bill palmteer book chapter

Introduction

Additive manufacturing (AM) also known as 3D printing has been around for

over 30 years and is now taking center stage in the medical manufacturing field.

Initially computing power hindered the technology but over the years various

3D printing technologies have emerged providing the ability to make complex

moving/interlocking products out of a wide range of material(s). These new

AM processes and materials, when used in the correct application are slowly

displacing a percentage of the more traditional subtractive processes like CNC,

machining and casting. Wide ranging applications for AM technologies exist in

the medical industry for customer specific surgical tools, soft prosthetics and

structural replacements for bone/joints each requiring unique material

properties. The first 510K was issued by the FDA in 2012 for an implantable

product and has cleared the way for this technology but the newcomers will still

need to validate the new processes/materials will meet biocompatibility,

sterilization, safety and efficacy standards.

Prosthetics

Implants

Custom Tools

Bill Palmteer 5/4/15

The Emergence of 3D Printing in Biomedical Applications 2

3D Printing Methods and Materials

Since the inception of 3D printing various forms of additive manufacturing have arisen here we

will only cover FDM/FFF (Fusion Deposition Modeling, Fusion Filament Fabrication), PLS

(Polymer Laser Sinter), DMS (Direct Metal Sinter) and lastly CLIP (Continuous Liquid Interface

Production Technology). Reviewing these 3 technologies will provide an adequate overview of

the potential medical products that can be manufactured, associated material and capital costs, and

time to market.

Imaging Requirements (Hardware/Software)

In order to make a 3D solid object using additive manufacturing processes you first need to acquire

a scan of the object you want to replicate or create a 3D computer model. The computer model

can be created using one of the many 3D rendering software’s such as Solid works, Google Sketch,

Pro-E or Rhinoceros. If the actual object

already exists that you want to replicate then it

can be scanned into the computer by using

various scanners on the market. For medical

applications, CT, MRI and even ultrasound

scans are capable of producing the data that is

needed. For dental applications one such

scanner is called iTero and is shown in Fig 1.

This is the latest technology being used in the

dental office replacing the need for

castings/impressions which are time

consuming and composed of distasteful

materials. Like most scanners, the iTero scanner digitally captures the structure of the teeth and

gums by projecting 100,000 points of laser light and capturing the reflective energy on a photo

sensor transmitting the data back to the computer. The data can then be used to 3D print accurate

crowns, veneers, Invisalign and other dental appliances (Itero, 2014).

At this point, in some customization to the file might be needed. In some cases to maximize

throughput rather than printing one part at a time the user may wish to make multiple copies of an

object or several different objects at a time. The space available for printing is based on the 3D

printer and its print base. Also, one may choose to add a spruce or support structures that can be

removed after the printing.

Fig 1: iTero Dental Scanner


Once a 3D computer model is completed the file needs to be manipulated into the industry standard

exchange format for additive

manufacturing is the STL

(STereo Lithography or

Standard Triangulation

Language) file. Basically, it is a

file that replaces the original

surface of solid, surface or

scanned model with a mesh of

triangulated surface segments.

Almost all of today's CAD

systems are capable of

producing a .STL file. Within

the software you can control the

variables that control the mesh

size. If you create a finer mesh

you will a larger .STL file but

you will end up with a better 3D

print. However, increasing the

resolution excessively may not

improve the quality of the

produced part and could cause

delays in processing because of

the larger size.

The next step is to convert the

*.STL file to G Code which the

3D printer understands. The G

Code establishes the “slicing” or

sectioning CAD design into

layers. The preprocessing

software calculates section and

slices the part design into many

layers, ranging from 0.005

inches (0.127 mm) to 0.013

inches (0.3302 mm) in height.

Using the sectioning data, the

software then generates “tool

paths” or building instructions, which will drive the type of 3D printer. We will touch briefly

again on additional capabilities of software under PLS/DLS (Hiemenz, 2014)

Fig. 2: Hardware/software flow chart, created by author


Fused Deposition Modeling

Depending on the type of equipment used most thermoplastic resins can be printed using FDM.

The most common thermoplastics are ABS (Acrylonitrile Butadiene) which is used to make Lego’s

and PLA (Polylactic Acid) which can be plant based and recyclable. Equipment manufacturers

like Makerbot replicator 2, shown in Fig. 3.1 specify the thermoplastic filament be supplied on a

reel with a specified filament diameter between 1.75-3mm thick(Hiemenz, 2014). One reel holds

about 2.2 lbs. of material enough to produce approximately 400 chess pieces. The filament (Fig.

