Mechanical properties of biomaterials

Last updated May 18, 2021

Materials that are used for biomedical or clinical applications are known as biomaterials. The following article deals with fifth generation biomaterials that are used for bone structure replacement. For any material to be classified for biomedical application three requirements must be met. The first requirement is that the material must be biocompatible; it means that the organism should not treat it as a foreign object. Secondly, the material should be biodegradable (for in-graft only); the material should harmlessly degrade or dissolve in the body of the organism to allow it to resume natural functioning. Thirdly, the material should be mechanically sound; for the replacement of load bearing structures, the material should possess equivalent or greater mechanical stability to ensure high reliability of the graft.

Introduction

The biomaterial term is used for materials that can be used in biomedical and clinical applications. They are bioactive and biocompatible in nature. Currently, many types of metals and alloys (stainless steel, titanium, nickel, magnesium, Co–Cr alloys, Ti alloys),^[1] ceramics (zirconia, bioglass, alumina, hydroxyapatite) ^[1] and polymers (acrylic, nylon, silicone, polyurethane, polycaprolactone, polyanhydrides) ^[1] are used for load bearing application. This includes dental replacement and bone joining or replacement for medical and clinical application. Therefore, their mechanical properties are very important. Mechanical properties of some biomaterials and bone are summarized in table 1.^[2] Among them hydroxyapatite is most widely studied bioactive and biocompatible material. However, it has lower young’s modulus and fracture toughness with brittle nature. Hence, it is required to produce a biomaterial with good mechanical properties.

Elastic modulus

Elastic modulus is simply defined as the ratio of stress to strain within the proportional limit. Physically, it represents the stiffness of a material within the elastic range when tensile or compressive load are applied. It is clinically important because it indicates the selected biomaterial has similar deformable properties with the material it is going to replace. These force-bearing materials require high elastic modulus with low deflection. As the elastic modulus of material increases fracture resistance decreases. It is desirable that the biomaterial elastic modulus is similar to bone. This is because if it is more than bone elastic modulus then load is born by material only; while the load is bear by bone only if it is less than bone material. The Elastic modulus of a material is generally calculated by bending test because deflection can be easily measured in this case as compared to very small elongation in compressive or tensile load. However, biomaterials (for bone replacement) are usually porous and the sizes of the samples are small. Therefore, nanoindentation test is used to determine the elastic modulus of these materials. This method has high precision and convenient for micro scale samples. Another method of elastic modulus measurement is non-destructive method. It is also clinically very good method because of its simplicity and repeatability since materials are not destroyed.^[3]

Hardness

Hardness is a measure of plastic deformation and is defined as the force per unit area of indentation or penetration. Hardness is one of the most important parameters for comparing properties of materials. It is used for finding the suitability of the clinical use of biomaterials. Biomaterial hardness is desirable as equal to bone hardness. If higher than the biomaterial, then it penetrates in the bone. Higher hardness results in less abrasion. As above said, biomaterials sample are very small therefore, micro and nano scale hardness test (Diamond Knoop and Vickers indenters) are used.^[3]

Fracture strength

Strength of materials is defined as the maximum stress that can be endured before fracture occurs. Strength of biomaterials (bioceramics) is an important mechanical property because they are brittle. In brittle materials like bioceramics, cracks easily propagate when the material is subject to tensile loading, unlike compressive loading. A number of methods are available for determining the tensile strength of materials, such as the bending flexural test, the biaxial flexural strength test and the weibull approach. In bioceramics, flaws influence the reliability and strength of the material during implantation and fabrication. There are a number of ways that flaws can be produced in bioceramics such as thermal sintering and heating. The importance is for bioceramics to have high reliability, rather than high strength.

The strength of brittle materials depends on the size of flaws distributed throughout the material. According to Griffith’s theory of fracture in tension, the largest flaw or crack will contribute the most to the failure of a material. Strength also depends on the volume of a specimen since flaw size is limited to the size of the specimen’s cross-section. Therefore, the smaller the specimen (e.g., fibers), the higher the fracture strength. Porosity of implanted bioceramic has a tremendous influence on the physical properties. Pores are usually formed during processing of materials. Increasing the porosity and pore size means increasing the relative void volume and decreasing density; this leads to a reduction in mechanical properties and lowers the overall strength of bioceramic.

To use ceramics as self-standing implants that are able to withstand tensile stresses is a primary engineering design objective. Four general approaches have been used to achieve this objective: 1) use of the bioactive ceramic as a coating on a metal or ceramic substrate 2)strengthening of the ceramic, such as via crystallization of glass 3) use of fracture mechanics as a design approach and 4) reinforcing of the ceramic with a second phase.

For example, hydroxyapatite and other calcium phosphates bioceramics are important for hard tissue repair because of their similarity to the minerals in natural bone, and their excellent biocompatibility and bioactivity but they have poor fatigue resistance and strength. Hence, bioinert ceramic oxides having high strength are used to enhance the densification and the mechanical properties of them.

