PLASTICS IN PROSTHETICS

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COV ER STORY

Its its amazing to have this movement, just gets me more excited about now, about the future

says Iraq war veteran Sergeant Juan Arredondo. One of the first recipients of the bionic hand

with movements controlled by the brain (i-LIMB), he compares the innovation to the bionics in the

films Star Wars and Terminator, what with its ability to deal with moving and stationary objects

with comparable ease. These smart prosthetics have become a divine intervention for numerous

victims of amputation, and plastics has enabled these devices in becoming a feasible option for the

common man. Sundeep Nadkarni discovers that for giving a new lease of life to people who have

lost hope without a limb, the word is not only prosthetics but also plastics. Read on

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Modern Plastics & Polymers April 200942


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April 2009 Modern Plastics & Polymers 43

in an excerpt from his book

Wings of Fire, the ace scientist

and past President of India

Dr APJ Abdul Kalam, quotes

the following incident as one of the

four milestones of his illustrious

career. In his own words, One day anorthopaedic surgeon from the Nizam

Institute of Medical Sciences visited my

laboratory. He lifted the material and

found it so light that he took me to his

hospital and showed me his patients.

There were these little girls and boys

with heavy metallic calipers weighing

over 3 kg each, dragging their feet

around. He asked me to remove the

pain of his patients. In three weeks,

we made these floor reaction orthosis

300 gm calipers and took them to theorthopaedic centre. The children did

not believe their eyes. From dragging

around a 3 kg load on their legs, they

could now move around! Their parents

had tears in their eyes. That was my

fourth bliss!

Plastics redefine prostheticsThis dream of not needing to carry

the burden of 3 kg metal prosthetics was

realised with the use of plastics. Polymeric

applications have redefined flexibilities

in prosthetic movements. Material

manufacturers are on an innovation spree,

due to the supreme heights reached

by the prosthetics industry in enabling

unimaginable limb replacements.

A number of companies and

research laboratories have successfully

employed degradable & non-

degradable structure-controlled,

dendritic macromolecules to

function as a scaffold to support

in-growth of human tissue culture

cells like osteocytes, chondrocytes, or

hepatocytes. This holds promise for

the eventual development of artificial

human organs without the need for

harvesting cadaver or living donor

organs, mentions Miguel Linares,

director - Medical Engineering and

Design, Linares Medical Devices.

The company is developing a large

number of plastic-metal, plastic-

ceramic, and plastic-mineral alloy

compounds to improve upon the

materials currently used for medical

applications. Our proprietary materials

are being blended to optimise their

functional lifespan, with an emphasis

on durability. The high fidelity imitationof nature is a perfect example of the

superiority of plastics over metals for

orthopaedic applications, Linares adds.

The company specialises in the

development of polymers for the

prosthetic & orthopaedic market,

and presently has over 50 patents

emphasising the use of plastics in

these applications.

Plastics and polymers are widely

used in the fabrication of prosthetics

or artificial limbs and orthotic

supports. The technique of moulding

a lightweight, load-bearing artificial

limb comprises the steps providing

a number of carbon-fibre sheets.

The prosthetic limb socket can be

designed by taking a negative cast

and then moulding a positive cast of a

patients residual limb, known as stump.

Thermoplastics are moulded over the

positive cast to form a socket, which is

worn over the stump. Another method

of fabricating a socket is the lamination

process in which a blend of resins,

accelerators and catalysts have wide

applications.

The socket fabrication is a

significant aspect because the entire

fitment of the artificial limb depends

on how the socket has been fabricated.

For giving the artificial limb a cosmetic

appearance, it is necessary to provide

it with a covering of polyurethane

foam that is shaped to give a natural

appearance similar to a real limb.

Polyethylene foam is also widely

used as it is suitable for skin contact,

does not absorb perspiration, does

not support the growth of micro-

organisms, is tough with a soft-feel,odourless and has increased resilience.

The combination of being lightweight

and having excellent shock dissipation

makes it a good padding material for

body protection between the rigid

plastics and the skin. These are easy-

to-fabricate, economical and improve

the performance of the prosthetic &

Miguel Linaresdirector - Medical Engineering and Design, Linares Medical Devices

Blending plastics with ceramics or metals at

the molecular level allows the best properties

of both substances to be retained. Novel

plastics have been developed resulting in

materials with lightweight and elasticity,

combined with strength and durability.


