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Ultra-High-Molecular-Weight Polyethylene manufacturing and application in Medical Textiles sector

This article is being published in two parts. This is the first part of those two.

 

Prologue:

Ultra High Molecular Weight polyethylene (UHMW-PE) is an engineering thermoplastic with a molecular weight of greater than 3.1 AMUs (Atomic Mass Units). The elevated molecular weight enhances a number of imperative physical properties, not the least of which are outstanding abrasion resistance, high impact strength and a low coefficient of friction. In addition, the polymer has excellent chemical resistance; excellent sound dampening characteristics, superior dielectric and electrical insulating properties and it doesn’t absorb moisture. From the above special properties of UHMWPE fiber has been the material of choice for years in medical textiles, particularly in the field of total joint arthroplasty.

In the orthopedic industry, UHMWPE has been the material of choice as a surface bearing material in total joint arthroplasty for decades. The material is applied in for example artificial hips, knees, as well as in shoulders, elbows, wrists, ankles and spinal disks. These devices require bearing materials that are strong, have a high resistance against wear and are of the highest purity levels, necessary for the application inside human bodies.
In this paper we have discussed about the morphological structure, properties of UHMW Poly Ethylene, Process of Producing Orthopedic Implants and its applications of Medical textiles.

Keywords: Biodegradation, Inflammation, HPPE, Implant, Orthopedic, Scaffold, UHMWPE

1. Introduction

The fate of a biomaterial is gritty by its reaction with the biological environment. The human body is so sensitive and an antagonistic that the properties of the biomaterial should be well-matched enough as not to disturb the various functions of human body. Biomaterials should be chemically and biologically inert to the surrounding cells and body fluids. It should also be hard and wear resistant with a low coefficient of friction for some applications. In addition to corrosion resistance, it should not release toxic and carcinogenic elements into the human body. It would be more desirable if the biomaterials have functions to provide a biomedical treatment.

The common metals that have been used for different biomedical applications are Titanium, Stainless Steel, Titanium Oxide, Titanium Nitride, Cobalt–Chromium Alloy, and Nitinol Alloys. These materials suffer from drawbacks in case of sustained and long-term use like cytotoxicity, release of metal ions, corrosion, and wear. For example, though stainless steel has been successfully used in many biomedical applications, it can become corroded and releases Cr, Ni, Mn, and Mo ions when the metal is placed in coronary vessels.

Previously, medical devices were selected based on its material and bulk properties. However, it is now recognized that the surface properties of the device mainly govern its biomedical applications. In most cases, a surface modification is considered to be a prerequisite for better biocompatibility. Ion beam processing or coating the medical devices with inert, corrosion resistant, adhesive, and biocompatible materials have been gaining importance from last decade. UHMWPE has found a wide application as a load bearing material in the majority of joint endoprostheses in combination with metal or ceramic counter-parts. Because of it has extremely low moisture absorption and a very low coefficient of friction, also self-lubricating, and is highly resistant to abrasion, in some forms being 15 times more resistant to abrasion than carbon and steel. Its coefficient of friction is significantly lower than that of nylon and acetal, and is comparable to that of polytetrafluoroethylene (Teflon), but UHMWPE has better abrasion resistance than PTFE, It is odorless, tasteless, and nontoxic.

2. Structure and properties of UHMWPE

Structure of UHMWPE, the molecular chain can consist of more than 200,000 ethylene repeat units. The average molecular weight typically lies in the range of 2–2.5 million.

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Figure 1: Morphological structure of UHMWPE

2UHMWPE is a type of polyolefin. It is made up of extremely long chains of polyethylene, which all align in the same direction. It derives its strength largely from the length of each individual molecule (chain). Vander Waals bonds between the molecules are relatively weak for each atom of overlap between the molecules, but because the molecules are very long, large overlaps can exist, adding up to the ability to carry larger shear forces from molecule to molecule. Each chain is bonded to the others with so many Vander Waals bonds that the whole of the inter-molecule strength is high. In this way, large tensile loads are not limited as much by the comparative weakness of each Vander Waals bond. When formed to fibers, the polymer chains can attain a parallel orientation greater than 95% and a level of crystalline of up to 85%.

The weak bonding between olefin molecules allows local thermal excitations to disrupt the crystalline order of a given chain piece-by-piece, giving it much poorer heat resistance than other high-strength fibers. Its melting point is around 144 to 152 °C (291 to 306 °F), and, according to DSM, it is not advisable to use UHMWPE fibers at temperatures exceeding 80 to 100 °C (176 to 212 °F) for long periods of time. It becomes brittle at temperatures below −150 °C (−240 °F).

The simple structure of the molecule also gives rise to surface and chemical properties that are rare in high-performance polymers. For example, the polar groups in most polymers easily bond to water. Because olefins have no such groups, UHMWPE does not absorb water readily, nor wet easily, which makes bonding it to other polymers difficult. For the same reasons, skin does not interact with it strongly, making the UHMWPE fiber surface feel slippery. In a similar manner, aromatic polymers are often susceptible to aromatic solvents due to aromatic stacking interactions, an effect aliphatic polymers like UHMWPE are immune to. UHMWPE does not contain chemical groups (such as esters, amides or hydroxylic groups) that are susceptible to attack from aggressive agents, it is very resistant to water, moisture, most chemicals, UV radiation, and micro-organisms. Under tensile load, UHMWPE will deform continually as long as the stress is present – an effect called creep.

