MEDICAL GRADE MATERIAL

For the designing of a medical device, each composing material should have certain characteristics, which should be in a harmony with the final properties of the medical device as well the target application.
The manufacturing companies take into consideration the following criteria:

• availability of the material in sufficient quantities and material cost, (including the costs of production, transportation, and amounts required for each device)
• matching between the material properties and the required specifications of the designed device.
• biocompatibility of the finally designed device, as well as its components. This can be considered one of the most important factors for selecting material, where the formation of any harmful products following the usage of the device will lead to its failure.
• required sterilization technique which can preserve the structure and properties of the constituting materials as well.
• sustainability of the medical device: choice of material, the manufacturing method and its related economic issues, and finally the disposal of the device.

ISO standard for medical grade materials

Because they come in contact with the human body, materials are tested for biocompatibility and safety in order to receive the “medical grade” designation
ISO 10993: biological evaluation of medical devices. The ISO 10993 set entails a series of standards for evaluating the biocompatibility of medical devices.

• Part 1: Evaluation and testing within a risk management process (2018)
• Part 3: Tests for genotoxicity, carcinogenicity and reproductive toxicity (2014)
• Part 4: Selection of tests for interactions with blood (2017)
• Part 5: Tests for in vitro cytotoxicity (2009)
• Part 6: Tests for local effects after implantation (2016)
• Part 7: Ethylene oxide sterilization residuals (2008)
• Part 9: Framework for identification and quantification of potential degradation products (2019)
• Part 10: Tests for irritation and skin sensitization (2021)
• Part 11: Tests for systemic toxicity (2017)
• Part 12: Sample preparation and reference material (2021)
• Part 13: Identification and quantification of degradation products from polymeric medical devices (2010)
• Part 14: Identification and quantification of degradation products from ceramics (2009)
• Part 15: Identification and quantification of degradation products from metals and alloys (2009)
• Part 16: Toxicokinetic study design for degradation products and leachables (2018)
• Part 17: Establishment of allowable limits for leachable substances (2002)
• Part 18: Chemical characterization of medical device materials within a risk management process (2020)
• Part 23: Tests for irritation (2021)

PLASTICS AND ELASTOMERS

Definition

Plastic is material consisting of any of a wide range of synthetic or semi-synthetic organic compounds that are malleable and so can be molded into solid objects. Most plastics contain organic polymers. The vast majority of these polymers are formed from chains of carbon atoms, ‘pure’ or with the addition of: oxygen, nitrogen, or sulfur. The chains comprise many repeat units, formed from monomers. Each polymer chain will have several thousand repeating units.

An elastomer is a polymer with viscoelasticity (i.e., both viscosity and elasticity) and very weak intermolecular forces compared with other materials. The term of elastic polymer, is often used interchangeably with rubber. Each of the monomers which link to form the polymer is usually a compound of several elements among carbon, hydrogen, oxygen and silicon.

Characteristics

They are generally lightweight, can have excellent flexibility, and are generally inexpensive. Approximately 75 percent of polymers used in medical device manufacturing are thermoplastics (plastics that, when heated, do not undergo chemical change in their composition and so can be molded again and again), allowing them to be molded to precise shapes. Unlike metals, polymers do not interfere with medical scanning devices such as MRIs.

Polymers used in medical device manufacturing must be sterilizable, resistant to contamination, and have acceptable low levels of toxicity.

Many of the properties of plastics are determined by intenational standards.

Plastic number code

The plastic symbols are the Resin Identification Coding System (RICS). They advise what type of plastic the item is. Recyclers use these plastic codes: they can visually identify plastics to sort if for recycling, but the symbol doesn’t tell you if it will be recycled or not.

Properties of some plastics

Polyvinyl chloride (PVC / V)

• inert, chemically stable and flexible polymer that is used in a wide array of products
• the material has a smooth surface and is firm enough for easy insertion, yet soft enough to be slightly pliable. It tends to warm to body heat which can also aid in overall pliability.
• it is often used for medical devices which are sterilized by the manufacturer, it is the most popular material used for intermittent catheters. It is clear, you can view the output.
• unplasticized PVC is hard and brittle at room temperature. A plasticizer (=phthalate) is typically added to increase the flexibility of the polymer. DEHP (= Di-ethylhexyl phthalate) is the plasticizer for most PVC medical devices.
• platelet affinity is significantly higher for PVC = more thrombogenic.
• incineration generates dioxins (carcinogenic) which escape in the atmosphere via the fumes of the incinerator, may also contain some lead (Pb).