Fracture toughness

Fracture toughness is required to alter the crack propagation in ceramics. It is helpful to evaluate the serviceability, performance and long term clinical success of biomaterials. It is reported that the high fracture toughness material improved clinical performance and reliability as compare to low fracture toughness.^[4] It can be measured by many methods e.g. indentation fracture, indentation strength, single edge notched beam, single edge pre cracked beam and double cantilever beam.

Fatigue

Fatigue is defined as failure of a material due to repeated/cyclic loading or unloading (tensile or compressive stresses). It is also an important parameter for biomaterial because cyclic load is applied during their serving life. In this cyclic loading condition, micro crack/flaws may be generated at the interface of the matrix and the filler. This micro crack can initiate permanent plastic deformation which results in large crack propagation or failure. During the cyclic load several factor also contribute to microcrack generation such as frictional sliding of the mating surface, progressive wear, residual stresses at grain boundaries, stress due to shear.^[3]

Table 1: Summary of mechanical properties of cortical bone and biomaterial

Material	Tensile strength (MPa)	Compressive strength (MPa)	Elastic modulus (GPa)	Fracture toughness (MPa. m^-1/2)
Bioglass	42^[5]	500^[5]	35^[6]	2^[6]
Cortical Bone	50-151^[5]	100-230^[7]	7-30^[6]	2-12^[6]
Titanium	345^[8]	250-600^[9]	102.7^[8]	58-66^[8]
Stainless steel	465-950^[1]	1000^[9]	200^[5]	55-95^[9]
Ti-Alloys	596-1100^[8]	450-1850^[9]	55-114^[8]	40-92^[8]
Alumina	270-500^[9]	3000-5000^[9]	380-410^[6]	5-6^[6]
Hydroxyapatites	40-300^[9]	500-1000^[7]	80-120^[6]	0.6-1^[6]

Fatigue fracture and wear have been identified as some of the major problems associated with implant loosening, stress-shielding and ultimate implant failure. Although wear is commonly reported in orthopaedic applications such as knee and hip joint prostheses, it is also a serious and often fatal experience in mechanical heart valves. The selection of biomaterials for wear resistance unfortunately cannot rely only on conventional thinking of using hard ceramics because of their low coefficient of friction and high modulus of elasticity. This is because ceramics are generally prone to brittle fracture (having a fracture toughness typically less than 1 MPa√m) and need absolute quality control to avoid fatigue fracture for medical device applications. The development of fatigue fracture and wear resistant biomaterials looks into the biocomposites of two or more different phases such as in interpenetrating network composites. The advantage of these composites is that one can incorporate controlled drug release chemicals, friction modifiers, different morphologies to enable better host–implant performance and chemical entities to reduce or aid removal of wear debris. Of equal importance are the tools developed to predict fatigue fracture/wear using new methodologies involving in vitro tests, computational modelling to obtain design stresses and fracture/wear maps to identify mechanisms.

Related Research Articles

A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing a nonmetallic mineral, such as clay, at a high temperature. Common examples are earthenware, porcelain, and brick.

Young's modulus $, the Young modulus, or the modulus of elasticity in tension, is a mechanical property that measures the tensile stiffness of a solid material. It quantifies the relationship between tensile stress and axial strain in the linear elastic region of a material and is determined using the formula:$

Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or $within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials the ultimate tensile strength is close to the yield point, whereas in ductile materials the ultimate tensile strength can be higher.$

In engineering and materials science, a stress–strain curve for a material gives the relationship between stress and strain. It is obtained by gradually applying load to a test coupon and measuring the deformation, from which the stress and strain can be determined. These curves reveal many of the properties of a material, such as the Young's modulus, the yield strength and the ultimate tensile strength.

The field of strength of materials, also called mechanics of materials, typically refers to various methods of calculating the stresses and strains in structural members, such as beams, columns, and shafts. The methods employed to predict the response of a structure under loading and its susceptibility to various failure modes takes into account the properties of the materials such as its yield strength, ultimate strength, Young's modulus, and Poisson's ratio. In addition, the mechanical element's macroscopic properties such as its length, width, thickness, boundary constraints and abrupt changes in geometry such as holes are considered.

$Toughness Material ability to absorb energy and plastically deform without fracturing$

In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing. One definition of material toughness is the amount of energy per unit volume that a material can absorb before rupturing. This measure of toughness is different from that used for fracture toughness, which describes load bearing capabilities of materials with flaws. It is also defined as a material's resistance to fracture when stressed.

Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics to characterize the material's resistance to fracture.

Work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. Work hardening may be desirable, undesirable, or inconsequential, depending on the context.

Titanium alloys are alloys that contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness. They are light in weight, have extraordinary corrosion resistance and the ability to withstand extreme temperatures. However, the high cost of both raw materials and processing limit their use to military applications, aircraft, spacecraft, bicycles, medical devices, jewelry, highly stressed components such as connecting rods on expensive sports cars and some premium sports equipment and consumer electronics.

