Residual toxicity assessment is a critical element of the biological evaluation of medical devices, verifying that residual solvents, sterilant by-products, monomers, heavy metals, and leachable additives do not expose patients to unacceptable toxicological risk. Guided by ISO 10993-1, ISO 10993-17, and ISO 10993-18, this evaluation combines chemical characterization, extractables and leachables (E&L) studies, and quantitative toxicological risk assessment to demonstrate device safety for regulatory submissions worldwide. This article provides a technical overview of the assessment framework, applicable sample types, testing items, analytical methods, and frequently asked questions.
Technical Overview
Residual toxicity assessment evaluates whether chemical substances that remain in, or migrate from, a medical device during clinical use could expose patients to toxicologically significant levels. These residues originate from multiple stages of the product life cycle: polymer synthesis (unreacted monomers, oligomers, catalysts), manufacturing and assembly (cleaning agents, solvents, mold-release agents, adhesives), sterilization (ethylene oxide and its reaction by-products), and product storage (leachables transferred from packaging, labels, and inks). Because such substances are frequently present at trace concentrations yet introduced directly into contact with tissue or the circulatory system, they represent a distinct hazard pathway that cannot be fully addressed by standard biocompatibility endpoint testing alone.
The evaluation is anchored in ISO 10993-1, which mandates biological evaluation within a risk management process, and is executed primarily through two standards: ISO 10993-18 for chemical characterization of materials and ISO 10993-17 for toxicological risk assessment of device constituents. Device-specific residue limits for ethylene oxide (EO), ethylene chlorohydrin (ECH), and ethylene glycol (EG) are defined in ISO 10993-7. Complementary guidance includes USP General Chapters <1663> and <1664> for extractables and leachables study design, ICH Q3D for elemental impurities, ICH Q3C for residual solvents, ISO 10993-12 for sample preparation, and ISO 14971 for integration into the overall device risk management file.
The typical workflow proceeds in four stages: first, materials are characterized to build a constituent inventory; second, extraction studies under simulated-use, accelerated, or exaggerated conditions generate extractables profiles; third, target analytes are quantified with validated analytical methods; and fourth, a toxicological risk assessment derives a tolerable intake for each constituent, estimates patient exposure from the device, and calculates a margin of safety. A device is considered acceptable when every identified constituent demonstrates a margin of safety greater than one, or when the level falls below the threshold of toxicological concern (TTC).
Testing Samples
Blood-Contact and Implant Devices
Devices with the highest exposure intensity and longest patient contact require the most rigorous residual evaluation. Representative samples include central venous and peripheral catheters, drug-eluting and bare-metal stents, heart valves, vascular grafts, orthopedic implants and bone cements, spinal fixation systems, dental composites, intraocular lenses, and absorbable sutures. For implantable and long-term circulating devices, degradation products and cumulative exposure over the full implantation period must be considered in addition to process residues.
Externally Communicating Devices
Samples in this category indirectly contact the patient through fluid or gas pathways: infusion sets, syringes and needles, insulin pen components, blood dialyzers and extracorporeal tubing sets, blood bags, enteral feeding tubes, oxygenators, heat-moisture exchangers, and anesthesia breathing circuits. The extraction medium and residence time should reflect the actual clinical pathway, such as drug carrier solutions, parenteral nutrition formulations, or ventilated gases.
Surface Contact Devices and Packaging Components
Wound dressings, hydrogels, surgical drapes and gowns, examination gloves, electrodes, and compression bandages are evaluated for skin-contact migration of plasticizers, preservatives, and adhesive residuals. Container closure systems, film laminates, sterilization pouches, secondary packaging, labels, and printing inks are also tested as potential sources of leachables that migrate into the device or its drug or fluid contents during shelf-life storage.
Raw Materials and Process Chemicals
Upstream assessment of polymer resins, masterbatches, colorants, thermoplastic elastomers, silicone tubing, adhesives, coatings, lubricants, mold-release agents, and cleaning solvents allows manufacturers to control residue sources before design freeze. Supplier certificates and formulation disclosure substantially reduce the analytical burden of finished-device characterization.
