Wherever there are repeated connections, disconnections and relative movement between parts, you will find hoses and tubing. However, the requirements for each application may differ enormously. For example, when dealing with biomedical or pharmaceutical markets, all components used in the delivery of drug or extraction of bodily fluids must not insert undesired chemicals in the fluid stream. As such, hoses and tubing are required to be chemically inert, odorless, non-wetting, non-leaching, and have a low level of impurities. Such hoses are often known as high-purity hoses, and the following five questions explore the fundamental aspects of these hoses.
By Angelica Pajkovic and Davi Correia
1: What Are High Purity Hoses?
High purity is a term that can mean many things, according to the application. Take oxygen, for example. In the United States, the USP has a listing containing the purity requirements for that gas. As such, oxygen for medical consumption requires a purity of no less than 99.0% (mole). One may think that means high purity, but when oxygen is used for silicon oxidation in semiconductor manufacturing, 99.9995% pure oxygen is ideal.
The same reasoning applies to hoses and tubing. It is important to know what market is being discussed and what the standards and legislation that govern the purity requirements for a specific product are; this leads to the next question.
2: What Type of Standards Are Followed?
The standards for the construction and use of high purity hoses varies considerably depending on its intended application. In general, many specialty hoses are compliant with both ISO and USP standards.
ISO 9001 Standard
The ISO 9001 sets specific requirements for a quality management system where an organization needs to demonstrate its ability to consistently provide product that meets customer and applicable regulatory requirements. At the same time, it helps consumers distinguish between companies, allowing them to make educated choices when choosing a vendor. It does this by identifying best practices for each industry, standardizing those practices and promoting adherence to those practices.1
The United States Pharmacopeia (USP) is a non-profit scientific organization that develops quality standards for prescription and over-the-counter medicines and other healthcare products manufactured or sold in the United States. USP sets widely recognized standards for food ingredients and dietary supplements, and for the quality, purity, strength, and consistency of these products.2
Both of these standards can be applied to any highly sterile application and have a range of specifications that coincide with the level of purity required by the application. When considering food grade hoses in particular, the 3-A sanitary standards are often followed.
3-A Sanitary Standards, Inc. is a not-for-profit company committed to food safety and hygienic equipment design. These standards provide design specifications for equipment and processes that come into contact with food. For hoses, this standard dictates that all of the elements used in a hose assembly must be 3-A compliant as separate pieces, meaning each part must meet 3-A Sanitary guidelines. More specifically, the materials used for hoses must meet certain hardness and absorption requirements in different chemical solutions and pass strict testing requirements.3
The Food and Drug Administration’s (FDA) 21 CFR 170-199 and 21 CFR 177.2600 are additional standards that purity hose manufacturers are often compliant with. The 21 CFR 170-199 regulates food contact materials, including: plastics, metal, paper, glass, additives, adhesives, inks, colorants, and coatings etc. During contact, molecules can migrate from these materials to the food and this FDA regulation is made to ensure food safety. The 21 CFR 177.2600 focuses on indirect food additives, substances that may come into contact with food but are not intended to be added directly to food. Molecules from food processing equipment, such as hose, are examples of indirect food additives. CFR 21 FDA 177.2600 dictates what materials are recognized as safe for food processing, how they should be cleansed prior to first use, and sets product extraction limits for aqueous and fatty foods.
3: What Materials of Construction Are Most Common?
Given how broad the topic can be, this article will focus on the inert polymers used in biomedical tubing. A list of the most common materials typically includes polyethylenes, polypropylenes, polyamides (nylons), polyurethanes and polysiloxanes (silicone).
Polyethylene polymers are obtained by chain polymerization processes. Depending on the reaction time, temperature and pressure, polyethylene with different molecular weight, crystallinity and degree of branching can be obtained. They are commonly employed as catheter tubes, facial implants, artificial tendons, or bearing components in total joint replacements. The melting temperature is typically in the range of 120–147°C and the glass transition temperature is on the order of 80°C. Polyethylene is generally classified in different subcategories based on its density and branching.4 High density (HDPE), low density (LDPE), and linear low density polyethylene (LLDPE) are common polyethylene polymers used for medical applications.