Bioglass 45S5 or calcium sodium phosphosilicate, commonly referred to by its commercial name Bioglass and NovaMin, is a glass specifically composed of 45 wt% SiO₂, 24.5 wt% CaO, 24.5 wt% Na₂O, and 6.0 wt% P₂O₅. Glasses are non-crystalline amorphous solids that are commonly composed of silica-based materials with other minor additives. Compared to soda-lime glass (commonly used, as in windows or bottles), Bioglass 45S5 contains less silica and higher amounts of calcium and phosphorus. The 45S5 name signifies glass with 45 weight % of SiO₂ and 5:1 molar ratio of calcium to phosphorus. This high ratio of calcium to phosphorus promotes formation of apatite crystals; calcium and silica ions can act as crystallization nuclei. Lower Ca:P ratios do not bond to bone. Bioglass 45S5's specific composition is optimal in biomedical applications because of its similar composition to that of hydroxyapatite, the mineral component of bone. This similarity provides Bioglass' ability to be integrated with living bone.

Bioactive glasses are a group of surface reactive glass-ceramic biomaterials and include the original bioactive glass, Bioglass. The biocompatibility and bioactivity of these glasses has led them to be investigated extensively for use as implant devices in the human body to repair and replace diseased or damaged bones.

In materials science, fracture toughness is the critical stress intensity factor of a sharp crack where propagation of the crack suddenly becomes rapid and unlimited. A component's thickness affects the constraint conditions at the tip of a crack with thin components having plane stress conditions and thick components having plane strain conditions. Plane strain conditions give the lowest fracture toughness value which is a material property. The critical value of stress intensity factor in mode I loading measured under plane strain conditions is known as the plane strain fracture toughness, denoted $. When a test fails to meet the thickness and other test requirements that are in place to ensure plane strain conditions, the fracture toughness value produced is given the designation . Fracture toughness is a quantitative way of expressing a material's resistance to crack propagation and standard values for a given material are generally available.$

Hardness is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard metals such as titanium and beryllium are harder than soft metals such as sodium and metallic tin, or wood and common plastics. Macroscopic hardness is generally characterized by strong intermolecular bonds, but the behavior of solid materials under force is complex; therefore, there are different measurements of hardness: scratch hardness, indentation hardness, and rebound hardness.

A biomaterial is a substance that has been engineered to interact with biological systems for a medical purpose, either a therapeutic or a diagnostic one. As a science, biomaterials is about fifty years old. The study of biomaterials is called biomaterials science or biomaterials engineering. It has experienced steady and strong growth over its history, with many companies investing large amounts of money into the development of new products. Biomaterials science encompasses elements of medicine, biology, chemistry, tissue engineering and materials science.

Filler materials are particles added to resin or binders that can improve specific properties, make the product cheaper, or a mixture of both. The two largest segments for filler material use is elastomers and plastics. Worldwide, more than 53 million tons of fillers are used every year in application areas such as paper, plastics, rubber, paints, coatings, adhesives, and sealants. As such, fillers, produced by more than 700 companies, rank among the world's major raw materials and are contained in a variety of goods for daily consumer needs. The top filler materials used are ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), kaolin, talc, and carbon black. Filler materials can affect the tensile strength, toughness, heat resistance, color, clarity etc. A good example of this is the addition of talc to polypropylene. Most of the filler materials used in plastics are mineral or glass based filler materials. Particulates and fibers are the main subgroups of filler materials. Particulates are small particles of filler which are mixed in the matrix where size and aspect ratio are important. Fibers are small circular strands that can be very long and have very high aspect ratios.

Bioceramics and bioglasses are ceramic materials that are biocompatible. Bioceramics are an important subset of biomaterials. Bioceramics range in biocompatibility from the ceramic oxides, which are inert in the body, to the other extreme of resorbable materials, which are eventually replaced by the body after they have assisted repair. Bioceramics are used in many types of medical procedures. Bioceramics are typically used as rigid materials in surgical implants, though some bioceramics are flexible. The ceramic materials used are not the same as porcelain type ceramic materials. Rather, bioceramics are closely related to either the body's own materials or are extremely durable metal oxides.

Artificial bone refers to bone-like material created in a laboratory that can be used in bone grafts, to replace human bone that was lost due to severe fractures, disease, etc.

Mechanical testing covers a wide range of tests, which can be divided broadly into two types:

those that aim to determine a material's mechanical properties, independent of geometry.
those that determine the response of a structure to a given action, e.g. testing of composite beams, aircraft structures to destruction, etc.

Polymer fracture is the study of the fracture surface of an already failed material to determine the method of crack formation and extension in polymers both fiber reinforced and otherwise. Failure in polymer components can occur at relatively low stress levels, far below the tensile strength because of four major reasons: long term stress or creep rupture, cyclic stresses or fatigue, the presence of structural flaws and stress-cracking agents. Formations of submicroscopic cracks in polymers under load have been studied by x ray scattering techniques and the main regularities of crack formation under different loading conditions have been analyzed. The low strength of polymers compared to theoretically predicted values are mainly due to the many microscopic imperfections found in the material. These defects namely dislocations, crystalline boundaries, amorphous interlayers and block structure can all lead to the non-uniform distribution of mechanical stress.

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