In 2002, the US FDA issued a Public Health Notification concerning PVC. The agency expressed concerns regarding exposure to the PVC plasticizer DEHP that is used in numerous medical devices. In view of the available animal data, the agency advised, “precautions should be taken to limit the exposure of the developing male to DEHP.” In the wake of public health concerns regarding phthalate plasticizers, PVC is becoming an outdated legacy material overtaken by others “better suited to the demands of healthcare applications.

For MD used by MSF: For new products: the potential benefits of products containing phthalates will be evaluated against their risks. Enteral nutrition for neonates seems to be the highest risk procedure at present time. For the existing articles: No phthalate eviction policy.

Polypropylene (PP)

Rugged material which resists very well against chemical solvents, bases and acids, as well as bacterial growth. It is therefore often used in the manufacture of medical devices and laboratory equipment.
Incineration of PP is less polluting than the one of PVC. Moreover, its components are elements that give off a lot of calorific value when incinerated. This allows helping incineration of other medical waste that doesn’t burn that well without having to add (too much) additional fuel. Polypropylene (PP) and polypropylene copolymer (PPCO) containers can be autoclaved many times.

Polycarbonate (PC)

• group of thermoplastic polymers containing carbonate groups in their chemical structures
• has high impact-resistance but also a low scratch-resistance
• can undergo large plastic deformations without cracking or breaking
• products made from polycarbonate can contain trace quantities of the precursor monomer bisphenol A
• polycarbonate (PC) products can be autoclaved with caution, as they should not be exposed to alkaline detergents or steam additives, and they may withstand only up to 30 – 50 autoclaving cycles. Sterlizing PC reduces the mechanical strength of the material.
• commonly used in eye protection

Polyester (PES)

• polyester is a synthetic polymer made of purified terephthalic acid (PTA) or its dimethyl ester dimethyl terephthalate (DMT) and monoethylene glycol (MEG)
• it most commonly refers to a type called polyethylene terephthalate (PET)
• polyester fabrics are highly stain-resistant, they have high water, wind and environmental resistance compared to plant-derived fibers but they are less fire resistant and can melt when ignited
• polyester fibers are sometimes spun together with natural fibers to produce a cloth with blended properties

Polystyrene (PS)

• a synthetic aromatic hydrocarbon polymer made from the monomer styrene
• can be solid or foamed (Styrofoam): “open cell” form, in which the foam bubbles are interconnected, as in an absorbent sponge or “closed cell”, in which all the bubbles are distinct
• general-purpose polystyrene is clear, hard, and rather brittle, is a poor barrier to oxygen and water vapor and has a relatively low melting point
• widely used in packaging, petri dishes, test tubes and microplates
• generally non-biodegradable, burns to give carbon dioxide and water vapor, it typically combusts incompletely as indicated by the sooty flame.

Polytetrafluorethylene (PFTE)

• synthetic fluoropolymer of tetrafluoroethylene, thermoplastic polymer
• best known brand name of PTFE-based formulas is Teflon
• commonly used as a graft material in surgical interventions.
• frequently employed as coating on catheters; this interferes with the ability of bacteria and other infectious agents to adhere to catheters. (catheters using PTFE cannot be sterilized by irradiation because the irradiation process degrades PTFE)

Polyurethane (PUR)

• polymer composed of organic units joined by carbamate. there are many types of thermoplastic polyurethanes available: polyester or polyether or polycarbonate based polyurethanes
• thermoplastic polyurethanes do not use plasticizers to obtain softness, the additives used are biocompatible
• they are key polymers in the vascular catheter market: Polycarbonate-based polyurethanes are first choices for long-term central venous catheter applications. Polyether-based polyurethanes soften considerably within minutes of insertion in the body: this promotes patient comfort and reduces risk of vascular trauma.
• larger internal diameter compared with same CH silicone catheter = increased flow. PUR requires less wall thickness for strength compared to silicone. PUR is easily coated with a variety of specialized coatings

Silicone (SI)

