Rapid Pro to Typing Technology

RAPID PROTOTYPING TECHNOLOGY –

Conjoined Twins Separation – A Model Surgery Using RPT,

“Turning Virtual Reality into Physical Reality”

ABSTRACT

In the manufacturing arena, productivity is achieved by guiding a product from

concept to market quickly and inexpensively. Rapid prototyping technology aids this

process. This physical model conveys more complete information about the product

earlier in the development cycle. The turnaround time for a typical rapid prototype part

can take a few days. Conventional prototyping may take weeks or even months,

depending on the method used.

RPT does not-and will not-replace completely conventional technologies such NC

and high-speed milling, or even hand-made parts. Rather, one should regard RPT as one

more option in the toolkit for manufacturing parts.

It is assumed, evidently, that the part can be manufactured by either technology

such that the material and tolerance requirements are met. The axis have no values; these

are company dependent. RPT offers clear advantages when more than one copy of a

complex part must be made.

Concerning material requirements, it is clear that when using milling one can

always obtain directly a part with the desired mechanical properties. This is usually the

choice when manufacturing production toolings. But, as mentioned earlier, using a chain

of processes that includes a RPT part, it is many times possible to obtain, indirectly, the

same results in a shorter period of time.

What Is Rapid Prototyping?

The past decade has witnessed the emergence of new manufacturing technologies that build parts on a layer-by-layer basis. Using these technologies, manufacturing time for parts of virtually any complexity is measured in hours instead of days, weeks, or months; in other words, it is rapid.

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Since then, Rapid Prototyping Technologies (RPT) have taken enormous strides. Nowadays, there are over 30 processes some of which are commercial, while others are under development in research laboratories.

The accuracy has improved significantly, and the choice of materials is relatively large, to the extent that the term prototype is becoming misleading; the parts are more and more frequently being used for functional testing or to derive tools for pre-production testing. It is very likely that a new term, or one of the numerous other expressions that are floating around, will replace it in the future.

Data transfer to RPT

As mentioned earlier, speed is one of the most distinguishing features of RPT when

compared to conventional methods. In fact, in many cases, the use of RPT can only be

justified if the part can be obtained quickly. Quite often, though, the limiting factor is the

time spent preparing the data. Once the data is correct, manufacturing time is known and

relatively fast. Figure 5 sketches a typical scenario.

The designer delivers the model to the manufacturer using surface mail or by

electronic means. The model will usually be represented in some neutral format,

e.g.VDAFS [27], IGES [25], or STL [1], or in some native format when both have access

to the same CAD system. The model is then verified for correctness and converted to a

suitable form if possible. Figure 6 depicts the state transitions of interest undertaken by a

model from the moment it is sent by the designer until it is manufactured.

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All the state transitions in the diagram are possible. For instance, one can take slices from medical imaging systems and interpolate intermediary slices that are subsequently used in a LMT process. In this case, a 3D model is never evaluated (path b+b).

On the other hand, some processes cannot effectively handle sliced models-or ``D'' models-therefore, a faceted model is created and then sliced again (path b+a,a+b). The reason is that in some cases it is important to be able to position the model arbitrarily in the workspace of the machine, and this cannot be done with sliced models.

The typical scenario is shown in Figure 7.

RPT In Manufacturing

During the development process, one is frequently faced with the choice of either

extending the development time or increasing the resources in order to meet the

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deadlines. Under these circumstances, time to market has been identified as a key factor

inprofitability; it is the development time and not the cost that is critical for the result.

RPT allows a physical model to be available as soon as a 3D CAD model is ready. The physical model is a perfect communication tool; if a picture is worth a thousand words, then a physical model is worth a thousand pictures.

In addition, parts produced via RPT are more and more frequently being used for functional tests and for obtaining tools that can be used for pre-series production tests. In this way, errors can be found at an earlier stage when changes are not so costly. Requirements can be refined and better understood leading to better products that meet the market demands. It has been estimated that using RPT effectively, the development time for toolings can be reduced by half.

