Biomaterials Unveiled: A Journey Through History, Qualification, and Diversity

For more than 100 years, metals and metal alloys (combinations of metal elements) have been widely used in medical implants across multiple clinical specialties including orthopedics, cardiology, dentistry, and neurology. These materials provide the strength, durability, and biocompatibility required for long-term implantation within the human body.

This article reviews current scientific understanding of metallic biomaterials, focusing on how metallic materials interact with physiological environments, how the immune system responds to implanted metals, and the potential clinical manifestations associated with metal exposure from implantable medical devices.

The discussion integrates concepts from biomaterials science, toxicology, physiology, and regulatory biocompatibility assessment frameworks, including principles used in ISO 10993 biological evaluation.

What are Biomaterials?

Biomaterials play an integral role in modern medicine by restoring biological function, supporting tissue healing, and replacing damaged structures. A biomaterial is any natural or synthetic material designed to interact with biological systems for medical purposes.

Historically, biomaterials have been used for thousands of years. One of the earliest documented uses dates back to ancient Egypt, where sutures made from animal sinew were used for wound closure. Today, the field of biomaterials integrates knowledge from medicine, biology, chemistry, materials science, and biomedical engineering.

Recent advances in tissue engineering and regenerative medicine have further expanded the scope of biomaterials research, enabling the development of sophisticated implants, bioactive scaffolds, and controlled drug-delivery systems.

Modern biomaterials may include metals, ceramics, polymers, glasses, and even biological tissues or living cells. These materials can be engineered into multiple forms including molded components, coatings, fibers, films, foams, and scaffolds used in biomedical devices.

Examples of medical products incorporating biomaterials include:

• Heart valves and cardiovascular stents
• Orthopedic implants such as hip and knee replacements
• Dental implants and prosthetics
• Contact lenses and ophthalmic implants
• Neural stimulation devices

How Biomaterials Are Used in Current Medical Practice

Doctors, biomedical engineers, and researchers utilize biomaterials across a wide range of medical technologies.

• Medical Implants – Artificial joints, heart valves, vascular grafts, dental implants, and neurostimulation devices.
• Wound Healing Materials – Sutures, clips, staples, and bio-absorbable dressings that facilitate tissue repair.
• Tissue Engineering – Biomaterial scaffolds combined with cells and bioactive molecules to regenerate tissues.
• Nanoscale Medical Technologies – Molecular probes and nanoparticles for imaging and targeted therapy.
• Biosensors – Devices capable of detecting biological signals such as glucose concentration or neurological activity.
• Drug Delivery Systems – Biomaterial systems designed to release therapeutic agents at targeted locations.

Recent advances in additive manufacturing (3D printing) and advanced metallurgy allow production of customized implants with optimized mechanical properties and patient-specific geometries.

Regulatory Insight – Additive Manufacturing of Medical Implants

Regulatory agencies including the FDA and European regulators recognize additive manufacturing (3D printing) as an emerging method for producing customized implants. However, manufacturers must demonstrate that additively manufactured metal implants meet requirements for:

• Material composition consistency
• Mechanical strength and fatigue resistance
• Surface finish and porosity control
• Biocompatibility and corrosion resistance

These evaluations are critical to ensure the long-term safety of patient-specific implants.

Materials in Human Physiology and Pathology

Several metals commonly used in medical implants also play essential roles in human physiology. Examples include iron, copper, zinc, manganese, and cobalt, which serve as cofactors for enzymes and proteins involved in metabolism, oxygen transport, and immune function.

These metals are required only in small amounts and are therefore classified as essential trace elements.

In contrast, other metals frequently encountered in implants such as nickel, titanium, or aluminum are not required for normal biological function. These are considered non-essential metals.

However, both essential and non-essential metals can disrupt biological systems when present at elevated concentrations, leading to toxicological responses including:

• Cellular oxidative stress
• Inflammatory immune reactions
• Tissue degeneration
• Organ toxicity affecting the liver, kidneys, nervous system, or cardiovascular system

Understanding these biological responses is critical for the biological safety assessment of medical devices.

Regulatory Insight – ISO 10993 Biological Evaluation

According to ISO 10993-1: Biological Evaluation of Medical Devices, the biological safety of materials used in medical devices must be evaluated based on the nature and duration of patient contact. For implantable medical devices, regulators require assessment of potential biological risks including:

• Cytotoxicity
• Sensitization
• Irritation or intracutaneous reactivity
• Systemic toxicity
• Genotoxicity
• Implantation effects

Metal-containing implants must also be evaluated for potential ion release, corrosion, and long-term tissue responses that may occur within the physiological environment. Reference: ISO 10993-1 Biological Evaluation of Medical Devices.