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orthotic devices, states Dr V J S Vohra,

co-founder and director, Nevedac

Prosthetic Centre. Dr Vohra is also

the honorary prosthetic advisor to

Governments of Punjab, Haryana,

Chandigarh, and Himachal Pradesh.

Biocompatible and mechanicalproperties

Plastics score more than any

conventional materials in terms

of performance, economics and

compatibility. The favourable

characteristics of plastics in healthcare

include flexibility, ductility, toughness

and lightweightedness. These materials

are inert, non-toxic and biocompatible,

and hence can come in contact with

blood, tissue, etc. Transparency of

the materials is vital to monitor flow

through the tube visually as well as

electronically. Flexibility in design,

forming & jointing, ease-of-sterilisation,

and low costs are other features

that work in favour of polymers in

prosthetic devices.

A range of screws and pins made

from re-absorbable polymers has

been recently launched to replace

metal screws and pins in orthopaedic

& trauma applications in order to

avoid the traditional requirement for

explantation. A device based on the

development of a monobloc polymer

material is under development. Actinglike a shock absorber, this material has

a flexible central part and rigid, upper

& lower outer parts. The product a

prosthesis for inter-vertebral discs is

manufactured from biocompatible

polymers and has greater compression

and torsional-tensile strength.

Currently, using the results of a similar

technology, intra-ocular lenses are also

being produced, states D L Pandya,

CEO, Medical Plastics Data Service.

Material selection is driven by

performance potential and ease of

processing. With a broad range of

medical-grade resins available, there are

greater possibilities for medical OEMs to

choose from. OEMs must apply the right

material to a device design to enhance

human interaction while reducing the

development cycle and minimising

scrap or production interruptions.

Test data helps confirm the

attributes necessary to deliver the

appropriate level of design flexibility,

strength, clarity, and chemical

resistance, which are key attributes

for devices with maximum integrity.

Comparing data makes materialselection simpler. Material properties

are also matched to application

performance, secondary operations,

and regulatory guidelines. In the

case of a product redesign that leads

to material conversion, companies

should seek out materials compatible

with their existing sterilisation and

validation processes to contain costs

and expedite processing, feels Mark

J Costa, executive vice president

- Polymers Business Group, EastmanChemical Company.

Down to the boneOne of the chief advantages

associated with the use of plastic-

based orthopaedic prosthetics and

implantable hardware is the possibility

to precisely match the strength,

hardness, and elasticity of plastics with

that of the bone. This is dramatically

different from the case where metal

is in direct contact with the bone. The

ability of plastics to mimic the native

bone is important for the long-term

health of the native bone that is in

direct contact with the prosthesis or

implanted hardware.

Normal bone is a living tissue,

which is capable of remodelling

and rebuilding itself continuously

in response to the conditions it

Dr V J S Vohraco-founder and director, Nevedac Prosthetic Centre

Manufacturers of basic resins produce different

polymers to meet the needs of prosthetic

patients. Manufacturing basic resins requires the

plastic to be heated to a molten state twice, once

to make the resin and a second time to blend in

the additives that affect its final characteristics.

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Courtesy: DuPont


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is experiencing. Just like muscles

become stronger when subjected

to weightlifting, bones, too, become

denser, larger, and stronger in response

to the stress of increased loads. The

opposite occurs when bones are

not stressed, ie, they become lessdense, thinner, and weaker when

not experiencing loads. The same

effect occurs in the long thighbone

following a hip replacement. There is

a loss of 10-45 per cent of bone mass

in the bone surrounding the implant

during the first few years after total

hip replacement. This is because the

titanium implant cemented in the

centre of the femur is much harder

and stronger than the surrounding

bone. As the titanium rod is hardand strong, the surrounding bone no

longer has to work hard to support the

patients weight, and thus is unloaded.

The removal of stress from the bone

by an implant is known as stress

shielding, and leads to bone thinning

& weakness, and eventually the bone

surrounding the titanium implant

breaks down, says Linares.

Replacing natureBy matching the strength and

hardness of the implant with that of

the natural bone, the loads should

be evenly distributed between the

implant and host-bone. Plastic-

based orthopaedic hardware has an

advantage the host-bone and the

implant can be adjusted so it matches

that of the bone. Linares Medical

Devices has developed a method of

transitioning so that the articulating

surface can be made softer and more

elastic to mimic cartilage, while the

bone segment can be made with the

same hardness as the natural bone.

This helps in creating a longer lasting

and better performing product and

improving patients satisfaction.

