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

For more than 100 years, metals and metal alloys (a combination of metal elements) have been commonly used for a host of medical implant applications across most medical specialties. This Article is written on the basis of considering available scientific information related to metals and their uses in medical implants, with focus on how metal materials are impacted by a physiological environment, expected and potential immune system responses to the metal associated with an implant, as well as subsequent clinical manifestations.

What are Biomaterials

Biomaterials play an integral role in medicine today—restoring function and facilitating healing for people after injury or disease. Biomaterials may be natural or synthetic and are used in medical applications to support, enhance, or replace damaged tissue or a biological function. The first historical use of biomaterials dates to antiquity, when ancient Egyptians used sutures made from animal sinew. The modern field of biomaterials combines medicine, biology, physics, and chemistry, and more recent influences from tissue engineering and materials science. The field has grown significantly in the past decade due to discoveries in tissue engineering, regenerative medicine, and more.

Metals, ceramics, plastic, glass, and even living cells and tissue all can be used in creating a biomaterial. They can be reengineered into molded or machined parts, coatings, fibers, films, foams, and fabrics for use in biomedical products and devices. These may include heart valves, hip joint replacements, dental implants, or contact lenses. They often are biodegradable, and some are bio-absorbable, meaning they are eliminated gradually from the body after fulfilling a function.

How are biomaterials used in current medical practice?

Doctors, researchers, and bioengineers use biomaterials for the following broad range of applications:
  • Medical implants, including heart valves, stents, and grafts; artificial joints, ligaments, and tendons; hearing loss implants; dental implants; and devices that stimulate nerves.
  • Methods to promote healing of human tissues, including sutures, clips, and staples for wound closure, and dissolvable dressings.
  • Regenerated human tissues, using a combination of biomaterial supports or scaffolds, cells, and bioactive molecules. Examples include a bone regenerating hydrogel and a lab-grown human bladder.
  • Molecular probes and nanoparticles that break through biological barriers and aid in cancer imaging and therapy at the molecular level.
  • Biosensors to detect the presence and amount of specific substances and to transmit that data. Examples are blood glucose monitoring devices and brain activity sensors.
  • Drug-delivery systems that carry and/or apply drugs to a disease target. Examples include drug-coated vascular stents and implantable chemotherapy wafers for cancer patients.
Recent developments in materials chemistry, metallurgy, and manufacturing continue to spur innovations in design and diversification in materials utilized in metal implants. For example, additive manufacturing (e.g., 3D printing), is increasingly being utilized to produce custom shapes and geometries using a variety of metals, adding to the potential applications of metal implants or material come in contact with Skin/blood/tissues.

Materials In Human Physiology and Pathology

Some metals which are commonly found in implants, such as copper, zinc, iron, manganese, and cobalt are examples of elements that are essential to our normal biological functions. These metals are required only in small amounts and are critical to the structure and/or function of many proteins and enzymes. Abnormal function may occur, along with associated signs or symptoms, when there is too little (deficiency) of an essential metal. Other metals used in metallic implants such as nickel, titanium and aluminum are nonessential for human health. Both essential and nonessential metals when present at sufficiently high concentrations can disturb normal biological functions and result in cellular stress responses known as metal toxicity. Metal toxicity may affect various tissues including the kidney, liver, heart, the immune and nervous systems.

The toxicity information is typically based on exposure to the element through dietary intake and/or occupational/environmental exposure (e.g., dermal contact, inhalation). However, since the in vivo implant environment and the form/composition the metal appears in that environment (e.g., chemical form (valence), physical form (particulate vs ionic), dose released over time, etc.), may be different or unknown the degree to which published toxicity data can be extrapolated to a patient implanted with a metal-containing device is not known. 
  1. Essential Trace Metal Elements
  2. Non-Essential Metals
  3. Non-Metal Materials
Essential Trace Metal Elements

Trace elements are those which are present in only small amounts within a given environment. When such an element is critical to the structure and/or function of a living organism, it is considered an essential trace element. An element is generally considered to be essential if it is present in living tissues at a relatively constant concentration; evokes similar structural or physiological anomalies when removed from the organism; and those anomalies are prevented or resolved by supplementation of the element.

Examples-  Cobalt (Co), Copper (Cu), Iron (Fe), Manganese (Mn), Molybdenum (Mo), Zinc (Zn),  Chromium (Cr), Vanadium (V), Stainless Steel and Alloys of these metals.

Non-Essential Metals
Although not considered essential to human health, other metal elements may impact human physiological processes.

Examples- Nickel (Ni), Titanium (Ti), Aluminum (Al), Silver (Ag), Gold (Au), Palladium (Pd), Platinum (Pt), Tin (Sn), Tungsten (W), Iridium (Ir) and Alloys of these metals.

Non-Metal Materials
  • Plastics: A wide range of medical devices, from catheters to IV bags, utilize medical-grade plastics due to their lightweight, flexible, and biocompatible nature.
  • Silicone Rubber: Commonly used for flexible tubing, seals, and medical implants due to its biocompatibility and durability.
  • Ceramics: Some medical devices, such as dental implants and artificial joints, use ceramic materials known for their durability and biocompatibility.
  • Biological Materials: Tissue-engineered and regenerative medicine devices can incorporate biological materials like collagen, fibrin, or tissue scaffolds.

Characteristics for Qualification as Biomaterial

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.

Reference - Studies
US-FDA  Industry Guidance, ICMR Studies, NCBI, 

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