3.1) is loaded into the equipment and then a gear pulls the filament to the print head and then

through the extruder nozzle (Fig. 3.2). Depending on the filament material the nozzle temperature

is between 215-230 oC. The nozzle orifice comes in diameters between .4mm-1mm. A smaller

orifice size equates to finer detail but slower build speeds while the larger sizes will produce faster

builds and thicker wall widths. In some equipment there might be two print heads so that you can

print two different colors and materials at the same time. The nozzle then extrudes the heated

plastic onto the printing surface commonly referred to the print bed (Fig. 3.3). For ABS and some

other plastics the print bed needs to be heated to optimize cooling rate and reduce shrinkage. The

print bed may also contain support structures that might be needed to support the 3d object. In

some cases this is also printed and is shown in Fig. 3.5 (Makerbot, 2014)

Fig 3: Makerbot & Stratasys FDM Process (graphics modified by author)


As sequential layers (Fig. 3.4-5) are added the print bed moves down. The secret to FDM’s

technology is the movement of the print bed, print head, and material flow rate are all constantly

changing in concert. The print head is suspended on the gantry system which is composed of two

metal bars that run left to right and two

more metal bars connected front to

back. This allows the print head to

move in four directions altogether.

The print head accelerates and

decelerates as it travels across the print

bed and as the head speed changes, the

drive wheels adjust the material flow

rate. The result produces each layer

composed of a flat ribbon of material

that measures from 0.008 inch to 0.038

inch wide (0.20 mm to 0.97 mm) and

as fine as 0.005 inch high (0.13 mm).

On the highest-performance FDM

machines, part accuracy can reach as high as 0.003 inch (0.08 mm), which rivals injection molding.

However, as you can see in Figure 4 a rather rough, stepped surface is produced (He, 2014))

While different printers have different build volumes the build volume for the Makerbot is 28.5 x

15.3 x 15.5cm. While the enclosure limits the overall print area it also has some benefits. First, it

is safety mechanism preventing probing hands from getting near the moving gantry and hot

extruder. Secondly, melting plastic can lead to unwanted and possibly toxic odors so it helps to

retain the fumes so they can be properly vented to an outside environment. Thirdly, it limits any

drafts that could enter the build area depositing any airborne particle in the thermoplastic.

PLS (Polymer Laser Sinter) and DLS (Direct Laser Sinter)

PLS and DLS both use the same method to manufacture

AM components. One major difference between laser

sinter and FDM is the use of a laser and the raw materials

come in powder form. In order to fuse or sinter high

performance polymers and metals like HP3 PEK, PEEK,

PEKK, and Titanium you need a high temperature laser.

Shown in Fig. 5 is one machine used to perform AM at

high temperatures, EOSINT P800 made by EOS.

The basic process begins with loading the material of choice

into the powder bed. In order to produce a final product

with uniform high density, consistent surface quality and Fig. 5: EOSINT 800 P

Fig 4: Rough surface created by FDM (He, 2014)


accuracy the particle size distribution has to

be tightly controlled. Fig. 6 depicts the

optimum particle distribution size roughly

centered on 63 micron (Berretta, 2014).

Without adhering to the particle distribution

curve various negative side effects like poor

flow, agglomeration and undesired sintering

can occur. Incorrect particle size can cause

difficulty removing the unsintered powder

from the finished product and fogging of the

optical elements inside the system. As

depicted in Fig 7 the powder is then spread

across evenly across the build platform by a

leveling roller. A 50 W CO2 laser beam is

then focused through optical lenses and

redirected downward to the appropriate coordinates by an x-y scanning mirror. As the laser hits

the powder it fuses it together. As the process is repeated the powder bed is raised and the build

platform is lowered to allow

the roller to push more

powder over the previously

sintered layer. While the

layer thickness capability is

dependent on the powder

material and equipment type

being used typical thickness

that can be achieved is .12mm

(.005 in). Typical part

accuracy can reach +/- 0.25

mm but can be improved with

post processing. Due to the

high sintering temperatures

required (< 385 oC) and to

limit the material from

oxidizing the entire process

takes place in a sealed chamber filled with an inert gas such as

Nitrogen or Argon gas. Unlike FDM no support structures are

required and parts can be built on top of each other as the

unsintered material will help support the sintered structures

during processing. The effective build volume of the EOSINT

P800 is 700mm x 380mm x 560mm (27.6 x 15 x 22.05 in).