Currently, there are only a select

few plastics that have been approved

by the FDA for permanent human

implantation. Because of the limited

choices for FDA-approved plastics,

the Linares Medical Devices team has

developed an entirely new class of

materials. These materials are blends,

or alloys, of FDA-approved plastics &

metals, or FDA-approved plastics and

ceramics. Individually, all these materials

have been approved and successfullyimplanted in humans for many years,

and hence its biocompatibility issues

have been addressed. What is new

and completely revolutionary, is

the Linares Medical Devices

proprietary method of combining

these substances in a way that

greatly enhances their biomechanical

usefulness, feels Linares.

According to him, blending

plastics with ceramics or metals at

the molecular level allows the best

properties of both substances to

be retained, while the undesirable

properties are minimised. Linares has

developed novel plastics resulting

in materials with lightweight and

elasticity, combined with strength and

durability normally found in metals

and ceramics. One major advantage

inherent to the use of plastic-based

hardware for orthopaedic surgery

applications is that it is radiolucent.

It does not interfere with radiological

imaging technologies like CT, X-rays,

or MRIs, which is extremely important

to better follow-up and evaluate the

patients after surgery, he adds.

Brawn with brainsSince decades now, scientists have

been working towards developing a

technique for interpreting brain activity

to motor output. In other words, this

D L PandyaCEO, Medical Plastics Data Service

Polymers, when combined with nano-

materials offer a range of polymer

compounds, composites & semi-finished partswith amazing functional properties, including

electrical & thermal conductivity, mechanical

properties and opto-electronic properties.

April 2009 Modern Plastics & Polymers 45

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Courtesy:SABICInnovativePlastics


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enables in deciphering the electric

patterns of the brain and converting

them into coherent thoughts. The

majority of motor functions in our

body are driven by electrical currents

originating in the brain motor cortex

and conducted through the spinalcord and peripheral nerves to the

muscles, where the electrical impulse

is converted into motion by the

contraction & retraction of

specific muscles.

For example, when the arm is bent

at the elbow joint, the bicep muscles

contract and the triceps relax. This

seemingly simple movement is the

result of the cumulative activity of

numerous brain cells in the area of the

cortex in charge of arm movement. The

neurons, following a cognitive decision

to bend the arm, generate an electric

impulse through the peripheral nerves,

causing the correct muscles to contract

or relax. Microprocessor technology

with a computer interface allows the

prosthetist and the patient to fine-tune

the adjustments to achieve maximum

performance of the artificial arm. The

i-LIMB Hand, a recent development, is

controlled by a unique, highly intuitivecontrol system that uses a traditional

two-input myoelectric (muscle signal)

to open and close the hands life-like

fingers. The modular construction of

i-LIMB means that each individually

powered finger can be quickly

removed by simply removing one

screw. This means that a prosthetist

can easily swap out fingers that require

servicing and patients can return to

their everyday lives after a short clinic

visit, states Dr Vohra.

Essentially until now, people using

the prosthetics have controlled one

motor at a time and had to think

carefully about what motor they

wanted to control and how to move it

instead of just thinking about moving

it and being able to do it. However, a

new technique called targeted muscle

reinnervation, involves taking the

nerves that remain after an arm is

amputated and connecting them toanother muscle in the body, often in

the chest. Electrodes are placed over

the chest muscles, acting as antennae.

When the person wants to move the

arm, the brain sends signals that first

contract the chest muscles, which send

an electrical signal to the prosthetic

arm, instructing it to move. This process

requires no more conscious effort

than it would for a person who has a

natural arm. The Touch Bionics i-LIMB

was developed using leading-edgemechanical engineering techniques

and is manufactured using high-

strength plastics. The result was a

next-generation prosthetic device

that is lightweight, robust and highly

appealing to both patients and

healthcare professionals.

Myoelectric controls utilise the

electrical signal generated by the

muscles in the remaining portion of the

patients limb. This signal is picked up by

electrodes that sit on the surface of the

skin. Existing users of basic myoelectric

prosthetic hands are able to quickly

adapt to the system and can master the

new functionality of the device within

minutes. For new patients, i-LIMB Hand

offers a prosthetic solution that has

never before been available, claims

Dr Vohra.

The nano effect in prostheticsThe use of both synthetic as well

as natural materials in medical and

pharmaceutical applications has been

successful due to the availability of a

wide variety of user-specific materials

at affordable prices. However, the

integration of nanotechnology into this

application has resulted in possibilities

beyond the conventional.