Testing Items
The scope of residual toxicity testing is tailored to device materials, manufacturing and sterilization processes, and patient exposure route and duration. Common testing items include:
- Ethylene oxide (EO), ethylene chlorohydrin (ECH), and ethylene glycol (EG) residues following gas sterilization, evaluated against ISO 10993-7 limits for the relevant device category and exposure duration.
- Residual process solvents classified under ICH Q3C, including methanol, ethanol, acetone, toluene, hexane, dichloromethane, dimethylformamide, and methyl ethyl ketone.
- Residual monomers and reactive starting materials, such as methyl methacrylate, styrene, acrylonitrile, vinyl chloride, bisphenol A, isocyanates, and acrylate crosslinkers.
- Plasticizers and phthalates, including DEHP, DBP, DINP, DINCH, adipates, and citrate esters.
- Antioxidants and polymer additives, such as hindered phenols, phosphite stabilizers, thioesters, UV absorbers, slip agents, and antistatic agents.
- Elemental impurities and heavy metals, including lead, cadmium, mercury, arsenic, chromium VI, nickel, cobalt, and tin, screened in alignment with ICH Q3D and ISO 10993-18.
- Nitrosamines and nitrosatable amines, particularly for elastomers, latex components, and drug-device combination products.
- Polycyclic aromatic hydrocarbons (PAHs) in carbon-black-filled rubber and elastomeric components.
- Per- and polyfluoroalkyl substances (PFAS) associated with fluoropolymer coatings, lubricants, and high-performance tubing.
- Radiolysis and process degradation products generated by gamma, electron-beam, or EtO sterilization.
- Hydrolytic and enzymatic degradation products of absorbable polymers, including lactide, glycolide, and caprolactone monomers and oligomers.
- Unknown extractables screened against toxicological thresholds such as the safety concern threshold (SCT) of 1.5 micrograms per day and the qualifying threshold (QT) defined in ISO 10993-18.
Testing Methods
Extraction Study Design
Extraction conditions are selected to bracket or simulate clinical exposure in accordance with ISO 10993-12 and ISO 10993-18. Simulated-use extraction replicates actual patient contact; accelerated extraction uses elevated temperature over shortened durations to model long-term contact; and exaggerated extraction maximizes residue release to establish a worst-case profile. Polar (water, saline, or aqueous ethanol), semi-polar, and non-polar (hexane or heptane) solvents are chosen to cover the polarity range of expected constituents, with extraction ratios normalized to device surface area or mass. Drug-device combination products and fluid-path devices may require extraction in the actual carrier fluid or pharmaceutical formulation.
Analytical Techniques
Validated instrumental methods are applied based on analyte volatility and polarity. Headspace gas chromatography with flame ionization or mass spectrometric detection (HS-GC-FID/MS) quantifies residual solvents and EO/ECH. Gas chromatography-mass spectrometry (GC-MS) covers volatile and semi-volatile organics including monomers, plasticizers, and PAHs. Liquid chromatography with ultraviolet, fluorescence, or tandem mass spectrometric detection (HPLC, LC-MS/MS) targets non-volatile additives, antioxidants, nitrosamines, and PFAS. Inductively coupled plasma mass spectrometry or optical emission spectrometry (ICP-MS, ICP-OES) quantifies elemental impurities, while ion chromatography addresses inorganic anions and cations. Fourier-transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR) support structural identification of unknown extractables, often complemented by high-resolution mass spectrometry. Method validation follows ICH Q2 requirements for specificity, linearity, accuracy, precision, detection limit, and quantitation limit.