Once the product is completed they are removed from the build

chamber and now they need to be cleaned. The process of

removing the unsintered powder can be simple or highly

complex and is based on the product structure and end use

application. Shown in Fig 8 is the most basic process of

blowing a compressed gas of nitrogen or air on the finished

Fig. 6: Peek particle size distribution

Fig. 7: EOSINT 800 P Build Area (graphic modified by author)

Fig 8: Compressed Air Clean


product. A more complex process that might be used for medical products containing porous

structures and/or hollow cavities can consist of multi-step blowing, rinse and ultrasonic cleaning

process (EOS Alphaform, 2015).

One of the drawbacks of laser sintering systems is the ability to switch materials quickly. While

the costly raw powder can be reused after sieving the material has to be removed entirely before

introducing a different raw material.

Additional Software Capability for PLS (Polymer Laser Sinter) and DLS (Direct Laser Sinter)

One unique capability that cannot be reproduced with FDM nor with standard subtractive

technologies is the ability to use

advanced software to control the

porosity and surface finish of the

product. Prior to printing,

import the design into the

proprietary software developed

by Within Medical allows you to

do just that. A screen shot of

the software, shown in Fig 9,

allows the user to create pore

sizes between 2µ to 200µ and

create various topologies. This

capability allows for products to

be produced with less weight

and the ability to allow ingrowth

of biomaterials (Within, 2015).

CLIP (Continuous Liquid Interface Production)

Just recently, during an episode of TED Talks in March of 2015 a new type of additive

manufacturing was made public that has the potential not only to disrupt current AM technologies

but also traditional commercial molding technologies. This new technology called CLIP

(Continuous Liquid Interface Technology), a variant of SLA (Stereo lithography), overcomes a

major hurdle associated with AM technologies, speed. The technology demonstrates that solid,

complex parts can be drawn out of polymer resins at rates 20-100X faster than current technologies

with feature resolution below 100 micrometers. The CLIP technology is currently being developed

by a startup company called Carbon3D whose founders (Joseph DeSimone, Edward Samulski, and

Fig. 9: Custom pore size and Topology (Within)

http://www.designnews.com/author.asp?dfpPParams=ind_183,industry_medical,bid_27,aid_270238&dfpPParams=ind_183,industry_medical,bid_27,aid_270238&dfpPParams=ind_183,industry_medical,bid_27,aid_270238&dfpLayout=blog&dfpLayout=blog&dfpLayout=blog&doc_id=270238&image_number=5



Ermoshkialled) tailored the innovative process by which it works. Shown in Fig. 10 is the simple

underlying architecture for the basis of the CLIP technique. The objects are built above a vat of

room temperature liquid UV curable resin (DeSimone, 2015).

The basic principle to the technology is maintaining a “Dead Zone”, a thin uncured liquid layer

between the oxygen permeable window and the cured part surface. UV light triggers photo

polymerization and oxygen

inhibits it. By carefully balancing

the interaction of light and

oxygen, CLIP continuously

grows objects from a pool of

resin. The object being built is

anchored to a platform which is

slowly raised out of the resin.

Beneath the build platform, are

adjustable optics and a high

power UV light source

continually altering its image by a

Digital Light Processing (DLP)

imaging unit. DLP imaging has

been a mainstream technology in

TV sets for many years now. The image is then projected through a thin membrane with the same

properties as contact lenses. In this case the material is amorphous fluoropolymer, Teflon AF 2400

which has excellent oxygen permeability, UV transparent and chemically inert. The object is

constructed as light from the projector hardens the layers of resin and as the platform rises upwards

produces suction

forces that constantly

renew reactive liquid

resin. Compared to

other additive

technologies the

projector draws entire

layers at once (hence

the speed). Figure 11

shows a comparison

at 130X between

CLIP technology

versus other 3D

printed process. Clearly, the latter does not exhibit the same “stepped” surface nor does it suffer

a deficit of mechanical properties in-between cured layers like that of FDM/PLS/DLS technologies

(Tumbleston, 2015).

Fig. 10: CLIP Setup

Fig. 11: 130x Electron microscope surface finish image.

100 Micron

http://3dprint.com/wp-content/uploads/2015/03/dia.png

http://3dprint.com/wp-content/uploads/2015/03/dia.png


While a video would

demonstrate the

technology better, Fig.

12 depicts a set of

concentric geodesic

structures with linkages

between each one. This

structure is made from

TriMethylolPropane

TriAcrylate or TMPTA

a room temperature UV

curable resin. The print

speed of this structure,

the size of a lacrosse

ball, took 6.5 minutes to

produce. A graphical

representation of print

speed versus other

technologies is shown

in figure 13. The print

speed for CLIP is

limited only by the resin cure rates and viscosity(Tumblston, 2015). Print rates up to 500mm/hr.

have been

demonstrated leaving

competitive

technologies in the

dust. Preliminary

work shows that the

CLIP process is

compatible of

producing parts from

soft elastic materials

and biological

materials however

challenges would arise

using metal since it

would need to be

heated to its melting

point.

