Medical Device Link.

MANUFACTURING

The driving force

Miniaturisation is the principal driver of advances in medical technology. This is particularly true for the less invasive medical procedures such as arthroscopic and heart surgeries that have become increasingly popular; incisions are smaller and the tolerances in tubing and other devices are correspondingly tighter than ever. The intensified requirement for more performance in less space has led to an acceleration in tooling research and development to meet the needs of the newest medical applications. Some of the most recent developments are reviewed below.

Tolerances continue to tighten

Tighter tolerances are essential for the newest medical extrusions. For example, “What used to work as a 6 French catheter is 5 French today; and people want it to be a 3 French tomorrow,” says the Chief Executive Officer of an extrusion company. “It has to have the same properties, carry the same amounts of fluid through it, or other accessory components have to fit with it, even though it is only half the size.”

In the past, customers were satisfied with tolerances of 0.001 in. (0.0254 mm). More recently, it was 0.0005 in. (0.0127 mm). Now they need to get even tighter than that, for example, two- or three-tenths on concentricity. In terms of percentage of the wall, previously the tolerance objective was 90 per cent. Today, it is typical to see tolerances of 95 per cent, and advanced medical extrusions are even running at 99–99.5 per cent.

More layers and more lumens

The increased sophistication of today’s medical devices now requires extruders to produce tubing with multiple layers and multiple lumens, sometimes up to eight lumens. Furthermore, more plastics with two or three compounds are appearing that also require three, four or five lumens. Some of the multilumen products may have wire inside, some may have braids and others may have different lining materials. One extruding company said that a year or two ago, approximately 25 per cent of its business was multilumen; today, this has increased to double that.

More coextrusion

As medical tubing and other products decrease in size and their wall thicknesses decrease, it becomes difficult to retain all the desired properties. Increasingly, the answer is coextrusion. By combining two materials, it is possible for a thin-wall tube to maintain the feel of a thicker wall tube.

Tubing with varying stiffness

There is also a growing demand for tubing with stiffness that varies with length, which makes tooling more complex. This is accomplished with two different extruders that oscillate from one durometer to the other and back again. Within one tube there are actually two durometers changing the length of the tube so that it can be guided up through the patient’s body with a harder durometer, and go around areas in the body with the soft durometer.

Changing durometer with length is just the beginning, says the head of one manufacturing company. “We do braided catheter work,” he explains, “with the requirement being to change the braid together with changing the hardness of the material over the braid, while at the same time changing the inner and outer diameter of the tube. It is not as simple as saying ‘I want a stiff shaft and a transitional area to a soft tip.’”

The latest development in this area is to move to three durometers when three different degrees of stiffness are required in the tube: soft, medium and firm. This process requires the use of three extruders.
Accelerating material changes There are some interesting new developments in medical polymers. Resin manufacturers are taking more interest in the medical device market and are making available materials that they previously did not manufacture, or were not able to manufacture for a variety of reasons.

In the polyetherketone family, there are resins that are actually championed by the resin manufacturers for use with short and long term implantables, and they provide United States (US) Food and Drug Administration master files for these US Pharmacopoeia Class VI materials.

Also of significance is an increasing demand for bioabsorbing materials, as well as some increase in the use of materials that promote bone regeneration, and proprietary compounds and mixtures for orthopaedic applications that promote bone growth and then will be absorbed as that growth occurs.

Phthalates and other plasticisers

In recent years, there has been increasing concern about the possible dangers of phthalate plasticisers such as di(2-ethylhexyl)phthalate (DEHP) used in poly(vinylchloride) (PVC) tubing. Users are reviewing alternatives. Many long time users of PVC seem content to stay with what they have. However, the customers of some companies, particularly those planning new products for sale in Japan or Europe, have begun to request DEHP-free materials. One company, for example, sells polyurethane and silicone as replacements for PVC; and polymers such as ethylene vinyl acetate, polyolefins, polyesters and polybutadiene have also been investigated as potential replacements.

For some manufacturers phthalates are not a concern because they do not use PVC much or their products are used in medical offices rather than in surgical units.

The role of tooling makers

The changes and developments discussed in this article have increased the challenges for, and the pressure on, extrusion technology. In the process, the tooling industry is having to respond with greater innovation, creativity and rigorous quality assurance programmes.

For extrusion tooling companies, these accelerating demands have accentuated the commitment to developing tooling for maximum performance with a high degree of precision, while at the same time seeking every way possible to keep costs under control.

These are difficult challenges in view of the critical requirements of the medical device industry. Yet, these challenges will help the industry maintain the delivery of health care at its highest level in the long term. The tooling industry, through intense commitment and extensive research and development, is up to meeting them.

Bill Conley is Technical Sales Manager at Guill Tool & Engineering Co. Inc., 10 Pike Street, West Warwick, Rhode Island 02893, USA, tel. +1 401 828 7600, e-mail: bconley@guill.com, www.guill.com.