Materials science is a relatively new field of study that has emerged at the intersection of physics, chemistry, and engineering. It involves the analysis of the properties of a physical substance that can be used in an application. The study seeks to comprehend the underlying structure of the material, its properties, how it behaves under various conditions, and how to alter its properties through processing. It is a key component in the research and design of medical devices. With the need to build smaller appliances increasing, materials science is crucial to developing and manufacturing medical devices that break the boundaries of what was formerly thought possible.
The Materials Paradigm
The materials paradigm divides the study of substances into four aspects: structure, properties, processing, and performance. Materials scientists seek not only to understand each aspect but also how the different aspects relate to and influence one another.
The study of structure deals with the composition of the material. It begins at the atomic level, describing the chemical composition of the material, including its classification as crystalline or non-crystalline and the bonding processes that create the material. Moving progressively to larger scales, the structure at the nano level (1–100 nm), micro level (100 nm–1 cm), and macro level (>1 cm) is explored. Most properties of any given substance are determined by its structure at these various levels.
In examining the properties of the material, scientists and engineers seek to understand the physical characteristics of the substance. The chemical properties determine how the material interacts with other substances on a chemical level. Electrical properties describe the ability of the object to conduct electricity. Mechanical properties include its strength, durability, friability, and ductility. How the material responds to various stresses (tension, compression, and shear) is examined. Other properties, such as thermodynamics, are cataloged as well. Vital to this aspect of materials science is the determination of how these properties arise from the structure of the material.
The third factor in the paradigm is the processing of materials. This explores how the history of the material (i.e., the processes to which it has been subjected) have altered its properties. It also attempts to determine how future processing could modify those properties. Understanding material processing is a critical element in the development of new materials. Processing that allows micro-materials to be developed is imperative to the design and development of medical devices that function even at sizes small enough to be implanted.
The first three aspects—structure, properties, and processing—combine to determine the performance of the material. The goal is to produce a material that has the performance parameters needed for a particular application. For medical devices, performance requirements can include durability, acceptable levels of toxicity, minimal thickness, and resistance to microbial growth, as examples. This is also the stage where materials are analyzed to determine if they meet appropriate international standards and comply with governmental regulation.
Types of Materials Used in Medical Device Manufacturing
Metals are solid, non-organic materials. They are highly ductile and malleable, exhibiting good compressive, tension, and shear strength. They have high electrical and thermal conductivity. They have long been the most common material in medical device manufacturing and are currently used in some way shape or form in 80 percent of all medical devices. The combination of metals with other materials allows the properties of the material to be modified through the creation of alloys. Because most metals oxidize easily, stainless steel—comprising iron, carbon, and chromium—is often the metal of choice for medical device manufacturers.
Materials science research on metals is seeking to develop new alloys and processing that would improve the properties of metal for use in medical device manufacturing. The use of titanium alloys is increasing, in part due to its modulus of elasticity which is closer to that of bone than that of steel. New titanium alloys—in particular alloys without nickel—are currently being researched.
Another promising area is research into bioabsorbable metals that are absorbed or eliminated by the body after performing their function. Currently, only polymers are bioabsorbable, but both magnesium and iron offer possible avenues for development of alloys with the same property. Other materials science engineers are working to develop metals that have lower susceptibility to magnetization, because current metal implants interfere with MRIs. Developments in surface modifications are moving toward metal materials that resist absorbing or binding with proteins, viruses, and other biological substances that can inhibit its function.
In materials sciences, the term ceramics applies to solid materials that are neither metallic nor organic. The category includes glass, clay, and concrete. They are usually oxides but can also be carbides, silicides, or nitrides. Most are crystalline in structure, although some, such as glass, are non-crystalline. Mechanically, they are hard and brittle with very low plasticity. They exhibit high compressive strength and low tension and shear strength. Ceramics generally have low electrical conductivity, although some function as semiconductors and a few become superconductors at extreme temperatures. They are chemically nonreactive.
Ceramics play an increasing role in medical devices manufacturing. Because they are good insulators, they can be molded at small sizes. And because they do not degrade within the body, they are ideal for implantable medical devices. Although aluminum oxide has been the most common ceramic material in medical device manufacturing, zirconium dioxide is being increasingly used. When stabilized with yttrium oxide, it has a greater strength than aluminum oxide, which allows the material to obtain the same strength as aluminum oxide at smaller sizes. Sensors made with piezoelectric ceramics are increasingly replacing metal sensors in many medical devices. Lead zirconate titanate is the most commonly used piezoceramic, although non-lead-based ceramics are also being studied for use in implantable medical devices.
Polymers are materials made up by multiple units of similar chemical compounds chained together. Common polymers are various forms of plastic and rubber. They are generally lightweight, can have excellent flexibility, and are generally inexpensive. Approximately 75 percent of polymers used in medical device manufacturing are thermoplastics, allowing them to be molded to precise tolerances. Unlike metals, polymers do not interfere with medical scanning devices such as MRIs. They can be made bioabsorbable and therefore are a material of choice for temporary uses. Polymers used in medical device manufacturing must be sterilizable, resistant to contamination, and have acceptably low levels of toxicity. By nature, polymers are open to improvement via processing, allowing their mechanical properties to be modified for new applications.
One of the most prominent new uses of polymers in medical device development is in 3D printing. Recent advances in the technology make the production of device components via 3D printers feasible. Acrylonitrile butadiene styrene and polylactic acid are two commonly used polymers for printing. In addition to its use in production, 3D printing has also made the process of prototyping medical devices easier, allowing for shorter development cycles. Even when the final medical device may include or be entirely constructed from metals or ceramics, prototypes can be printed using polymers.
Composites are one of the newest materials being utilized for medical devices. Composite materials are a combination of materials from two or more of the groups above. Such materials are a way to take advantage of the desired characteristics of a material while compensating for unwanted properties. For example, a composite of polymers and metals can retain the light weight and moldability of a plastic while exhibiting enhanced strength due to the incorporation of metallic fibers. The combination of the two materials usually takes place at the macroscopic layer. Many of the tissues in the human body—including skin, bones, muscles, and teeth—are composite materials, so synthesized composites can be ideal when the function of such tissues needs to be replicated or reinforced.
Biomaterials are not a distinct class of material. Rather, they are subsets within each of the material classifications above. The term biomaterial refers to any material—natural or synthetic—that interacts with biological systems within the body. Historically, most materials used in medical device manufacturing have been inert by design, due to the need to prevent the absorption of the medical device’s material by surrounding tissue or the degradation of the medical device through contact. In recent decades, however, materials science has begun to explore ways that materials can be made to interact with the body in positive ways. As previously mentioned, some materials are now being made to be bioabsorbable, allowing implantable medical devices to perform their function while needed, and then be absorbed or eliminated by the body without the necessity of removing the medical device through additional surgery. Even more cutting edge are materials that are intended to actually become part of the body. Materials that assist in the healing of wounds by forming part of the new tissue, injectable gels that carry biochemical signals, and implantable medical devices that encourage the growth of certain types of cells are just a few of the new paths being explored by materials science engineers.
The Future of Materials Science & Medical Device Manufacturing
Medical device design and manufacturing is benefitting from ongoing advances in materials science. The creation of new materials with improved properties is allowing medical devices to perform better and provide functionality that until recently was thought impossible. At the same time, the needs of medical device manufacturers are driving materials science to engineer new materials with improved mechanical, chemical, and electrical properties. Innovation in one field spurs innovation in the other. Proven Process has the engineering expertise and years of experience needed to take advantage of new materials and to work with engineers to create the materials needed to meet our clients’ requirements.