Technology Comparison Overview

Listed in Table 1 below is a review of the technologies presented herein. When choosing the

appropriate technology one must be careful to consider the application and the intended volume.

Is the component going to be under load or does it just need to look aesthetically pleasing? Do you

Fig. 13: CLIP vs. Other technologies

Fig. 12: Geodesic structure being pulled from resin


need to manufacture just one or thousands? Does the material need to be biocompatible or hold up

to sterilization requirements? These are just some of the questions one should ask themselves.

Material Properties

Table 2 indicates some of the important properties of a select few of the readily available materials

that can be used in additive printing. Also, while not every variant is biocompatible within each

class each major category does have a version that is biocompatible. PLA and ABS are the most

economically priced materials while Titanium is the upper echelon of materials. Compared to

ABS, PLA demonstrates much less part warping and curling and, for this reason, can be

successfully printed without a heated build plate. Details such as sharp corners and edges print

well with PLA and generally retain a glossier look and feel.

While PLA is the easiest to print it does not have retain the mechanical properties to be used for

structural or mechanically functionality but is quite suitable for molds. ABS, PEEK and of course

Titanium can all be used in demanding mechanical and high temperature applications. Two unique

properties of ABS is its solubility in Acetone which allows one to readily smooth the surface or

weld components together, also ABS does not have a true melting point since it is amorphous.

FDM

• Capital: $1-50K

• Material: $$

• Speed: Medium

• Strength: Med.

• Finish: Rough

• Resolution: Low

• May require support structures

• Poor Z-axis strength

• Easily create complex shapes

PLS/DLS

• Capital: $.2-1.5M

• Material: $$$

• Speed: Slow

• Strength: High

• Finish: Rough

• Resolution: High

• Can customize porosity

• Create moving & interlocked geometries

• High temp. applications above 180 OC

CLIP

• Capital: <$5000

• Material: $

• Speed: Fast

• Strength: High

• Finish: Smooth


• New Technology risk

Molding &

Machining

• Capital: $1.5M

• Material: $

• Speed: Fast

• Strength: High

• Finish: Smooth


• Requires cutting oils or mold release agents

• Requires mold draft angles

Table 1: AM Technology Review (created by author)


MATERIALS

PROPERTIES PLA ABS PEEK Titanium (Ti6AlV4)

Glass Transition Temp. (oC) 60-65 105 157 n/a

Melt Temp. (oC) 150-160 n/a 372 899

Young's Mod. (GPa) 3.5 1.7-2.8 4.25 110

Tensile Strength (MPa) 50 33-110 90 1150

Hardness 77-81 R 110 85 45.9 C

Biocompatibility Yes Yes Yes Yes

Cost $/kg 48 48 70 250-400

One of the major differences in PLS/DLS vs. traditional injection/compression molding

technologies is the variation in mechanical/dimensional

properties of the part based on part orientation and

location within the build platform. A study

demonstrating these differences was conducted using the

EOSINT 800P and EOS HP3 PEK material (Ghita,

2014). HP3 PEK is a recent formulation developed by

EOS specifically for PLS and has a higher melting point

then traditional PEEK. The study conducted destructive

tensile testing of a 162 industry standard (ISO 527-2-1A)

dog bone shaped test coupons laser sintered at various

locations/directions on the test bed. The shape and

orientation of the dog bone coupons can be seen in Figure

14. X (Ghita, 2014). The resultant Ultimate Tensile

Strength (UTS) based on print direction is shown plotted

against traditional Injection Molding (IM) in Fig. 15 (Ghita, 2014). Furthermore, Fig. 16 (Ghita,

2014) depicts the percent shrinkage map

based on Z-direction of the build samples

that occurred in the build chamber of the

P800. The plot indicates the maximum

shrinkage occurs towards the center of the

chamber where the light colors are with the

least shrinkage occurring around the edges

where the darker colors are. You will

notice that top right of the plot does not

indicate any color as test coupons were not

built in this quadrant. During this

evaluation it is expected the build chamber

to have symmetry in terms of thermal

gradients and that 3 quadrants would be adequately predict the build chamber profile. During

Fig. 15: Tensile strength direction dependent Plot

Fig 14: Test Coupon Distribution

Table 2: Common AM Material Properties (created by author)


cooling, polymer shrinkage

occurs in both injection

molding and additive

manufacturing and can be

countered by predictive

software modeling and

increasing the geometry size

before manufacturing.

