Mechanical properties are the most important properties to be considered for the characterization and design of efficient biomaterials.
The mechanical properties are;
1. Stress-Strain Behaviour
2. Viscoelasticity
3. Surface properties
STRESS-STRAIN BEHAVIOUR
For a material that undergoes a mechanical deformation – Stress is defined as a force per unit area Newtons per square meter (Pascal, Pa) or pounds-force per square inch (psi).
A load (or force) can be applied upon a material in tension, compression, and shear or any combination of these forces (or stresses). The deformation of an object in response to an applied load (stress) is called strain(ε). It is also possible to denote strain by the stretch ratio, i.e., deformed length/original length. The deformations associated with different types of stresses are called tensile, compressive, and shear strain.
Stress-strain behavior of an idealized material
- The stress-strain curve of a solid sometimes can be demarcated by the yield point (σy or YP) into elastic and plastic regions.
- In the elastic region, the strain increases in direct proportion to the applied stress (σ) (Hooke’s law).
- Peak stress is followed by an apparent decrease until a point is reached where the material ruptures. This peak stress is known as the Tensile or ultimate tensile strength (TS).
- The stress where failure occurs is called the failure or fracture strength (FS).
- In the plastic region, strain changes are no longer proportional to the applied stress – applied stress is removed, and the material will not return to its original shape but will be permanently deformed which is called plastic deformation.
Deformation characteristics of metals and plastics under stress;
- Metals rupture without further elongation after necking occurs.
- In Plastics the necked region undergoes further deformation called drawing.
2. VISCOELASTICITY;
VISCOELASTIC MATERIAL BEHAVIOR
- Real materials that have both elastic and viscous aspects to their behavior are known as viscoelastic materials.
- Viscoelastic materials – those for which the relationship between stress and strain depends on time.
- Materials – stiffness will depend on the rate of application of the load.
- Mechanical energy is dissipated by conversion to heat in the deformation of viscoelastic materials.
- All materials exhibit some viscoelastic response.
- Metals – steel or aluminum, quartz at room temperature undergo small deformation is almost purely elastic.
- Metals can behave plastically at large deformation.
- Plastic deformation occurs only if threshold stress is exceeded.
- By contrast, materials such as synthetic polymers, wood, and human tissue show significant viscoelastic effects, and these effects occur at small or large stress.
- Creep is a slow, progressive deformation of a material under constant stress.
3. SURFACE PROPERTIES
Surface properties are important since all implants interface with the tissues at their surfaces.
The surface property is directly related to the bulk property since the surface is the discontinuous boundary between different phases.
- Atoms at the surface have special organization and reactivity, hence requiring special methods to characterize them.
- They drive many of the biological reactions, protein adsorption, cell adhesion, cell growth, blood compatibility, etc.
SURFACE FREE ENERGY;
Molecules in the bulk of a material (e.g., crystal lattice) have a low relative energy state due to nearest-neighbor interactions (e.g. bonding).
Molecules at a surface are in a state of higher free energy than those in the bulk. This is in large part due to the lack of nearest neighbour interactions at a surface.
The unusual properties of surfaces and interfaces are due to unbalanced intermolecular interaction forces or energies across the surface and interface.
SURFACE COMPOSITION;
The surface composition of materials is usually different from their bulk composition due to,
- adsorption of environmental contaminants.
- desorption of surface-active compounds or impurities from bulk phase.
SURFACE PROPERTIES;
- Roughness
- Wettability
- Surface mobility
- Chemical composition
- Electrical charge
- Crystallinity
- Heterogeneity to biological reaction
- Topography
- Thickness
HOST REACTIONS TO BIOMATERIALS
Biomaterials are now commonly used as implants and other tissue contacting medical devices over a wide range of applications – prostheses
✓ Cardiovascular
✓ Orthopaedic
✓ Dental
✓ Ophthalmological
✓ Reconstructive surgery
✓ Minimal invasive interventions – stent placement in the biliary tree or blood vessels.
✓ In extracorporeal devices – hemodialysis membranes.
✓ Surgical sutures or bioadhesives and controlled drug-release devices – some implants and extracorporeal devices ultimately develop complications, and adverse interactions to the patient with the device.
Effects of both implant on the host tissues and host on the implant – important in mediating complications and device failure.
The most important host reactions to biomaterials and their evaluation are
› Non-specific inflammation
› Specific immunological reactions
› Systemic effects
› Blood- materials interactions
›Tumour formation
› Infection
Inflammatory and potential immunological interactions occur with biomaterials
✓ Most biomaterials elicit the foreign-body reaction (FBR) – a special form of non-specific inflammation.
✓ Prominent cells in the FBR are macrophages, which attempt to phagocytose the material, but complete engulfment and degradation are often difficult.
✓ Macrophages activated in the process of interacting with a biomaterial, may elaborate cytokines that stimulate inflammation or fibrosis.
Macrophages – are also the first line of defense against pathogens and the mode of activation will determine the success or failure of the host’s response to pathogens.
[The more “biocompatible” the implant, the more quiescent (less inflammation) is the ultimate response]
- The nature of the reaction is largely dependent on the chemical and physical characteristics of the implant.
- For most inert biomaterials, the late tissue reaction is encapsulation by a fibrous tissue capsule (composed of collagen and fibroblasts).
- Tissue interactions can be modified by changing the chemistry of the surface, surface–active agent to chemically bond the tissue.
- Bioresorbable component to allow slow replacement by tissue to simulate natural healing properties, or by systemic or localized administration of corticosteroid drug.
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