Devices made from animal tissue
A device built from porcine or bovine tissue carries risks of bio-contamination and bio-incompatibility that come from its animal source, and the evidence a manufacturer must write to answer them is specified in its own series of standards. That evidence turns on why animal tissue is used, the hazards the risk analysis has to address, the controls on sourcing the tissue, how viral and prion safety are demonstrated, and the zoonosis surveillance that keeps the analysis current.
Animal-derived tissue and its uses
Regenerative medical products derived from animal tissue have been used to successfully treat millions of patients. The use of animal-derived tissue, as a stand-alone regenerative construct or as a delivery platform for biological agents, is an area of extensive research and development in regenerative implants. Clinically, regenerative applications of animal-derived tissue are in widespread use in wound care, orthopaedics, dentistry, cardiology, general surgery, urogynaecology, neurology and other fields. Some commonly known animal-derived products include advanced wound dressings, synthetic bone grafting materials, dural substitutes and biological heart valves. Tissue from pigs and cattle is by far the most used in commercially approved animal-derived products, in Europe and elsewhere, given its abundance, relatively low cost and the controls that governmental and industrial regulations place on it.
The fundamental scientific reason animal-derived tissue makes an ideal implantable material is the extensive homology between animal and human extracellular matrices. That homology is mainly due to collagen and other macromolecules such as elastin, hyaluronic acid and sulphated glycosaminoglycans, which are the principal components of all extracellular matrices across species and are remarkably similar in structure among mammals. Given that similarity, the main concern in implanting animal tissue is the human antigenic response to the animal cells and nucleic acids within it. An entire field has therefore emerged that engineers and manufactures extracellular matrices and collagen so as to remove those cellular and nucleic acid components, with the goal of leaving the extracellular matrix intact. Decellularised animal-derived extracellular matrix tissue becomes almost humanised in its resemblance to its human counterpart.
The ISO 22442 series
Because they are manufactured from animal sources, animal-derived regenerative products carry bio-contamination and biocompatibility risks that must be addressed in accordance with ISO 22442 for the CE mark and subsequent commercialisation in the EU. Whatever the animal source, compliance with several important standards must be demonstrated for the CE mark, and of these the animal tissue sourcing requirements of the ISO 22442 series are the most relevant. Those requirements include risk management, animal health controls, quality system elements and the demonstration of safety with respect to potentially transmissible pathogens, and the information they call for needs to be presented as reports, risk analyses and other documentation. To protect patients from bio-contamination and bio-incompatibility risks, device manufacturers must adhere to the animal tissue sourcing requirements of ISO 22442. One developer of such products, with more than 25 years in the field, holds that compliance with these standards will ensure that animal-derived devices remain safe for patients.
Risk analysis under ISO 22442-1
ISO 22442-1 sets out risk management requirements that relate primarily to the risks of bio-contamination and bio-incompatibility arising from the use of animal-derived tissue. The device manufacturer needs to provide a risk analysis that considers these possible product hazards:
- Parasites and unclassified pathogenic entities
- Bacteria, moulds and yeasts
- Viruses
- Transmissible spongiform encephalopathy agents
- Pyrogenic, immunological and toxicological reactions
ISO 22442-1 compliance also involves a justification of why animal tissue is required rather than a synthetic alternative, a material from a less risky animal species or a material of human origin. Such a justification typically involves a scientific and clinical review of the relevant literature and an overall assessment of the product's risk-to-benefit ratio.
Sourcing, collection and handling under ISO 22442-2
ISO 22442-2 provides the controls for animal-derived tissues and substances at every level of the supply chain, setting requirements for farms, abattoirs and device manufacturers, with traceability requirements defined on the basis of risk management. At the farm, veterinary oversight is required to monitor animal health, and animals must be judged fit for human consumption in a post-mortem inspection by an animal-health official. Procedures must be in place to prevent cross-contamination and to give specific instructions for the collection and handling of tissue, its storage and transport, labelling requirements and auditing responsibilities.
Viral safety under ISO 22442-3
Viral safety for a device derived from animal sources must be demonstrated through a concept similar to the sterility assurance level for sterile devices. The manufacturer must establish that its manufacturing process inactivates or eliminates potential viral contaminants to a level that makes the device a low viral risk, a level quantified as a log reduction value from a viral clearance study. Elimination removes viruses from the tissue substrate while they remain intact, whereas inactivation alters a virus through the manufacturing process so that it is no longer infectious.
