Overview of Biomaterials By: Dr. Murtaza Najabat Ali (CEng MIMechE P.E.)

Types of Biomaterials Biomaterials Metals Natural Ceramics Composites Biomaterials can be divided into four major classes of materials Biomaterials Polymers Metals Natural Ceramics Composites Fifth Class of Biomaterials

Types of Biomaterials contd..

Types of Biomaterials contd..

Types of Biomaterials contd.. Polymers Consists of repeating units strung together in long chains Represent the largest class of biomaterials Natural Synthetic Hydrophobic Hydrophilic Water swelling materials (Hydrogels) Biostable Biodegradable

Types of Biomaterials contd.. Application of Polymers as Biomaterials The hip joint is the largest load bearing joint. A hip joint is lined with a layer of cartilage that reduces friction and acts as a shock absorber. When the bone is exposed to arthritis and injury, this protective layer is damaged, causing extreme pain. Polymers used as femoral head and acetabular cup in total hip replacement

Types of Biomaterials contd.. Polymers By using a Polymethyl-methacrylate (PMMA) cement to adhere the metal to the bone By using a porous metal surface to create a bone ingrowth interface The acetabulam and the proximal femur ca be also replaced, where the femoral side is completely metal. The acetabular side is composed of the polyethylene bearing surface

Bonding Time: Cemented Vs. Cementless Types of Biomaterials contd.. Polymers Bonding Time: Cemented Vs. Cementless Cemented Approximately 10min Cementless One year or more for good ingrowth Healing Time Cemented Cementless Weight bearing Next day with crutches Next day depending on stability Walk (crutches) 2 day after surgery 6-12 weeks depending on surgeon Pain Free 5 days in some cases Depends on stability Normal Walk 2 weeks (6 weeks with crutches is recommended) Couple of months to 1 year depending on ingrowth rate Everything good Around six months for a knee replacement 2 years onward

Types of Biomaterials contd.. Polymers PMMA and silicone elastomers are used as Intraocular lenses to treat cataract Hard contact lenses for clear vision

Types of Biomaterials contd.. Polymers Ultra high molecular weight Polyethylene, as Femoral head, acetabular cup or insert Polyethylene, Polytetrafluoroethylene (PTFE) and silicone used as medical tubing for drains and catheters

Types of Biomaterials contd.. Polymers The healthy human knee joint is also lined with articular cartilage. Arthritis and injury can similarly damage this protective layer of cartilage causing extreme pain Hyaluronic Acid (HA) Injections restores lubrication and fluid in the joint and creates a shock absorber between the bones

Types of Biomaterials contd.. Polymers Pacemakers leads are insulated by silicone PTFE and Polyurethane used as Vascular and non-vascular grafts Polyvinylchloride (PVC) and silicone used as Tubing, blood storage bags, in blood transfusion, feeding and dialysis

Types of Biomaterials contd.. Metals Load bearing implants and internal fixation devices; When processed suitably contribute high tensile, high fatigue and high yield strengths; Properties depend on the processing method and purity of the metal. When metallic biomaterials were first used in biomedical applications, the only requirement was to achieve a suitable combination of physical properties to match those of the replaced tissue with a minimal toxic response of the host Few concepts were gradually introduced with time as requirements for metallic biomaterials in the design of implantable devices, such as Foreign body reaction (particularly due to wear debris) Stress shielding Biocompatibility Bioactivity, and Osteoinduction

Types of Biomaterials contd.. Metals Although many metals and alloys are used for medical device applications, the most commonly employed are Stainless steels Cobalt-base alloys Commercially pure Titanium and Titanium alloys Nickel- Titanium alloys, and some Noble metals (Au, Pt, Pd)

Types of Biomaterials contd.. Metals

Types of Biomaterials contd.. Metals

Types of Biomaterials contd.. Metals Ti alloy Pacemaker case and metallic leads encapsulated in silicone tubing Metal electrodes In Phrenic stimulator for respiratory control

Types of Biomaterials contd.. Metals Tooth metal crown For securing removable denture Ti alloy single teeth replacement Metal wires and braces

Types of Biomaterials contd.. Metals

Types of Biomaterials contd.. Ceramics Bone bonds well to them Exhibit minimum foreign body reaction High stiffness Low friction and wear coefficients But Low fracture toughness Low impact resistance Owing to its Brittle nature

Calcium Phosphates (CaPs) Bioactive Glasses (BGs) Types of Biomaterials contd.. Ceramics Alumina (Al2O3) Zirconia Several porous ceramics (CaCO3) The most common ceramic materials can be classified (both as cements and ceramics) as ; Calcium Phosphates (CaPs) Bioactive Glasses (BGs) Glass Ceramics Hydroxyapatite Ca10(PO4)6(OH)2 A range of suitable CaPs (amorphous CaP (ACP), dicalcium phosphate (DCP), tricalcium phosphate (α-TCP, β-TCP), tetracalcium phosphate (TTCP) e.t.c.) Several BGs formulations containing e.g. SiO2 as network former and Na2O, CaO and P2O5 as modifying oxides (Bioglass 45S5)