• inherent chemical and thermal stability, low surface tension and hydrophobicity, does not contain proteins and are non-allergenic, can be autoclaved ➔ intrinsic biocompatibility and biodurability.
• super-smooth material, softness may reduce vein trauma
• its flexibility as an intermittent catheter lies somewhere between vinyl and rubber latex. It is firmer than latex, which helps with an easy insertion, but it can feel slightly more flexible than vinyl materials.
• kink resistant
• greater wall diameter compared to PUR
• it is clear, so you can view your output.
• extensive application in catheters and other medical products: short- and long-dwelling catheters, drains, and shunts
• compared to PVC: does not contain phthalates or another organic plasticizer which might leach out. Show significantly less sepsis, prolonged service life and fewer catheter insertions per patient.
• mineral incrustation with urinary catheters: the all-silicone and silicone-coated catheters remained patent the longest (compared to latex and latex-coated catheters)

Polyamide (PA)

• macromolecule with repeating units linked by amide bonds, occur both naturally (proteins) and artificially
• synthetic polyamides are classified according to the composition of their main chain: Aliphatic polyamides, Polyphthalamides and Aramids = aromatic polyamides.
• nylon is a generic designation for the synthetic polymers, based on aliphatic polyamides. In common usage, the prefix ‘PA’ (polyamide) or the name ‘Nylon’ are used interchangeably and are equivalent in meaning
• nylon is a thermoplastic silky material that can be melt-processed into fibers films or shapes.
• nomenclature used for nylon polymers uses numbers to describe the number of carbons in each monomer unit

Natural Rubber (NR)

• Rubber and latex are not the same, but many people use the words as if they both referred only to natural rubber
• a latex is a dispersion of extremely small particles of an insoluble liquid or solid material in a liquid (the particles are almost always polymeric, and the liquid is usually water, most latexes also contain a surfactant)
• natural Rubber Latex refers to the white sap that comes from the hevea brasiliensis tree. The latex then is refined into rubber, also called India rubber or caoutchouc, ready for commercial processing.
• thermos-sensitive: warms up to the surrounding temperatures and becomes easily pliable.
• very soft, has a large stretch ratio and high resilience, and is extremely waterproof.
• is almost always given a tint with special dyes while it is still in a liquid state.
• Latex allergy: prevalence among medical professionals is estimated at 8 – 17%, in the general population: 1 – 6%. Latex allergies usually present as a type I (IgE-mediated) immediate allergic reaction to proteins contained in the natural rubber. Several of the additives used during manufacture have also been implicated as causal agents. Latex allergies are regarded as a major healthcare issue.

Synthetic rubber

Synthetic rubber is a man-made rubber which is produced in manufacturing plants by synthesizing it from petroleum and other minerals. It has the property of undergoing elastic stretch or deformation under stress but can also return to its previous size without permanent deformation. Depending on the chemicals added and the properties associated with it, the synthetic rubber can be as hard as a bowling ball or as resilient as a rubber band or as soft as a sponge. Approximately 70% of all rubber used today are one from many synthetic rubber varieties. Some of the popular synthetic rubber types include the following:

• Isoprene Rubber (IR)
• Nitrile Rubber (NBR)
• Polychloroprene (CR)/ Neoprene
• Silicone Rubber (SiR)
• Styrene Butadiene Rubber (SBR)

Ethylene-vinyl acetate (EVA)

• also known as poly (ethylene-vinyl acetate) (PEVA) = copolymer of ethylene and vinyl acetate
• elastomeric polymer that produces materials which are “rubber-like” in softness and flexibility. The material has good clarity and gloss, low-temperature toughness, stress-crack resistance, hot-melt adhesive waterproof properties, and resistance to UV radiation. EVA is competitive with rubber and vinyl polymer products in many electrical applications.
• used in: Hot melt adhesives, in biomedical engineering applications as a drug-delivery device, EVA foam is used as padding or as a shock absorber in physiotherapy equipment.