Shrinkage can only be

corrected only if it is

predictably consist across the

build platform and lot/batch

processed. The findings of the

paper demonstrate while there

is some improvements to be

made with respect to materials

and methods that overall High Temperature Laser Sintering can rival the expected quality of

traditional manufacturing means (Ghita, 2014).

Slowly, over the past ten years new AM

materials for medical applications are

displacing traditional materials such as

titanium, stainless steel and ceramics in a

variety of components spinal fusion, hip

replacement, craniofacial and dental implants.

Oxford Performance Materials (OPM), a

company based in Oxford, Connecticut has

been working on a new version of a polyketone

for medical applications they are calling PEKK,

or PolyEtherKetoneKetone. OPM states that

“PEKK has a density and stiffness similar to

bone, is lighter than traditional implant

materials such as titanium and stainless steel,

and is chemically inert.” Unlike the aforementioned metals, PEKK is radiolucent, meaning it does

not interfere with diagnostic imaging equipment. According to OPM, other advantages are said to

be a lower coefficient of thermal expansion and superior dimensional stability (Hanson, 2013).

Some of PEKK’s properties are shown in Table 3 (OXPEKK®-IG datasheet, Author 2015).

The importance of utilizing biocompatible materials that contain biomimic constituents like

stiffness and porosity can have an active effect on the attachment, size, migration, proliferation

and function of both endogenous and exogenous cells (Atala, 2014). OPM has successfully shown

biocompatibility testing for 52-weeks per ISO 10993 for OXPEKK®-IG. Perhaps its most

exciting attribute is bone’s affinity to the material. “PEKK as a molecule is unique,” says DeFelice,

President and CEO of OPM (EOS OPM, 2014) “Based on research studies, it is osteoconductive,

Fig. 16: EOSINT 800P shrinkage map of HP3 PEK

Table 3: PEKK Properties (created by author)


meaning bone cells will grow onto it, unlike some the other materials. Without this beneficial

property it is likely that the surrounding tissue will pull away overtime and you will have to rely

on screws to hold it in place. Depicted in Figure 17 is a before and after SEM micrograph taken

3000X resolution of PEKK material deposited with human osteoblast plated at 50,000 cells/cc.

After 10 days of incubation it is apparent that the focal adhesions (FA) are responsible for cell

attachments and are instrumental in translating cell phenotype. The ability of PEKK to reduce

motility demonstrates a value inherent to the material that accentuates osteoblast transduction and

appears to retain cell performance (Ganey, 2011). However, all these new materials will have to

demonstrate their ability to be sterilized and meet the majority of Biocompatibility testing per ISO

10993-1 shown in Table 4. (ISO 10993-1, 2009 ed.)

Fig. 17: PeKK Osteoconductive testing (modified by author)

After Before

Table 4: Biocompatibility Test Matrix (modified by author)


Cranial Implants

Estimates are that 300-500 U.S. patients are in need

of skull bone replacements every month (Felix,

2013). The most common need for cranial implants

are caused by severe trauma to the head from

falling, motorcycle accidents but are also caused by

infection, tumors and rare bone ailments that cause

the skull to grow too thick. The need to remedy the

situation is dire to combat the effects of pressure on

the brain caused by hematoma (Figure 18), which

left untreated decreases blood flow and oxygen to

the brain causing dizziness, nausea, blurred vision,

headache, and left untreated memory loss and

cognitive thinking.

To better comprehend the impact 3D printed implants will have in the 21st century we need to

understand the pitfalls traditional methods of cranial implant manufacturing/installation encounter.

Cranial defects have traditionally been repaired with metal plates or bone cement. During the

mixing and hardening of the bone cement (polymethyl-methacrylate) in the operating theatre it

produces toxic gases. Furthermore, the disadvantage of the conventional method is a perfect fit

Fig. 18: Representation of a hematoma


and curvature can be

difficult to obtain in large

reconstructions, especially

when the defect involves the

craniofacial junction

(Sundseth, 2013). Even with

recent material advances like

Medpor, depicted in Fig. 19

user met needs are still

missing. While Medpor, a

porous polyethylene

material is touted to have

excellent biomaterial

attributes, with a structure

that allows rapid fibro-

vascular growth and

incorporation of the patient’s

own tissue, it does not come

in “one size fits” but rather

20 different sizes! The

author has taken the liberty

to highlight in blue the so

called “Features and

Benefits” of the material, but

with the onset of 3D printing

makes them detriments. The

Medpor literature suggests

that while in the operating

room, with your skull open,

the surgeon will be cutting,

bending and forming the

material to try and

reconstruct the complex

curvature of the skull. Not

only does this process

consume expensive

operating room time but

leaves the patient’s brain

exposed to debris from

cutting Medpor, air borne

pathogens and increased risk

of contamination of the implant due to handling. All of the aforementioned factors can exacerbate

the foreign immune response.