Types of Biomaterials contd.. Ceramics Alumina Femoral head and Acetabular liner

Types of Biomaterials contd.. Ceramics Zirconia dental crown and bridges Hydroxyapatite coated metallic implants Zirconia artificial tooth root

Calcium Phosphate Scaffold Types of Biomaterials contd.. Ceramics Calcium Phosphate Scaffold All-carbon mono-leaflet mechanical heart valves consist of a rotatable housing ring within a compliant suture ring HA scaffold (light purple colour) with bone (yellow colour) that has grown in the scaffold

Types of Biomaterials contd.. Example 6 : Which of the following classes of biomaterials would be most appropriate for use to fabricate an artificial tendon, a tissue that must sustain substantial deformation at low forces and return rapidly to its original dimensions upon release of the stress ? WHY ? (a) Metals (b) Ceramics (c) Polymers

Important Characteristics The selection of the biomaterial and selection of the appropriate processing techniques for a given application is determined primarily by Surface Bulk, and Degradative properties of the material

An interface is the boundary region between two adjacent layers Important Characteristics contd.. Surface Properties of Biomaterials What constitutes a surface? An interface is the boundary region between two adjacent layers L V G S L = Liquid G = Gas S = Solid V = Vapor We recognize (S/G), (S/L), and (L/V) as surfaces

Important Characteristics contd.. Surface Properties of Biomaterials Biomaterial surface Protein adsorption The effect of protein size on interaction with a surface. Notice that the larger protein composed of more amino acids is capable of making more interactions Proteins (red) adsorb differently to the different materials and are depicted as elongated on metal and globular on polymer. Cells (blue) interact with the materials via the adsorbed proteins and conformation of the adsorbed proteins dictates how the cells will respond (adhere, proliferate, differentiate, etc.).

Important Characteristics contd.. Surface Properties of Biomaterials The surface of a material is defined to be the few atomic layers on the exterior of the object Surface properties can be different from the bulk properties Surface properties include both chemical and physical characteristics

Important Characteristics contd.. Surface Properties of Biomaterials Surface Chemical Property Contact Angle The contact angle is the angle at which a liquid-vapor interface meets the solid-liquid interface The contact angle is determined by the resultant between Adhesive and Cohesive forces As the tendency of a drop to spread out over a flat solid surface increases, the contact angle decreases A shows a fluid with very little wetting, while C shows a fluid with more wetting. A has a large contact angle, and C has a small contact angle. Wetting Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together Water bead on a fabric that has been made hydrophobic Thus, the Contact angle provides an inverse measure of Wettability

Important Characteristics contd.. Surface Properties of Biomaterials Surface Tension Surface tension is a measurement of the cohesive energy present at an interface This forms a surface “Film” which makes it more difficult to move an object through the surface Hydrophilicity If the liquid is very strongly attracted to the solid surface The droplet will completely spread out on the solid surface Hydrophobicity If the solid surface is hydrophobic, the contact angle will be larger than 900C

Important Characteristics contd.. Surface Properties of Biomaterials Surface Physical Property Surface Roughness

Important Characteristics contd.. Surface Properties of Biomaterials Example 2.1: Would a hydrophobic or hydrophilic polymer be a more appropriate choice for a Contact lens application? WHY ? Would a melting temperature (Tm) of the polymer above 37oC or below 370C be more appropriate for this application ? WHY ? Example 2.2: A 1ml droplet of distilled water is dropped onto each material “A” and “B” as shown below. Which material is more hydrophilic ? Justify your answer. Example 2.3: Why do atoms at the surface of a crystalline material generally possess higher energy than those inside of the crystal and what is the term for this heightened energy ?

Important Characteristics contd.. Bulk Properties of Biomaterials Most important parameter They may play a less significant role in initial biological interaction/response, but they have a large long-term impact They can be altered to allow the biomaterial to mimic the physicochemical properties of the biological system Bulk characteristic of biomaterials include Mechanical Properties Physical Properties Chemical composition

Important Characteristics contd.. Bulk Properties of Biomaterials Mechanical Properties Such as, Strength Stiffness Anisotropy Fatigue properties/strength Mechanical properties of materials are highly affected by their physical and chemical characteristics

Important Characteristics contd.. Bulk Properties of Biomaterials Physical Properties Such as, Crystallinity Thermal Transition Points Melting Point (Tm) Glass Transition Point (Tg) Chemical Composition SEM Micrograph of steel showing grain boundaries It varies when chemicals are added or subtracted, and when the ratio of substances changes It determines the (bulk) properties of the substance It also dictates other properties such as chemistry at the surface

Important Characteristics contd.. Degradative Properties of Biomaterials Factors that affect biomaterials’ degradation in vivo The size and shape of the implant Its location in the body Chemical, physical and mechanical (both bulk and surface) properties Although the temperature and pH of body environment/fluids are relatively mild, BUT ! Degradation can be undesired or desired The biocompatibility of degradation by-products is as important as the biocompatibility of the intact material