TEXTILES USED IN LINEN AND CLOTHING

Surgical drapes and materials are a key element when preventing postoperative wound infections. Surgical fabrics have to fulfill the following requirements:

• Maximum protection for patients, users and third parties
• High microbiological and hygiene standards to prevent the risk of infection
• Good wearing comfort of the clothing to maintain the high performance
• Liquid-absorbing
• Easy handling of the drapes

​Types of fabrics

• Woven fabrics have an over/under structure of machine direction (MD) and cross machine direction (XD) threads-produced on a loom that feeds thread in only two directions and are generally inelastic.
• Knit fabrics are constructed through series’ of interlocked loops, which make a knit fabric more flexible in multiple directions.
• Non-woven fabric is a fabric-like material made from staple fiber (short) and long fibers (continuous long), bonded together by chemical, mechanical, heat or solvent treatment.

Non-woven textiles

Non-woven fabrics are structures of textile materials, such as fibres, continuous filaments, or chopped yarns of any nature or origin, that have been formed into webs by any means, and bonded together by any means, excluding the interlacing of yarns as in woven fabric, knitted fabric, laces, braided fabric or tufted fabric (Film and paper structures are not considered as non-wovens)
Non-woven fabrics are engineered fabrics that may be single-use, have a limited life, or be very durable.
Different manufacturing processes classify non-woven fabrics: Staple non-wovens, Melt-blown non-wovens, Spunlaid (also called spunbond) non-wovens, Flashspun fabrics

• SMS non-woven fabric = fabric consisting of 3 layers (polypropylene is mostly used): 1 layer of meltblown non-woven in sandwich between 2 layers of spunbond non-woven.
• SMMS non-woven contains 4 layers: spunbond non-woven fabric + meltblown non-woven fabric + meltblown non-woven fabric + spunbond non-woven fabric

With special treatments during the production process, different specifications can be obtained: hydrophilic, antibacterial, anti-mite, anti-static, alcohol and blood repellent, oil repellenty, hydrostatic pressure resistance.

Medical use:

• medical packaging: porosity allows gas sterilization
• isolation gowns, surgical gowns, surgical drapes and covers, surgical masks, surgical scrub suits, caps, gloves, shoe covers, wound dressings, plasters,..

Regulations for non-woven fabrics:

• ISO 9092: 2019: Textiles — Nonwovens — Definition
• ISO 9073: Textiles -- Test methods for nonwovens – different parts

Woven fabrics

Woven fabric is any textile formed by weaving. Woven fabrics are often created on a loom, and made of many threads woven on a warp and a weft. Technically, a woven fabric is any fabric made by interlacing two or more threads at right angles to one another.

• Polyester/cotton: 50% polyester-50% cotton fabric
• Cotton: 100 % cotton cretonne fabric

Comparative results of the tests made on reusable woven textiles

• Dusting rates: Polyester/cotton gives off 50% less particles than cotton and is therefore particularly recommended for surgical linen.
• Physical properties : Polyester/cotton shows the best physical properties. However, its resistance decreases in proportion to use and to the number of sterilization cycles.
• Barrier properties (bacteriological and physical resistance to liquids): New cotton can absorb the equivalent of its weight: high permeability, immediate bacterial passage. After the 1st sterilization cycle, polyester/cotton shows a high level of absorption, but the permeability is limited thanks to waterproofing treatments, performed by the manufacturer.

Tychem® used in SARS or VHF contexts

Tyvek is a non-woven product consisting of spunbond olefin fiber. It is a brand of flashspun high-density polyethylene fibers, a synthetic material; the name is a registered trademark of DuPont. The material is very strong; it is difficult to tear but can easily be cut with scissors or a knife. Water vapor can pass through Tyvek, but liquid water cannot.
Tychem® fabric is a Tyvek® brand fabric with additional coating to increase chemical protection.

• Barrier against infectious agents
• Impervious to many inorganic chemicals and particles < 1 μm
• Impervious to liquids and aerosols
• Resistant to splashes up to 2 bars
• Permeable to air and steam
• Antistatic treated
• Stitched and covered seams for better strength and protective barrier

Selection of textiles within MSF

• The single-use non-woven textile is used for single use surgical masks and for the single use surgical linen module → intended for emergencies or kept in “reserve” in surgical programmes.
• Tychem® is used for items intended for the personal protection of medical/non-medical staff and patients’ attendants in contact with suspected or known cases of SARS or viral hemorrhagic fever (Ebola, Marburg, Lassa, etc.).
• Polyester/cotton fabric is used for surgical gowns and drapes (comfort of cotton and long life of polyester)
• 100 % cotton fabric is used for all other articles (easy local supply).