Classification of Implant-Related Materials

Materials encountered in medical implants can broadly be categorized into three groups:

1. Essential Trace Metal Elements
2. Non-Essential Metals
3. Non-Metal Biomaterials

Essential Trace Metal Elements

Trace elements are metals present in small concentrations within biological systems but essential for proper physiological functioning. These elements support enzymatic reactions, metabolic pathways, and structural functions of proteins.

Regulatory Insight – FDA Guidance on Metallic Implants

The U.S. FDA recommends that manufacturers of metallic implantable devices evaluate potential risks associated with:

• Metal ion release
• Wear debris generation
• Corrosion mechanisms
• Hypersensitivity reactions
• Systemic metal exposure

Risk assessment typically includes material characterization, corrosion testing, wear testing, and biological safety evaluation under the ISO 10993 framework. Reference: U.S. FDA – Biological Evaluation of Medical Devices Guidance.

An element is typically considered essential if:

• It is consistently present in living tissues
• Its deficiency produces physiological abnormalities
• These abnormalities can be reversed by supplementation

Examples include: Cobalt, Copper, Iron, Manganese, Molybdenum, Zinc, Chromium, and Vanadium.

Many implant alloys such as stainless steel and cobalt-chromium alloys contain these elements.

Metal	Major Physiological Roles of Proteins Utilizing the Metal	Key Manifestations of Deficiency	Potential Toxicities or Manifestations of Excess
Cobalt (Co)	Metabolism of purines and pyrimidines Amino acids, fatty acids, folate metabolism	Anemia Neuropathy Neurocognition changes	Allergic contact dermatitis (ACD) Cardiomyopathy Polycythemia Altered thyroid function
Copper (Cu)	Collagen cross-linking Bone formation Iron metabolism Hemostasis / thrombosis Neurotransmitter synthesis Free radical control	Iron-refractory anemia Neutropenia / infection Osteoporosis Neurological dysfunction	Gastrointestinal symptoms Hemolysis Cardiac failure Renal failure Hepatic dysfunction Association with Alzheimer’s disease
Iron (Fe)	Oxygen transport Oxygen storage DNA synthesis and repair RNA transcription Collagen and neurotransmitter synthesis Energy metabolism Immune function	Microcytic anemia Diminished thyroid function Impaired neutrophil function Impaired cognition	Free radical generation Acute gastrointestinal symptoms Hemochromatosis Cardiomyopathy Cirrhosis Diabetes Arthritis
Manganese (Mn)	Metabolism of carbohydrates and lipids Neurotransmitter synthesis Bone and cartilage formation Urea metabolism Control of free radicals	Dermatitis Weight loss Growth retardation Abnormal bone/cartilage formation Dyslipidemia Glucose intolerance	Headache Psychiatric symptoms Gastrointestinal symptoms Parkinson-like neurological symptoms
Molybdenum (Mo)	Amino acid metabolism Purine and nucleotide metabolism Drug and prodrug metabolism Neurotransmitter metabolism	Urinary tract stones Acute renal failure Myositis Mental changes / coma	Elevated uric acid / gout Secondary copper deficiency Reduced testosterone
Zinc (Zn)	Protein and carbohydrate metabolism Immune function Wound healing DNA synthesis and repair Free radical control Protein stabilization Intracellular signaling	Skin and mucosal changes Reduced immune function Delayed wound healing Neurological dysfunction Bleeding abnormalities Osteoporosis Delayed growth	Acute gastrointestinal symptoms Copper deficiency Myeloneuropathy
Chromium (Cr)	Glucose metabolism and tolerance Lipid metabolism	Impaired glucose tolerance Abnormal lipid profiles Peripheral neuropathy	Cr³⁺: Potential liver and kidney issues Cr⁶⁺: Respiratory symptoms, dermatitis, GI symptoms, lung cancer
Vanadium (V)	Phosphate metabolism Insulin enhancement Lipid metabolism	—	Gastrointestinal symptoms Headache Weakness Tremor

Abbreviations: ACD – Allergic Contact Dermatitis; GI – Gastrointestinal; UA – Uric Acid.

Non-Essential Metals

Although not required for human physiology, several metals are widely used in medical device manufacturing due to their favorable mechanical properties and corrosion resistance.

Examples include: Nickel, Titanium, Aluminum, Silver, Gold, Palladium, Platinum, Tin, Tungsten, and Iridium.

These metals may still influence biological processes under certain exposure conditions and therefore must be evaluated carefully during the biocompatibility assessment of implantable medical devices.