Another important aspect is the cost of introducing changes in the design of a

product. In this respect, development of a physical product does not differ from software

development: the cost of introducing changes increases significantly as one reaches the

final stages of development.

ENGINEERING APPLICATIONS

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RPT in Medical Applications

Applying RPT in the medicine is a new and exciting field. Many applications have

become possible due to the convergence of three distinct technologies, namely Medical

Imaging, Computer Graphics and CAD, and RPT.

Computer-Assisted Tomography (CT) and Magnetic Resonance Imaging (MRI)

provide high resolution images of internal structures of the human body, e.g.bone

structures and organs. Once these images have been processed by suitable software tools,

it is possible to transfer the result to a RP process and obtain a physical part, called a

medical model. Figure 13 depicts this process.

Together, these technologies provide doctors and surgeons with a new tool-

physical models of human internal structures-to better plan and prepare complex

surgeries. If the surgeries can be carried out more successfully, less costs associated to

post-operative treatment are expected, in addition to reduced risks, reduced patient

suffering, and improvements in the quality of the results.

Another recent application has been the manufacturing of a human chromosome.

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Conjoined Twins' Separation a Model Surgery

Operation takes on-quarter the time, using rapid prototyping

Craniopagus twins — those who are fused at the tops of their heads —

are one of the rarest types of conjoined twins, occurring an estimated 1

in 2.5 million births.

The good news was that the twins’ brains were separate and complete,

with separated arteries and a dividing membrane. The veins draining

the blood, however, were interwoven and fed into each others’ circulatory system.

Separating conjoined twins is a highly complicated procedure, but surgeons determined

that an operation was possible. The members in the surgical team all had different

visualization needs in order to plan their role in the surgery. The plastic surgeons were

most interested in the structure of the skull, while the neurosurgeons were particularly

concerned with the arrangement of the blood vessels.

Compiling data

Because the blood vessels were

crisscrossed, tracking them

using standard, two-

dimensional x-rays would be

impossible. They suggested the

use of 3D rapid prototyping to

help the plastic surgeons

practice how to separate the

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girls’ brain skulls, reroute the blood supply and plan skin grafting to cover the

separated brains.

The UCLA team had to supply BMI with three CT scans at different angles — it

wasn’t possible to arrange the twins for one scan. The company registered and

combined the scans of the twins’ brains and the intersection of the two skulls into

a single, 3D model. BMI then used Materialise’s MIMICs software to merge the

scans and process the data, including a biomodel of the skulls that included the

maze of blood vessels.

Building an image

The QuadraTempo system builds parts by

selectively jetting tiny droplets of acrylic

photopolymer creating layers and then curing

layer-by-layer, using UV light. A second, gel-like

photopolymer material is used for support and is

wiped off or removed by water jets.

BMI and InterPRO produced three biomodels — one of each of the twins' skulls,

which could be studied separately or combined to provide the surgical team with

a replica of the conjoined anatomy, and the third showing the region where the

twins were joined, enabling the surgeons to easily see the complex architecture of

arteries and veins. Dr. Clearihue adjusted each biomodel, orienting them to

correct their position and connecting them so that the models could be

manipulated in the operating room at the time

of surgery.

The biomodels enabled them to make

important discoveries about the twins'

anatomy which were not apparent from X-

rays, or CT and MRI scans.

Besides defining internal structures, models from BMI have been applied in the

creation of implants.

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The continuing progression, says BMI president Crispin Weinberg, in the quality

of medical imaging “is providing us with better views of soft tissues.

This means that, in future, not only will we be rendering models of bone, but more

preparations for procedures involving blood vessels, nerves and internal organs.”

"We're so pleased to see that our technology was chosen for this humane

undertaking”

"We've had doctors reporting that we've saved them as much as 20 to 30

percent of their operating room time”. "The models have also allowed them to

perform multiple surgeries that they wouldn't have otherwise felt comfortable

doing."