MEDPOR Cranial The MEDPOR Cranial / Neurosurgical portfolio contains more than 20 different implant size and shape options for use in a wide variety of surgical procedures. Many are available with or without embedded titanium mesh for additional cranioplasty solutions. Features & Benefits

Proven material has a long history of use for cranial repair

Interconnected, omni-directional pore structure allows for native tissue in-growth for enhanced biocompatibility

MEDPOR Implants may be contoured with a scalpel, burr, or scissors in the sterile field to fit the individual needs of each patient.

TITAN Implants may be cut with surgical scissors. MEDPOR TITAN™ Implants have titanium mesh

embedded within the porous structure of the MEDPOR biomaterial. This may minimize sharp edges normally associated with cutting titanium mesh implants.

MEDPOR TITAN™ Sheet implants substantially hold their shape when bent, due to the titanium mesh embedded within the porous structure of the MEDPOR biomaterial.

Fig. 19: Courtesy of Stryker (Modified by Author)


Just over 2 years ago, in Feb. of 2013

the FDA approved the first 510(k)

clearance for the use of a 3D printed

cranial implant to Oxford Performance

Materials (OPM), (Hanson, 2013). The

OsteoFab Patient Specific Cranial

Device (OPSCD) was PLS

manufactured out of their proprietary

formulation of PEKK, which they call

OXPEKK. OPM began to formulate the

AM version of PEKK back in 2006.

Within two weeks of the announcement

the OPSCD was implanted in a patient

who underwent a 3D CT-reconstruction

of a 6 inch cranial defect caused by

acute subdural hematoma similar to the

one depicted in Fig. 20(Felix, 2013). The engineers at OPM then use the scan to construct the

cranial implant shown in Fig 21. Once the design is reviewed by the surgeon the implant is printed,

cleaned, and then undergoes rigorous mechanical and analytical testing. OPM employs a light

scanner to run 100% line of site metrology to confirm dimensional accuracy. It is then sterilized

and shipped to the hospital all in less 2 weeks (EOS OPM, 2014).

Shortly thereafter, in May 2014 Alphaform AG and Novax DMA designed, constructed and

successfully implanted a 95% porous Titanium cranial implant in a patient in Argentina (EOS

Alphaform, 2015). Although they chose a safe, reliable biocompatible material, they would have

to contend with how to integrate biological

function with heat dissipation. With any

medical device or procedure one should always

identify what the patients’ needs are. In this

case, one of them would be the ability to

withstand periods of time in the sun without the

chance of conducting unwanted heat through

the metal to the cerebral tissue. In order to

facilitate implant fixation and optimize heat

flow it is important to have tissue and fluid

permeability within the implant. After all, the

brain is continually bathed in 100-160 ml of

Cerebrospinal Fluid (CSF). Since only a

porous structure would produce the required

characteristics the engineers at Alphaform

utilized the Within software to create a lattice-structured implant with integrated screw-in fixings.

The fine geometry of the implant is shown in Fig. 22. The dimensions of the pores themselves are

approximately 1mm and amazingly the cell-links are approx. 0.2mm thick. Although this implant

only took a few hours to be laser sintered, the fact it has numerous small hollow spaces meant

Fig. 20: CT scan of 6 inch cranial defect

Fig. 21: PEKK laser sintered implant

http://www.ncbi.nlm.nih.gov/core/lw/2.0/html/tileshop_pmc/tileshop_pmc_inline.html?title=Click on image to zoom&p=PMC3&id=3616596_jcnsd-5-2013-019f1.jpg

http://www.ncbi.nlm.nih.gov/core/lw/2.0/html/tileshop_pmc/tileshop_pmc_inline.html?title=Click on image to zoom&p=PMC3&id=3616596_jcnsd-5-2013-019f1.jpg


developing a complex cleaning process to remove all of the unsintered material that is left behind.

It took Alphaform 6 months

to develop a multi-step

process of abrasive and

mechanical cleaning,

rinsing, and ultrasound in

order to arrive at the

medically required level of

purity. Christoph Erhardt,

Director of Additive

Manufacturing at Alphaform

AG. “We are proud of this

implant, not only because of

the precise realization of the

form, but above all because

we were able to optimize the

cleaning processes.”

