Plastic in various forms has improved nearly every field of medicine and serves a variety of roles in a medical facility. For example, medical plastics can be incorporated in everything from single-use surgical instruments or tubing, to long term spinal and cardiovascular implants.
Some of the most common medical plastics include:
- Polyetheretherketone (PEEK)
- Polypropylene (PP)
- Polyethylene (PE)
- Polystyrene (PS)
- Polyvinyl chloride (PVC)
- Polyethylene terephthalate (PET)
Of these, only a couple medical plastics can be used for long-term implantable devices. Those include some resorbable plastics, like polylactic acid polyglycolic acid, PE and PEEK. While UHMWPE (Ultra-high molecular weight polyethylene) is used in orthopedic applications, PEEK’s material properties make it a frontline choice in an array of more demanding applications. That includes spinal fusion, cardiovascular, dental and trauma fixation applications.
PEEK and CFR PEEK
PEEK can be used in its unfilled form or with the addition of chopped carbon fiber to create CFR PEEK. Both are appropriate in many medical applications, but each possesses its own advantages. Unfilled PEEK is more flexible while still offering strong tensile strength. Unfilled PEEK is a frontline choice in spinal fusion and cardiovascular applications. Both grades are fully radiolucent.
CFR PEEK provides added tensile strength and a greater modulus and thereby is used for trauma applications like bone plates and screws. Both PEEK options are similar to cortical bone’s tensile strength making each an excellent choice in loadbearing or load sharing applications, including applications where bone healing is necessary. CFR PEEK can also be used in some dental implants.
Why are medical plastics replacing other biomaterials?
There are only a handful of materials that are biocompatible and effective enough to be used in the human body. Until the introduction of medical plastics, metal and ceramic were the biomaterials of choice, and titanium remains a primary biomaterial today. High-performance polymers like PEEK and UHMWPE, though, are replacing those biomaterials in many important applications. There are a couple reasons for this, including:
- Stress shielding and subsidence – PEEK and CFR PEEK, due to their cortical bone-like moduli, can be used as load sharing materials. While titanium and ceramics are structurally strong enough to work as implants, their high moduli mean they bear too much weight in many instances, robbing nearby bone of important, constructive stresses. Over time, this can lead to loss of bone mineral density, and possible subsidence, or caving in. In multiple studies comparing PEEK and titanium, titanium has demonstrated higher rates of subsidence.
- Efficient manufacturing – In some applications, including arthroscopic and trauma fixation applications, PEEK has shown resiliency, tensile strength and pull out strength comparable or favorable to other biomaterials. However, PEEK possesses a processability advantage over metal and ceramic, as it can be easily injection molded.Injection molding is an economical manufacturing method that is particularly cost-efficient when used to create high component volumes such as anchors, screw systems and any other component needed in large quantities.
What makes PEEK a medical plastic?
PEEK, like every biomaterial, has been subjected to extensive biocompatibility testing to ensure it is safe for use in the human body. ISO 10993 details the most up-to-date testing protocols available to medical device manufacturers, and several of those protocols are relevant to PEEK implant testing. They include:
Cytotoxicity testing –
Cytotoxicity testing assesses whether the material is likely to cause cell damage. This is usually done by exposing a cell culture directly to a material sample, and then incubating both. Following incubation, the cells are checked for any lysis or malformation, as this potentially indicates cytotoxicity.
Sensitization testing –
Sensitization testing checks for an immunological response to the material, which may indicate the material is causing adverse systemic effects to the subject. To perform this test, material extracts are applied to subjects and monitored for a response.
Irritation testing –
Irritation testing looks for the material’s irritant potential, either to the skin or mucosal membranes. For implantable materials, this test is normally done intracutaneously, so extracts are injected under the skin, and any signs of redness or swelling are noted.
Acute system toxicity testing –
Acute systemic toxicity testing checks for systemic (in contrast to local) effects to the body following extended exposure. Material extracts are injected in a group of test subjects, and are checked immediately for any adverse signs. The subjects are checked several more times before the test’s conclusion, to verify that the material is not affecting the subject’s vitals.
Genotoxicity testing –
Genotoxicity testing considers the material’s mutagenic potential, or its potential to cause genetic damage to the subject. During genotoxicity testing, the test subjects are salmonella bacteria, which are sensitive to mutagens. Following exposure to the material, only mutated salmonella will survive, so researchers can note the prevalence of mutations among the bacteria.
Implantation testing –
Implantation testing is designed to simulate the implantation environment, so the material is exposed to the tissues it is likely to contact when used in a human body. After extended implantation in an animal subject, tissue samples are taken from the subject and inspected microscopically for any signs of disease.
There are many medical plastics, but only a couple, like PEEK grades, are safe enough to use in the body. These plastics have undergone comprehensive biocompatibility testing to earn that distinction, and have proven their effectiveness in an array of medical applications.
What is the difference between PEEK and CFR PEEK?
PEEK is an unfilled high-performance polymer, while CFR PEEK is PEEK polymer with the addition of chopped carbon fiber. CFR PEEK offers additional stiffness and strength, while unfilled PEEK provides additional flexibility. Both are radiolucent.
What standard is used to regulate medical plastics?
Medical plastics are tested for biocompatibility, and the recommended biocompatibility testing procedures are outlined in ISO 10993. ISO 10993 and ISO 13485 (which regulates medical device manufacturers) meet the regulatory guidelines of European and Asian agencies. The FDA also considers ISO standards to be sufficient for most medical devices, and will consider it during premarket approval.
Why is PEEK replacing titanium in bone healing applications?
Titanium, ceramic and other loadbearing biomaterials are strong and durable, but they may rob neighboring bone of important, bone-stimulating stresses. This is called stress shielding, and it can result in bone mineral density loss in nearby bone. That can lead to subsidence, or caving in of bone.
PEEK subsidence rates are much lower, according to several research studies. PEEK can also be injection molded, which gives it a major processability advantage in many medical applications.