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Biomedical Modeling, Inc. works with surgeons and physicians worldwide to

analyze medical imaging data and produce accurate 3D Biomodels for surgical planning

and prosthesis fabrication. By offering superior service and the ability to work with

different RP service bureaus, Biomedical Modeling provides biomodels to meet a broad

range of medical needs. They have successfully produced biomodels from CT, MRI, and

ultrasound to visualize a variety of hard and soft tissue structures.

“We have the Technology” – The Story of Ganga & Jamuna

To help the surgeons visualize and understand the interlocking structures of the twins’ brain tissues, Nanyang Polytechnic was called upon to produce separate models of the twins' skulls, brains, skin and blood vessels.

To deliver the “goods,” the Polytechnic had to draw on its capabilities in Computer-Aided Design and Rapid-Prototyping technologies as well as its experience in similar projects for medical applications.

Working closely together, our specialist engineers and the SGH surgeons meticulously converted data from CT and MRI scans into 3-dimensional CAD images. These images were then transferred to the Rapid-Prototyping equipment to produce the life-size models using a process called “laser-sintering”.

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“Although we have done more than 30 such models, none was as complex. For this

project, we had to deal with a lot of soft tissue. The biggest challenge was in identifying

which part of the interlocked brains belonged to which twin. Creating the model of the

blood vessels was equally challenging because the arteries and veins were very intricate

and fine,” remarked by the Manager of Rapid Prototyping Centre.

Advantages of Biomedical Models

Accurate three dimensional solid models have proven to be invaluable tools in all stages

of the successful planning and completion of complex surgeries. Moreover, their utility in

producing perfectly matched prosthetic implants is unsurpassed by any other available

technology.

Doctors and surgeons who wish to examine possible operating scenarios prior to

entering the operating room, as well as enjoy the opportunity to work with a physical

model which illuminates solutions to complications encountered during the surgical

process, may now do so. In addition, the use of a physical model as a precursor to surgery

has shown significant reductions in operating time, which translate to reduced trauma to

the patient as well as cost savings to the patient, hospital and insurance carrier.

Scientists and engineers wishing to explore "what if" scenarios in the areas of

medicine and biology can now create physical models for testing and experimentation of

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subjects under study. This provides the opportunity to gather scientific information using

non-evasive techniques. This information has the benefit of retaining the specific

properties of the subject which are lost when using generic models.

Fabrication of Biomedical Models:

At BMI we process and interpolate C.T. and M.R.I. images, in close cooperation

with the radiologist and surgical team to fabricate extremely accurate 3D physical

models. We use these highly detailed cross-sectional images to create a series of very thin

layers of resins or other specially engineered materials. Their accuracy is determined by

the sharpness and contrast of the original images. Our processes allow us to fabricate

solid models to greater tolerances possible with the most accurate C.T. or M.R.I.

equipment. For the final product, we utilize materials and coloring deemed most

appropriate for the procedure.

Visualization and Assessment:

 M.R.I. and C.T. scans are currently the diagnostic basis for most complex surgeries. At

BMI we employ those identical M.R.I. and C.T. files to produce highly accurate three

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dimensional solid models to any scale desired. These models are produced to tolerances

that exceed those of the original M.R.I. and C.T. files. Leading surgeons who employ

accurate biomodels as assessment and communication tools find them to be invaluable

aids, increasing knowledge and confidence.

Conclusion

It is impossible to cover all aspects of these relatively new manufacturing

processes without being brief at times. More important, though, is their effective

introduction in the current working practices of companies.

It is clear that these technologies, when applied correctly, can bring benefits in the

form of better products in shorter lead times, and at reduced costs.

Many researchers feel that within a few years, there will be a range of products

from 1 million to 10 million using this technology.

Thus this new technology has a ample scope for future research and development,

thus making it truly a rapid prototyping technology.

References:

www.wohlers.com

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www.biomodel.com

www.howstuffworks.com

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