Machine Design

Where foam fits in medical design

A variety of different chemistries and manufacturing processes can fine-tune characteristics in foam.

James R. Dovorany
Vice President illbruck inc.
Minneapolis, Minn.

Edited by Stephen Mraz

Foams give engineers a range of different and customizable materials for designing medical equipment.

Specially cut polyurethane foam is used in this oral swab so it isn't abrasive enough to harm patients' mouths and gums.

Closed cell

Open cell


Foam can be processed to change its structure and properties.

Foam offers designers a full palette of potential materials. They can use it to absorb, seal, filter, wick, cushion, insulate, or support loads. And to solve even more application problems, you can heat it, compress it, and laminate it to change its characteristics. Foam plays a crucial role in surgery masks, cervical collars, thermal insulation and wound-care products, fluid regulators, and acoustic absorbers and dampers. This kind of versatility makes foam unique as a design material. And thanks to innovative fabrication techniques and the latest in polymer chemistry, foam can take on a wide range of different properties, including shape retention, water resistance or absorbency, porosity, density, and a myriad of physical characteristics.

Variations on a basic recipe
In its simplest form, flexible polyurethane foam — the most common type of foam — is the product of petro-chemistry and resins. More precisely, it's made of toluene diisocyanate, polyol and water. These ingredients are mixed, poured onto a linear conveyor, then left to "rise" and cure. Additives are blended in for specific characteristics such as retarding bacterial growth or flame, adding color, absorbing or repelling liquids, and inhibiting the effects of UV radiation.

Once it cures, the foam consists of individual cells (also called pores) which have completely polymerized and solidified to form a skeletal structure. The cells can be open or closed.

Open-cell foams, such as polyurethane ester and polyurethane ether, have interconnected cells with a thin membrane between the skeletal ribs and the foam itself. Closed-cell foams, such as polyethylene, silicones and neoprenes, have intact and separate cells, making them nonpermeable and resistant to moisture and oil. Closed-cell foams also insulate against heat and cold and absorb shocks and vibrations.

Cell walls within the foam can be removed through a thermal or chemical process called reticulation which increases the foam's permeability. Reticulation dissolves the cell membranes, leaving only cell strands, or a skeletal structure. This opens more room for air and liquids. Reticulation also increases the foam's surface area, letting it wick and dispense water and other liquids. Reticulated foams can capture and hold particulates of various sizes, which provides possibilities for filtering.

Varying a foam's cell structure alters its physical properties, letting engineers tailor foams to meet specific applications. For example, closed-cell polyethylene, with densities ranging from 4 to 12 lb/ft3, is often used for floatation devices. And open-cell polyurethane, with densities of 2 to 6 lb/ft3, can be found in filters and sound-absorption materials.

Expanding foam's performance
Postproduction processing can change foam's characteristics and give medical engineers a new set of options. Almost all flexible foams can be impregnated, coated or faced with laminates, foils, and films. For instance, adding a protective outside layer of Mylar, Tedlar, or Tyvec increases the foam's resistance to dust, dirt, oil, and surface abrasion.

Fabrication includes a variety of methods for shaping and cutting foam. Bulk foams are made in large blocks or "buns," and rolls. Both forms are made in a variety of sizes. From this original state, it can be put through many different processes. For example:

Cutting transforms foam buns into precise sheets as thin as 0.06 in. CNC cutting machines can slice it into intricate shapes and contours while nesting parts for the most economical material usage and lowest cost.

Laminating adds a layer of woven or nonwoven fabrics, films, foils, or other foams to improve the performance characteristics for a particular application. Some layers or liners, such as pressure-sensitive adhesives chosen for bond strength, skin sensitivity, or biocompatibility, will have an integral release liner.

Flame-laminating bonds roll products together by melting a thin layer of the foam and pressing or "nipping" another material into the liquefied foam. It is typically used in high-volume production to avoid using adhesives in adding film, fabrics, and barrier materials, as well as other foams, to the original foam.

Convoluting uses patterned rollers to compress foam into convoluted shapes such as egg crate or sine wave for packaging, cushioning, and sound absorption.

Impregnating and top-coating foams improve some performance capabilities of open-cell foams. Impregnating open-cell foams with carbons, water repellents and oils, or top coating them with Hypalon and flame-retardant paint, helps them resist dust, dirt, oil, water, and other substances that would otherwise affect performance.