Polypropylene is a thermoplastic polymer also generated through chain polymerization of propylene in presence of suitable catalysts; generally aluminum alkyl and titanium tetrachloride. Polypropylene polymers have a chemical structure similar to polyethylene but have demonstrated certain advantages, such as improved strength, stiffness, and higher temperature capability.4 Due to its low coefficient of friction and high heat tolerance, it is a commonly used in tubing applications. The material is a versatile, cost-effective alternative to fluoropolymers and other engineered resins.
Polyamide polymers (Nylons) are obtained by step-growth polymerization. These materials have a broad range of properties, depending on the specific chemistry and processing, and may be amorphous or semicrystalline. These polymers are susceptible to swelling in aqueous solutions but are often employed in short-term applications such as catheters and catheter balloons in angioplasty procedures or stent deployment.4
Polyurethanes represent a major class of synthetic elastomers and are constituted of chains of organic units joined by carbamate links. They are generated through a step-growth polymerization reaction of three basic components: a diisocyanate, a sort chain diol, and a long-chain diol. Although they require sophisticated, and quite expensive manufacturing processes, the employment of polyurethanes in biomedical applications is growing dramatically as they can be used in applications where other materials do not work.4 When used for tubing, polyurethane is high biocompatible and offers great comfort for a patient if used as catheter, for it softens with body heat.
Silicone polymers, more properly called polysiloxanes, are inorganic-organic polymers with the chemical formula [R2SiO] n, where R is an organic group such as methyl, ethyl, or phenyl. By varying the -Si-O- chain lengths, side groups, and crosslinking, silicones can be synthesized with a wide variety of properties and compositions. They can be classified as fluids, elastomers, and resins. Silicones have a number of medical applications because of their biocompatibility and chemical inertness. The FDA has approved many medical silicone-based medical devices such as catheters, tubing, gastric bags, drains, and endoscopic windows. The gel form is used in bandages and dressings, breast implants, testicle implants, pectoral implants, contact lenses, and a variety of other medical uses.4
Polyvinyl chloride (PVC)
PVC is one of the most used plastic material for single use systems (SUS) medical devices. ‘Medical Grade PVC Tubing is resistant to corrosion, as well as many chemicals and solvents. Although often used in deposable applications, it is also capable of withstanding repeated sterilizations. Tubing made out of PVC has low friction and provides excellent flow capabilities. As a result of the flow performance, sediment accumulation is prevented, which prevents the growth of bacteria.5
4: How Are They Made?
The most common manufacturing process for hoses and tubing is extrusion. In this process, raw material in the form of pellets, or powder, is fed into a hopper, melted, and forced under pressure through an extrusion die orifice that defines the shape of the product’s cross section (see Figure 1). The extruded material is then cooled and removed at a compatible speed.
The raw material fed in the hopper is not only the polymer from the tubing will be made out of, but also additives. Additives are used to facilitate the processing and improve the properties. For example, ‘additives may be used to increase the stability of polymers during thermomechanical treatments and to modify their properties. However, most of these products are small molecules compared with the size of the polymers. Some of the products are therefore susceptible to migrating and inducing unwanted reactions in the surrounding living tissues.7
The extrusion process is not only a matter of getting the engineering right. The FDA regulates the quality of hoses and tubes for food and biomedical uses via a legislation called current good manufacturing practices (CGMP’s). For example, the manufacturing may be required to be carried-out in a cleanroom (an isolated environment, strictly controlled with respect to air contamination, temperature, humidity, air pressure, air flow, air motion, and lighting).
5: In Which Applications Are These Hoses Typically Used?
It has been made evident throughout the article that there are three sectors in which high purity hoses are primarily used: pharmaceutical, food & beverage and biotechnological processing. In the pharmaceutical industry, high purity hoses are used for the transfer of pharmaceutical, cosmetic, alimentary and chemical products in various areas. Similarly, the food & beverage and biotechnological processing industries use special grade hoses for the transfer of food items and production health care (medical) items, crop production and agriculture, and environmental uses.
As the susceptibility to contaminants for the end products of these sectors is so high, it is vital that the hoses and tubing used in the creation of these products be as sterile as possible.
- Francesco Puoci (Editor), Advanced Polymers in Medicine, Springer, 2015.
- Denis J-P Labarre & Gilles Ponchel & Christine Vauthier, Biomedical and Pharmaceutical Polymers, Pharmaceutical Press, 2011