Overall, it took 3 weeks to deliver the product to the hospital, only 2.5 days to design and print it,

one week for transportation, and the remaining time logistics (EOS Alphaform, 2015).

Hip Implant Applications

A titanium trabecular cup is another example of a complex small implant structure that can be

designed and built with a

customized, optimized

structure and surface texture

quickly and cost-effectively

with additive manufacturing.

Shown in figure 23 is an

acetabular cup that is the part

of a hip implant. It is adhered

to the pelvis, providing a

solid fixation for the ball of

the joint replacement to sit in.

For implant stability

traditional implant designs

used bone cement and

multiple screw fixations, however superior mechanical strength can be achieved by inducing

osseointegration. The trabecular lattice featured in the software is designed to encourage this. A

wide distribution of pore sizes is thought to encourage a more secure fit, with larger pores favorable

for stress transfer and smaller pores beneficial for initial fixation. A stable initial fixation of an

implant secures the implant and allows for bony ingrowth (Within, 2014).

Fig. 22: Titanium Cranial Implant (modified by author)

Fig. 23: Titanium acetabular cup, improved porosity




External Prosthesis

It is an unfortunate fact of life that various forms of cancer, fire, frost bite, and warfare all

contribute to a medical need to replace soft tissue with prostheses for the nose, ears and eyes.

While this is a perfect application for low volume, personalized replacements the technology to

3D print soft flexible materials such as silicone has not advanced enough to be suitable for medical

prosthetics. In recent years it has been shown that AM manufactured prostheses are still quite

costly to produce and exhibit poor mechanical properties and untested biological responses (Bibb,

2012). However, the cost and time to completely fabricate and install a new customized prosthesis

can still be significantly reduced today. He, demonstrates that in Fig. 24 that rather than employing

AM manufacturing processes directly to produce the prosthesis but rather using it to produce a

casting mold for which liquid medical grade silicone can be poured/cured into. Using this process

one can reduce the current 5-7 hospital visits over 5-10 week period at the cost of $4,000

(prosthesis only) to only one or two visits to hospital with a cost of $30.00 for the prosthesis. The

process would start as previously described for the FDM process where by a scan is produced of

the needed part. Hu, however, does not discuss how this could be done if the feature is not

available for scanning. For example, if someone needed a new nose how would they scan it?

However, if it was an ear, and they had another one it could be scanned, and a mirror image could

then be replicated. Once the scan is completed and rendered he used a low cost bench top FDM

printer such as RepRap or Fab@home to print the mold out of ABS. The benefit of using ABS is

the ability to refine the surface from rough to smooth by using Acetone. Acetone is heated above

its boiling point of 56.5oC to 80oC creating acetone vapor that fairly uniformly and slowing

Fig. 24: Low cost fabrication method for ear prostheses


dissolves the ABS. Since it is done in an enclosed chamber once the vapor pressure is reached,

the acetone will no

longer evaporate.

This setup is

demonstrated in

figure 25 (He, 2013).

Several tests were

performed to

determine the length

of time the ABS

needed to be exposed

to the acetone vapor

in order to receive

uniform and

acceptable roughness.

According to Figure

26, roughly 12

minutes indicates that

the ridges in the

material was

successfully removed

but at roughly 15

minutes the ABS

begins to deteriorate.

After having determined the best polish time a 2-component resin of medical grade silicone was

mixed to a 1:1 ratio by weight. The mixture was then stirred for 2 minutes. Prior to casting it is

important to put into a vacuum chamber to remove any bubbles. Any bubbles left in the material

before casting could leave to internal defects/voids. Depending on the material being cast and the

mold it is fairly common to spray a mold release agent prior to pouring. After the silicone has

been fully cured the perfectly replicated ear can is removed from the mold and shown in Figure

27.

Fig. 25: Vaporized Acetone on ABS

Fig. 26: Roughness vs. Polish time for ABS Fig. 27: Silicone ear cast from ABS mold

http://www.bing.com/search?q=deteriorate&FORM=AWRE


Summary

Within the coming years we will begin to see

the cost of 3D printing continue to come

down enabling more applications within the

medical industry. While applications here

focused on non-bio based materials we will

begin to see additive technologies mature that

will evolve from current bench top practice to

commercial applications for printing biology

based tissues and replacement organs.

Depicted in Figure 28, Ted Talks, recently

highlighted the ability to print a kidney. The

market need is there as there is not enough

replacement organs to fill the demand. In

order to help accelerate the ability of these

products on the market, we will need to

accelerate the medical approval process.