Table: 1 Chemical and Physical Properties of HPPE (High Performance Polyethylene)

Water and chemicals
Moisture regain Zero
Attack by water None
Resistance to acids Excellent
Resistance to alkalis Excellent
Resistance to most chemicals Excellent
Resistance to UV light very good
Thermal
Melting point 144–155°C
Boiling water shrinkage <1%
Thermal conductivity (along fiber axis) 20W/mK
Thermal expansion coefficient -12 ¥ 10-6 per K
Mechanical
Axial tensile strength 3GPa
Axial tensile modulus 100GPa
Creep (22 °C, 20% load) 1 ¥ 10-2 % per day
Axial compressive strength 0.1GPa
Axial compressive modulus 100GPa
Transverse tensile strength 0.03GPa
Transverse modulus 3GPa

UHMWPE has outstanding physical and mechanical properties such as high abrasion resistance, high impact toughness, good corrosion and chemical resistance, resistance to cyclic fatigue, and resistance to radiation. Due to its excellent properties UHMWPE finds use in highly demanding applications including artificial implant materials. The basic requirements for any medical implant material are: biological stability, biocompatibility, high toughness, high creep resistance, low friction, and low wear.

2.1 The Process of Producing Orthopedic Implants

Polyethylene is to proceed from ideal abstractions to actual physical implants, three real-world steps need to occur. First, the UHMWPE must be polymerized from ethylene gas. Second, the polymerized UHMWPE, in the form of resin powder, needs to be consolidated into a sheet, rod, or near-net shaped implant (Figure:1) Finally, in most instances, the UHMWPE implant needs to be machined into its final shape.

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The polymerization of UHMWPE was commercialized by Ruhrchemie AG, based in northern Germany, during the 1950s. UHMWPE powders have been produced by Ruhrchemie (Ticona) using the Ziegler process. The main ingredients for producing UHMWPE are ethylene (a reactive gas), hydrogen, and titanium tetra chloride (the catalyst). The polymerization takes place in a solvent used for mass and heat transfer.

UHMWPE is produced as powder and must be consolidated under elevated temperatures and pressures because of its high melt viscosity. UHMWPE does not flow like lower molecular weight polyethylenes when raised above its melting temperature. For this reason, many thermoplastic processing techniques, such as injection molding, screw extrusion, or blow molding, are not practical for UHMWPE. Instead, semi-finished UHMWPE is typically produced by compression molding and ram extrusion.

3. UHMW-PE for medical Application

3.1. Total Hip Replacement (THP)

4Introduced clinically in November 1962 by Sir John Charnley, UHMWPE articulating against a metallic femoral head remains the gold standard bearing surface combination for total hip arthroplasty. Considering how rapidly technology can change in the field of orthopedics, the long-term role that ultrahigh molecular weight polyethylene (UHMWPE) has played in joint arthroplasty. UHMWPE has a long, successful clinical track record in total hip replacements dating back to the 1960s. However, the in vivo wear rate of conventional UHMWPE for some THA patients falls above the threshold for osteolysis and may ultimately lead to the need for long-term revision. Although accurate computer-assisted methods have now been developed to track the progression of in vivo wear of UHMWPE, these techniques reveal a range of performance within the human body.

3.1.1 Artificial hip joints Basic materials: Stainless steel, titanium & its alloys & UHMWPE.

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A Total Hip Replacement (THR) is a surgical procedure whereby the diseased cartilage and bone of the hip joint is surgically replaced with artificial materials. The normal hip joint is a ball and socket joint. The socket is a “cup-shaped” bone of the pelvis called the acetabulum. The ball is the head of the thigh bone (femur). THR involves surgical removal of the diseased ball and socket and replacing them with a metal ball and socket and replacing them with a metal ball and stem inserted into the femur bone and an artificial plastic cup socket.

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Contemporary hip bearings may be generally classified as either hard-on-hard or hard-on-soft. In the 1990s, growing concern over long-term osteolysis provoked by UHMWPE wear debris from hard-on-soft bearings led to the concurrent development of highly crosslinked     UHMWPEs, as well as hard-on-hard bearing alternatives. Over the past decade, hip bearings incorporating Metal-On-Metal (MOM) or Ceramic-On-Ceramic (COC) articulations were widely adopted in orthopedics due to their “ultra-low” wear rates in hip simulator studies, even when clinical factors such as subluxation are taken into account. In recent years, a novel ceramic-on-metal bearing (COM) combination has also been clinically introduced.

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In the laboratory, current MOM, COC, and COM bearings have been reported to reduce the production of wear debris by two to three orders of magnituderelative to conventional UHMWPE.When compared against highly crosslinked UHMWPE,COC and COM (but not necessarily MOM) exhibit substantiallylower wear in a laboratory setting.

3.2. Surgical suture (Non absorbable)

It is specially engineered to deliver the outstanding performance required to meet the demands of cardiovascular and orthopedic surgery procedures. Suture is prepared from an ultra-high molecular weight polyethylene (UHMWPE) material that is incredibly strong and durable. Yet for all its strength, this exhibits a flexibility and pliability that makes it easy to work within the surgical suite. The suture is nonabrasive and has a silk-like feel that is gentle on tissue and gloves. Take a closer look at UHMWPE Suture, and you will immediately notice it is flatter than most polyester and poly-blend sutures. Its unique profile allows smaller knots and for each successive knot to better lock up against the last. For the surgeon, this translates to outstanding knot security with precise knot placement and a smooth tie down. The smoothness of the fiber material helps to improve the way in which the sutures slide through both tissue and anchors; furthermore, the abrasion resistance of the UHMWPE offers higher resistance to fraying. Tensile strength and knot break strength for UHMWPE suture more than 2.5 times that of the state-of-the-art polyester suture.

To be continued…..

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