Metal	Potential Adverse Effects	Other Commercial Uses
Nickel (Ni)	Delayed hypersensitivity reactions Acute exposure: gastrointestinal symptoms, headache, vertigo, vision changes Chronic exposure: Altered iron metabolism Cardiovascular, respiratory, or kidney disease Alteration in hemostasis of calcium, magnesium, manganese, and zinc	—
Titanium (Ti)	Suppression of osteogenic differentiation Yellow nail syndrome	Used in sunscreens and anti-tumor preparations
Aluminum (Al)	Osteomalacia Hepatic dysfunction Anemia Dialysis encephalopathy (dementia, myoclonus) Association with Alzheimer’s disease	Frequently used in antacids, toothpaste, antiperspirants, and sunscreens
Silver (Ag)	Local argyria (blue-grey skin or organ discoloration)	Sometimes used for antimicrobial properties
Gold (Au)	Bone marrow suppression Dermatitis Glomerulonephritis Vasculitis Hepatotoxicity Neuropathy	Intramuscular gold therapy historically used for rheumatoid arthritis
Palladium (Pd)	Lip edema Itching Respiratory symptoms	—
Platinum (Pt)	Respiratory symptoms Kidney toxicity Hearing loss Bone marrow damage	Cisplatin used in cancer therapy
Tin (Sn)	Acute gastrointestinal symptoms Headache Altered metabolism of zinc, iron, and copper Changes in cholesterol metabolism	—
Tungsten (W)	Certain compounds may antagonize molybdenum	—
Iridium (Ir)	Some salts may cause allergic reactions (ACD)	—

Non-Metal Biomaterials

• Medical Plastics – Used in catheters, IV bags, and tubing due to flexibility and chemical resistance.
• Silicone Rubber – Widely used for flexible implants and tubing due to durability and biocompatibility.
• Medical Ceramics – Used in dental implants and orthopedic devices because of high wear resistance.
• Biological Materials – Collagen, fibrin, and tissue scaffolds used in regenerative medicine.

Characteristics Required for Qualification as Biomaterials

The essential requirements governing medical devices have already been outlined. The best possible material to be used for a medical device in any particular case will be determined by its technical design and constructional features. However, under certain circumstances one single material will not be able to meet these requirements. Depending on the required properties, several materials may be combined and therefore in devices composed of many single parts, these individual components will be made of different materials.

The requirements governing the properties of the materials used to manufacture medical devices are determined by the intended use and processing method as well as by the manufacturing process. The following factors are taken into account: 

• Biocompatibility (ISO 10993): Nature of contact, duration of contact when device is used as intended > biological effect, e.g. cytotoxicity, genetoxcity
• Diagnostic properties: XAray transparency
• Thermal properties: Thermal conductivity, thermoforming and temperature resistance
• Chemical properties: Hydrolysis and chemical resistance during reprocessing (washer-disinfector), stress cracking susceptibility
• Electrical properties: Insulation properties – breakdown strength, conductivity, surface resistance, etc.
• Optical properties: Colours, transparency, reflexion, etc.
• UV resistance properties: Colour fastness, mechanical properties
• Processing properties: Injection moulding, extrusion, mechanically workable, malleable, rollable, temperable
• Mechanical properties: Tensile strength, stiffness, toughness, impact resistance, etc.

Based on knowledge of the properties of the materials and their behaviour, when also exposed to the effects generated by the medical device circuit such as temperature, pressure, chemical and mechanical influences, materials are chosen in accordance with their intended purpose from the range of materials available. After verifying the actual properties and only if these specifications are met are the raw materials forwarded to the manufacturing process. The initial inspections conducted to determine suitability include:

• Precise shape and dimensions
• Surface quality (cracks, pores)
• Material analysis (quantitative determination of alloy elements)
• Mechanical and technical properties (hardness, strength)
• Structural condition (grain size, purity degree, evenness)
• Malleability, temperability, corrosion resistance

Various nonclinical and clinical assessments can be used to understand whether materials used to manufacture medical devices can cause adverse biological responses. Corrosion and other physical or chemical processes can lead to the release of metal ions and small particles, which may cause adverse tissue responses at the site of the implant, as well as in other places in the body. For metal devices, immunological reactions and local changes in tissues surrounding an implant are the most reported issues.

Material Evaluation and Quality Verification

Before materials are used in manufacturing medical devices, several quality assessments are performed to ensure they meet design specifications.

• Dimensional accuracy
• Surface integrity (cracks or pores)
• Material composition and alloy analysis
• Mechanical strength and hardness
• Grain structure and material purity
• Corrosion resistance

Biological Safety of Implant Materials

Corrosion and mechanical wear of metallic implants may release ions or particles into surrounding tissues.

These released substances may cause:

• Local tissue inflammation
• Immune hypersensitivity reactions
• Metallosis
• Systemic metal exposure

Therefore, regulatory frameworks require extensive biological evaluation of implant materials through non-clinical testing and clinical evidence.

References

US FDA Industry Guidance ICMR Research Studies NCBI Scientific Literature ISO 10993 Biological Evaluation Standards