Fig. 28: 3D Printed Kidney


References:

Atala, A. 2011. 3D Printed Kidney, Ted Talks, March

Atala, A; Murphy S., 2014 “3D Bioprinting of Tissues and Organs”, Nature biotechnology, Vol.

32, number 8, pgs. 773-785, August.

Berretta, S., Ghita, O., Evans, K.E., 2014, “Morphology of polymeric powders in Laser Sintering

(LS), From Polyamide to new PEEK powders”, European Polymer Journal, Volume 59,

October, Pages 218-229

DeSimone, J., 2014 “What if 3D printing was a 100X faster”. Ted Talks,

http://www.ted.com/talks/joe_desimone_what_if_3d_printing_was_25x_faster

Eggbeer, D., Bibb, R., Evans, P. & Ji, L. Evaluation of direct and indirect additive manufacture

of maxillofacial prostheses. P. I. Mech. Eng. H. 226, 718-728 (2012).

EOS and OPM case study, 2014. July. http://www.eos.info/case_studies/customized_3D-

printed_polymeric_cranial_implants

EOS and Alphaform case study, 2015, Jan. http://www.eos.info/case_studies/cranial-implants-

produced-with-additive-manufacturing

Felix, 2013. “3D-Printed Polymer Skull Implant Used For First Time in US”. Science World

Report. http://www.scienceworldreport.com/articles/5441/20130307/3d-printed-polymer-skull-

implant-used-first-time.htm

Ganey, T., 2011. “Cell proliferation and vitality determination of osteoblasts on different

materials and surface characteristics; Interpretation of laboratory data”, Confidential OPM

Report- March.

Ghita, O., E. James, R. Davies, S. Berretta, et al., 2014 “High Temperature Laser Sintering (HT-

LS): An investigation into mechanical properties and shrinkage characteristics of Poly (Ether

Ketone) (PEK) structures”, Materials & Design, Volume 61, September, Pages 124-132

Hahn, B.D., Park, D. S., Choi, J. J., et al, 2013. “Osteoconductive hydroxyapatite coated PEEK

for spinal fusion surgery”. Applied Surface Science. Volume 283. 1. October, Pages 6-11

Hanson, K., 2013. “First 510(k) Clearance for Customized 3D-Printed Polymeric Cranial

Implants”, Medical Design Technology, November.

He, Y., Xue, G. & Fu, J. 2014. “Fabrication of low cost soft tissue prostheses with a desktop 3D

printer”. Nature. Science Rep. 4. 6973. DOI:10.1038/srep06973.

http://www.eos.info/case_studies/cranial-implants-produced-with-additive-manufacturing

http://www.eos.info/case_studies/cranial-implants-produced-with-additive-manufacturing

http://www.scienceworldreport.com/articles/5441/20130307/3d-printed-polymer-skull-implant-used-first-time.htm

http://www.scienceworldreport.com/articles/5441/20130307/3d-printed-polymer-skull-implant-used-first-time.htm


References continued

Hiemenz, J. 2014, “3D Printing with FDM, How it Works” white paper Stratasys Inc.

http://www.stratasys.com/~/media/main/files/white%20papers/ssys-wp-3dp-howitworks-09-

11.aspx

Itero. 2014, website http://www.itero.com/download/M20240RevA_122012.pdf

Leaversuch, R., 2001 “Demand Surge Tightens PEEK Supply”. Plastics Technology. July.

Makerbot website, 2014. http://www.makerbot.com/uses/for-professionals

Stryker website, 2014, http://www.stryker.com/en-

us/products/Craniomaxillofacial/MEDPOR/MEDPORCranialNeurosurgicalImplants/index.htm#

Sundseth, J., & Berg-Johnsen, J., 2013. Prefabricated Patient-Matched Cranial Implants for

Reconstruction of Large Skull Defects. Journal of Central Nervous System Disease, 5, 19–24

Tumbleston, J., et al., 2015 “Continuous liquid interface production of 3D objects”. Science 347.

1349.

Within Technology, 2014 “Osseointegration by Design”

http://www.stratasys.com/~/media/main/files/white%20papers/ssys-wp-3dp-howitworks-09-

http://www.stryker.com/en-us/products/Craniomaxillofacial/MEDPOR/MEDPORCranialNeurosurgicalImplants/index.htm

http://www.stryker.com/en-us/products/Craniomaxillofacial/MEDPOR/MEDPORCranialNeurosurgicalImplants/index.htm

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