Die-cutting creates various shapes and sizes. A "clicking" agent added during manufacture lets the foam bounce back from the pinching effects of die cutting.

Felting is a thermal-compressing process that "densifies" foam and provides increased internal surface area. Felted foams are often used for wicking, dispensing, and filtering.

Thermoforming and vacuum forming apply heat, pressure, and vacuum to cold or hot materials to form a specific shape, pattern, or logo. Flowing contours and radii can be die cut in the same process, providing many possibilities for packaging, gasketing, and damping.

Requirements for device-specific, cost-effective, and timely products will continue to influence technological developments in foam. And process improvements and a wider variety of additives mean that foam

will keep pace with the demands for a versatile, high-tech material that meets a range of application needs.

It filters
The permeable cell structure of reticulated, open-cell urethane makes it well suited for filtering, primarily because the open-pore structure leaves plenty of room to capture and hold airborne or liquid contaminants. The high volume of void space lets the foam filter with relatively little air resistance or pressure drop. Despite porosities ranging from 3 to over 100 pores per linear inch, urethane foams have high tensile strength and tear resistance. As a result, they can be cut or molded into complex shapes without damage. Polyurethane ether's hydrolitic stability prevents swelling or degradation, which enhances liquid filtration. On the other hand, polyurethane ester is more useful to air filtration because of its good resistance to UV degradation and stronger tensile and tear properties.

Multilayer filters can capture a range of different-sized particulates. They can be made either by combining foams of different porosities, or by combining foam with other media, such as nonwoven materials. Multiple layers also let biomedical engineers design systems that filter specific sizes of particulates from liquids that must be reclaimed.

It wicks and dispenses

Applicator pads use several layers of foam so they can dispense liquids without dripping.

In some medical applications, opencell reticulated foams act as reservoirs. They both wick and dispense liquid without letting it puddle on the applicator or drip. Clean-room wipes, swabs for gels and liquids, and other fluid applicators made with foam, wick and dispense at controlled rates. You can control foam's wicking and dispensing rates by varying how much it is compressed or felted. Compressed foam retains its high void volume, so it can hold a relatively large amount of liquid and still retain its shape. Even a foam compressed to one-tenth its original thickness will have a void volume of 70% (compared to uncompressed foam, which has a 97% void volume). Foam holds liquids on the surface and within its skeletal structure. Engineers can fine-tune the density, compression, and capillary action to match the viscosity of the liquid and how fast it must flow through the foam.

A pad made out of a series of foam screens, or layers with different porosities, will disperse liquids throughout its surface rather than stream through the middle. Urethane foams are also among the softest and lightest of all foams and therefore the most "patient friendly."

It insulates
Foams that insulate, whether acoustically, thermally, or both, can be used in a variety of medical devices. They act as barriers to protect heat-sensitive components or help retain heat inside a machine. Foam reduces noise from fans, pumps, compressors and other machinery. Generally, insulating foam is made from melamine. Compared to polyurethane or polyethylene, melamine is fiber-free, and its base chemistry lets it withstand much higher temperatures. For example, illbruck's proprietary melamine foam, willtec, is rated to withstand constant temperatures to 300°F, and short-term temperatures up to 482°F. It is an open-cell, low-density foam (0.7 lb/ft3) that dampens sound over a wide range of frequencies. It can withstand moisture and prevents microbial growth (UL 181) and fungus (ASTM G21), and resists organic solvents and a variety of diluted acids and alkali, including isopropanol, glycerin, sulfuric acid, citric acid, ammonia, water and caustic soda.

It cushions and supports

Combining polyethylene and hydrophillic foam makes an absorbent covering that keep wounds warm and protected.

Whether used as cushioning on a face mask or on a neck collar, foam conforms to body contours. It also covers the spectrum from lightweight and low density to firm and supportive, depending on the required function. A variety of foam fabrication processes, such as cutting, laminating and felting, make open or closed-cell foams applicable to just about any cushioning or support requirement.

Generally, the medical situation determines how long the device will be used and, therefore, what type of foam should be used. Shortterm devices, those used only for recovery and then discarded, are made from both open or closed-cell materials. Combining either with coverings can improve the user comfort. You can also attach hook-andloop closures to foam for maximum adjustability.

TAGS: Materials
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