the curing process employs a laser beam, ultra-fine resolution can be achieved. Vendors guarantee layers formed as fine as 0.001” (0.025mm), with features as small as 0.002” (0.050mm), for certain fine resolution material choices. More general-use materials with enhanced mechanical properties offer layers of 0.002” (0.050mm), with features of 0.005” (0.125mm).

SLS/DMLS (Selective Laser Sintering/Direct Metal Laser Sintering)

SLS comprises a high-powered, pulsed CO2 laser, powdered material (plastics, glass, ceramics), and a bed-roller assembly. Powdered material is applied by the roller in thin sheets. Sweeps of the laser then sinters (or fuses) together material line-by-line, creating parts with up to 100% in-fill density. DMLS employs the same process, but instead uses applicable metals (aluminum, titanium, stainless steel, etc.). An advantage of the SLS process is that it allows for the highest strength plastic parts across all three processes. Engineering-grade plastics with high stiffness and strength along with the potential for carbon-fibre or glass-fibre in-fill result in extremely tough SLS parts. These parts can be employed in the most demanding of applications, with functional features such as snap fits providing assemblies with mechanisms that reduce the need for separate fastening components.

Standard surface finish is a matte/sandy texture due to the fabrication process, but this can be polished or blasted to a desired final finish. Repeatability across a small volume of parts is high when compared to machined components, as parts are created on the same bed.

A distinct advantage of SLS over FDM or SLA is that since parts are supported by unsintered surrounding material, there is no need for support structures for features. This allows a consistent surface without the need for post-processing to remove supports. Parts can also be made within parts, which allows for the exploration of alternative designs; the only caveat here is that unsintered material within parts must be removed after fabrication. This can be accomplished by adding drain holes or the equivalent to access void spaces.

Process Comparison
Process	FDM	SLA	SLS
Part Quality	Low to Medium	High	High
Plastic Type	Thermoplastic	Thermoset	Thermoplastic
Surface	Layered finish	Matte to smooth/clear finish	Matte to smooth/clear finish
Transparency	Opaque material	Opaque to transparent material	Opaque material
Mechanical Strength	High tensile strength	Brittle	High overall strength
Layer Thickness	0.125–0.5 mm	0.025–0.05 mm	0.10–0.25 mm
Support Structure	Needed	Needed	Not needed
Airtight	No	Yes	Yes
Chemical Resistance	Not suitable for fluids	Not defined	High
Materials Available	ABS, PLA, nylon, polycarbonate, polyamide, polystyrene, elastomers	Photocuring resins	Polystyrene, polyurethane, nylon, aluminum, titanium, stainless steel, cobalt chrome, inconel