The work typically begins with a formal literature review conducted to a protocol whose requirements ISO 22442-3 stipulates. The review identifies the viruses relevant to the zoonosis situation of the tissue source. This lets the manufacturer identify specific viral risks and determine the approach its manufacturing process needs to eliminate any zoonotic viruses identified, and the same information is used to select the virus panel for the viral inactivation studies that follow. The review also determines which manufacturing steps may inactivate or eliminate viruses and what theoretical reduction levels the literature suggests those steps may achieve. Sterilisation usually plays a key role in viral inactivation, and radiation and chemical sterilisation methods, for example, usually offer significant inactivation capability. In this planning phase key considerations for viral inactivation are also evaluated, including the effects of the tissue substrate and of preceding manufacturing steps on the efficiency of viral inactivation. Finally, the review identifies the key processing parameters that affect viral inactivation, which are essential for establishing the critical process controls that must be maintained for eventual validation of viral inactivation.
In most cases the manufacturer then proceeds with a viral inactivation study that evaluates how effectively the selected manufacturing steps inactivate or eliminate viruses, with the goal of generating log reduction values. In some circumstances a manufacturer may instead rely on the literature and a risk-based approach. This may occur when literature studies are so highly relevant that they closely match a viral inactivation step of the manufacturing process and their log reduction values are directly applicable. Manufacturers may also take a hybrid approach that is both literature-based and uses viral inactivation studies.
The design and execution of viral elimination and inactivation studies must follow the requirements of ISO 22442-3. Before formal studies are run, a protocol should be written that documents:
- The risks identified under ISO 22442-1
- The anticipated zoonotic viral agents
- A relevant virus panel, which usually includes four virus types: RNA and DNA viruses, both enveloped and non-enveloped
- The manufacturing processes selected to eliminate or inactivate viruses
- A demonstration that the scaled-down process used in a viral testing laboratory is valid in comparison with the actual full-scale production process
- The methods for calculating viral reduction factors and, when applicable, for estimating reduction kinetics
The studies are then conducted in accordance with the protocol, and log reduction values are obtained for each manufacturing step evaluated. The last requirement of the ISO 22442-3 process is a final report that includes the literature review and the information from the executed studies. The report gives the efficacy of the overall manufacturing process in reducing the four virus types, and it identifies the critical manufacturing parameters, with limits, that need to be maintained during production for assurance of viral inactivation.
Transmissible spongiform encephalopathy agents under ISO 22442-4
For animal sources that represent a transmissible spongiform encephalopathy (TSE) risk, such as cattle, ISO 22442-4 provides information on inactivating TSE agents analogous to the viral inactivation requirements of ISO 22442-3. TSE inactivation studies are rarely performed for tissue and collagen materials, because the proven methods of inactivating the agent, the abnormal prion protein, are impractical in making a device: incineration, chlorination, and strong alkali combined with substantial heat all effectively destroy tissue and collagen preparations. When such studies cannot be performed, the risk of TSE contamination is mitigated by an alternative risk management strategy under ISO 22442-1, which usually depends on sourcing controls. Almost all tissue and collagen preparations are generally derived from connective tissue, which is known to carry a low TSE infectivity risk. Combined with strong governmental agricultural controls, sourcing control is usually accepted by regulatory bodies as demonstrating an acceptable risk of TSE transmission.
Zoonosis surveillance
ISO 22442-1 requires surveillance of animal zoonoses, to provide ongoing reassessment of the risk analysis and to show whether the risk profile associated with sourcing the tissue has changed. For example, porcine materials are inherently less risky than equivalent bovine materials, because a TSE agent, bovine spongiform encephalopathy, has been found in cattle while, as of 2022, no TSE agent had been found in pigs. A manufacturer of porcine materials used in its device must therefore continuously monitor the scientific literature for evidence of a possible porcine spongiform encephalopathy agent, which, if ever found, could greatly affect the risk profile of the device.
In 2022 the major zoonosis concerns were the SARS-CoV-2 coronavirus and, for porcine materials, the African swine fever virus. Manufacturers had incorporated SARS-CoV-2 into their surveillance, and commercial pig and cattle livestock had not been shown to be infected with it. Even if livestock were infected, it is likely that most tissue and collagen manufacturers would find their existing methods highly effective at inactivating SARS-CoV-2, because coronaviruses are enveloped RNA viruses highly susceptible to the processes typical of collagen manufacture, which include alcohol and alkaline treatments, radiation sterilisation and other physicochemical methods.
African swine fever is a deadly disease of pigs that, as of 2022, had seen widespread outbreaks across Africa, Asia and Europe since 2007. Its causative virus threatens pigs but has no impact on human health, so the main concern for manufacturers is a supply chain in which raw tissue may become limited. As of 2022, the animal controls instituted by governmental agencies had prevented any significant contamination of commercial pig livestock in most geographic markets, but vigilance and monitoring for the disease need to be maintained.