METALS USED IN MD MANUFACTURING

Metals are solid, non-organic materials. They have long been the most common material in medical device manufacturing and are currently used in some way shape or form in 80 percent of all medical devices. The combination of metals with other materials allows the properties of the material to be modified through the creation of alloys. Because most metals oxidize easily, stainless steel—comprising iron, carbon, and chromium—is often the metal of choice for medical device manufacturers. The use of titanium alloys is increasing, in part due to its modulus of elasticity which is closer to that of bone than that of steel.
Steels used in the manufacture of surgical instruments: see introduction ESUR family.

Types of stainless steel

In metallurgy, stainless steel, also known as inox, is a steel alloy with a minimum of 10.5% chromium content by mass and a maximum of 1.2% carbon by mass. Stainless steel is divided into different families depending on its metallurgical structure.

Austenitic steel

These steels are the most common. Their microstructure is derived from the addition of Nickel, Manganese and Nitrogen. It is the same structure as occurs in ordinary steels at much higher temperatures. Corrosion resistance can be enhanced by adding Chromium, Molybdenum and Nitrogen. They cannot be hardened by heat treatment but have the useful property of being able to be work hardened to high strength levels whilst retaining a useful level of ductility and toughness. Higher nickel austenitic steels have increased resistance to stress corrosion cracking. They are nominally non-magnetic but usually exhibit some magnetic response depending on the composition and the work hardening of the steel.

• Grade 302 and 304 =18/8: containing a minimum of 18% chromium and 8% nickel, combined with a maximum of 0.08% carbon
• Grade 316: the carbon content is held to 0.08% maximum, while the nickel content is increased slightly with addition of molybdenum up to a maximum of 3% (it increases the corrosion resistance)

Martensitic steel

These steels are similar to ferritic steels in being based on Chromium but have higher Carbon levels up as high as 1%. This allows them to be hardened and tempered much like carbon and low-alloy steels. They are used where high strength and moderate corrosion resistance is required. They are more common in long products than in sheet and plate form. They have generally low weldability and formability. They are magnetic.

• Grade 410 is the basic martensitic type: 11.5 – 13.5 % chromium, carbon max 0.15%, Manganese max 1%, Silicon max 1%, Nickel 0.75%
• Grade 416 is a modification of type 410: sulfur or selenium have been added

Ferritic steel

These steels are based on Chromium with small amounts of Carbon usually less than 0.10%. These steels have a similar microstructure to carbon and low alloy steels. They are usually limited in use to relatively thin sections due to lack of toughness in welds. However, where welding is not required they offer a wide range of applications. They cannot be hardened by heat treatment. Ferritic steels are also chosen for their resistance to stress corrosion cracking. They are not as formable as austenitic stainless steels. They are magnetic. (Steel grade 430 is the basic type)

Designation of steel types

Steel grades to classify various steels by their composition and physical properties have been developed by a number of standards organizations.

• SAE steel grades: a standard alloy numbering systems for steel grades maintained by SAE International (Society of Automotive Engineers (a U.S.-based professional association and standards developing organization for engineering professionals in various industries)
• British Standards
• International Organization for Standardization ISO/TS 4949:2016
• European standards - EN 10027: designation system for steels:

• Part 1 (2016): Steel names: specifies rules for designating steels by means of symbolic letters and numbers to express application and principal characteristics, e.g. mechanical, physical, chemical, so as to provide an abbreviated identification of steels.

• Part 2 (2015): Numerical system: sets out a numbering system, referred to as steel numbers, for the designation of steel grades. It deals with the structure of steel numbers and the organization for their registration, allocation and dissemination. Such steel numbers are complementary to steel names set out in EN 10027-1.

• Japanese steel grades : Japanese Industrial Standards (JIS) standard
• Germany steel grades : DIN standard
• China steel grades : GB standard

CERAMICS USED IN MD MANUFACTURING

Ceramics play an increasing role in medical devices manufacturing. In materials sciences, the term ceramics applies to solid materials that are neither metallic nor organic. The category includes glass, clay, and concrete. They are usually oxides but can also be carbides, silicides, or nitrides. They are ideal for implantable medical devices as they are chemically nonreactive, good insulators, can be molded at small sizes and do not degrade within the body.