Polymers the wonder material

of the century when combined

with nano-materials offer a range

Mark J CostaEVP - Polymers Business Group, Eastman Chemical Company

Material properties are also matched to

application performance, secondary operations,

and regulatory guidelines. Companies should

seek out materials compatible with their

existing sterilisation & validation processes to

contain costs and expedite processing.

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of nanotechnology-based polymer

compounds, composites and semi-

finished parts with amazing functional

properties including electrical

conductivity, thermal conductivity,

mechanical properties and opto-

electronic properties for variousunique application opportunities in the

medical device sector. The electrically

conducting compounds made from

carbon nanotubes or fibres have tiny

nano-structures about 80,000 times

smaller in diameter than a human

hair. These materials smart Nano

conductors in a polymer matrix can

be intelligently used as smart-sensors

or commonly known as electrodes

for bio-potential detection. Most

common applications are for brainwave detection, cardio measurements,

and muscle movement detection in a

human body, states Pandya.

What prothetists seek?Thermoplastics over the past several

years have increased in popularity in

both orthotic as well as prosthetic

applications. Clarity, flexibility, rigidity,

faster processing times, localised

adjustment by the use of heat, inert

material and surface quality are just

a few of the benefits associated with

thermoplastics. Thermoplastics, like

any other type of technology, are

subject to lack of understanding

and failures do occur. This process

quite often discourages the user and

brands the material or process as an

unacceptable method of fabrication.

Through education and detailed study,

thermoplastics & resins have become

one of the most valuable technologies

available to prosthetists and orthotists.

Handling the plastic sheets is critical

for the success of the finished device. The

manufacturers of basic resins produce

different types of polymers to meet the

needs of prosthetic and orthotic patients.

Manufacturing the basic resin requires

the plastic to be heated to a molten

state twice, once to make the resin and

a second time to blend in the additives

that affect its final characteristics. The

rigidity, strength and resistance to fatigue

allow polypropylene to be typically used

in lower extremity prosthetic applications

like above-knee sockets and below-knee

sockets. The typical shrinkage of these

devices is around 1.5-2 per cent. Co-

polymers have less rigidity and can beprocessed at slightly lower temperatures.

These qualities allow copolymers to be

spot modified more easily. Co-polymers

are more commonly used in orthotics,

but are now also gaining acceptance for

above-knee and below-knee sockets,

explains Dr Vohra.

Linear low-density polyethylene finds

applications in all areas of traditional

polyethylene usages. It has better tensile

& puncture resistance, impact and tear

properties making linear low-density

polyethylene a good choice for above-

knee and below-knee socket liners,

thereby improving prosthetic fitment to

a great extent, which further increases

the comfort level for the patients using

the prosthetic and orthotic devices. The

stress relieving process occurs during

the extrusion process, and there are

several methods of accomplishing stress

relieving. Stress relieving has no effect

on the quality of the part produced in

the orthotic & prosthetic (O&P) industry

for several reasons. First, the stress

relieving process occurs under the

minimum forming temperature. In the

O&P industry, the plastic is heated above

this temperature, which allows the plastic

to flow freely and relieve its internal

stresses. Stress may be reintroduced intothe plastic during the forming process.

Second, it is not apparent that there are

any chemical additives added to the

plastic to change the molecular structure

so as to relieve stress. It is possible that

some of the commercial methods for

stress relieving could be adapted to

relieve stress in formed parts, which

would allow for a more reliable finished

part, mentions Dr Vohra.

Realising dreams for a bettertomorrow

Arms have become a particular

focus in the prosthetics industry, as

science has long had success with

prosthetic legs. Recent developments

on this front include more flexible

and sensitive skin & arm designs,

and wireless devices implanted in

prosthetic arms to allow more natural

movement. In recent experiments,

researchers have also used sensors

implanted in the brain to enable

monkeys control a mechanical arm, and

a paralysed man to move a cursor on a

computer screen.

Some of these methods, if

perfected and approved by regulatory

agencies, may eventually become

more viable for amputees. Polymers

are bound to drive these initiatives

forward, particularly in making this

sophisticated equipment economically

feasible for the common man. While

the reinnervation technique does not

require regulatory approval because

it is done using surgery and existing

devices, it has limitations, which even

its creators acknowledge - including

the fact that it is not feasible for every

patient and is expensive. However,

there is a possibility of these negatives

getting completely eradicated with the

advent of advanced technologies in the

world of medical plastics.


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