Toxicological Risk Assessment
Per ISO 10993-17, each reported constituent undergoes a structured evaluation. Hazard identification draws on authoritative sources including ECHA registration dossiers, IARC monographs, US EPA toxicity databases, and peer-reviewed literature. Dose-response assessment identifies the no-observed-adverse-effect level (NOAEL) or benchmark dose (BMD), from which a tolerable intake (TI) is derived by dividing the point of departure by uncertainty factors accounting for interspecies and inter-individual variability. The tolerable exposure (TE) is further adjusted for device-specific exposure duration and route. Patient exposure is estimated from the measured leachable concentration, the volume of fluid contacting the device, the number of devices used per day, and patient body weight. The margin of safety (MOS) equals the tolerable exposure divided by the estimated patient exposure; a value greater than one indicates an acceptable risk. For constituents lacking adequate toxicity data, the threshold of toxicological concern approach using the 1.5 microgram per day safety concern threshold, supported by ISO/TS 21726 dose-duration extrapolation, provides a conservative screening criterion.
Applications
Residual toxicity assessment outputs directly support regulatory submissions and product lifecycle management across major jurisdictions:
- Regulatory dossiers for FDA 510(k), De Novo, and PMA submissions, EU MDR technical documentation and CE marking, NMPA registration in China, and PMDA submissions in Japan.
- Biological evaluation reports and gap assessments demonstrating conformity with the ISO 10993-1 test matrix justified through chemical characterization.
- Sterilization validation and routine release testing for ethylene oxide residuals in accordance with ISO 10993-7 and ISO 11135.
- Material, supplier, or manufacturing site change control, including comparability studies that verify the absence of new or elevated residues.
- Design verification for drug-device and biologic-device combination products, where extractables and leachables interact with product safety and stability.
- Support of ISO 14971 risk management files, benefit-risk analyses, and biological safety assessments required by notified bodies and competent authorities.
- Post-market surveillance activities, including complaint investigation, adverse event analysis, and periodic safety review of long-term implantable devices.
FAQ
When is a residual toxicity assessment required?
An assessment is required whenever a device contains substances that may migrate to the patient during clinical use, including all body-contacting devices under ISO 10993-1. It is mandatory for devices sterilized with ethylene oxide, devices manufactured with known toxic solvents or monomers, drug-device combination products, devices with extended contact durations exceeding 30 days, and any device where chemical characterization reveals constituents exceeding toxicological screening thresholds.
What is the difference between extractables and leachables?
Extractables are compounds released from device materials under exaggerated laboratory conditions such as elevated temperature and aggressive solvents, representing a worst-case inventory. Leachables are compounds that actually migrate into the patient-contacting fluid or tissue under normal storage and clinical-use conditions. Leachables are typically a subset of extractables, and simulated-use leachables data feed directly into the patient exposure calculation.
How is the margin of safety calculated?
The margin of safety equals the tolerable exposure divided by the estimated patient exposure for a given constituent. The tolerable exposure is derived from the tolerable intake adjusted for exposure duration and route, while patient exposure is calculated from the measured leachable concentration, contact volume, daily device usage, and patient body weight. A margin of safety greater than one indicates the residue level is toxicologically acceptable.
What happens if the margin of safety is less than one?
If a constituent exceeds the tolerable exposure, manufacturers can reformulate the material, modify the sterilization or cleaning process, add a degassing or post-curing step, extend the aeration cycle for EO-sterilized devices, redesign the device to reduce contact, or justify a higher tolerable intake with more robust toxicological data. A transparent benefit-risk rationale consistent with ISO 14971 may support acceptance where the clinical benefit outweighs the residual risk.
Can literature data replace analytical testing?
Complete formulation disclosure from suppliers combined with published migration data can sometimes justify a reduced testing scope, but regulatory authorities generally expect at least confirmatory analytical verification on the finished device for critical residues such as EO/ECH and known genotoxicants. Documented material characterization per ISO 10993-18 remains the foundation of any biological evaluation justification.
What sample quantities and turnaround times are typical?
Sample requirements depend on device geometry, but testing generally requires finished devices or material specimens equivalent to 5 to 25 grams or 100 to 500 square centimeters of contact surface. Analytical testing typically requires two to eight weeks, while a full chemical characterization and toxicological risk assessment program, including method development and reporting, generally spans eight to sixteen weeks. Accredited third-party laboratories with dedicated E&L and toxicology teams can provide protocol design, testing, and assessment as an integrated package to streamline regulatory review.