Introduction to Mechanics and Biomechanics

Tissue Biomechanics

• BM 525 course at Bogazici University, Biomedical Engineering Institute
• Focuses on the mechanics of tissues in the human (and animal) body, from cells to organs.
• Studies tissue deformation under mechanical loading, related strains and stresses.
• Aims to predict physiological function, failure criteria and mechanisms, and tissue adaptation to loading.

Introduction to Biomechanics

• Introduces fundamental concepts of biomechanics.
• Specifically focuses on mechanics as the study of motion and forces acting on matter.
• Mechanics involves concepts like time, space, force, energy and matter.

Definitions for Biomechanics

• Biomechanics is a scientific discipline relatively new and established.
• Various definitions of biomechanics exist ranging from broad to limited.
• Biomechanics can also be described as combining "biology" and "mechanics."
• Biomechanics is essentially mechanics applied to biology.
• Examining forces within and acting upon biological structures, along with resultant effects.

Divisions of Biomechanics

• Traditionally, biomechanics is divided into three parts:
  • Tissue biomechanics
  • Motion biomechanics
  • Fluid biomechanics

Tissue Biomechanics

• Focuses on the mechanical behavior of various tissues (cell level to organs).
• Studies tissue deformations under mechanical loads.
• Attempts to model physiological function and failure mechanisms.
• Examines how tissues adapt to these loads.

Motion Biomechanics

• Analyzes movements of structures in the neuromusculoskeletal system.
• Centers on the role of muscles, bones, joints, sensors, as well as the central and peripheral nervous system.

Fluid Biomechanics

• Studies the mechanics of all biological fluids, with a particular focus on the cardiovascular system.
• Examines blood transport through vessels.
• Analyzes the heart's role in pumping blood throughout the body.

Introduction to the Musculoskeletal System

• Diagrams of the leg showing the peroneus longus, tibialis anterior, extensor digitorum longus, peroneus tertius, ext hallucis longus, cruciate crural, and transverse crural ligaments

Bone

• Hard tissue forming the skeleton's main structural component.
• Key properties are stiffness, strength, toughness, and resilience.
• Primary function is load carrying, especially compressive loads.
• Composition: 30% organic material (primarily collagen fibers embedded in a matrix), 60% inorganic material (mainly hydroxyapatite crystals), 10% water.
• A composite material, exhibiting greater strength than its components.
• Ihomogeneous (mechanical properties depend on composition and distribution).
• Anisotropic (mechanical properties differ in different directions).
• Viscoelastic (mechanical properties are time-dependent).
• Dynamic, adaptive material; properties are influenced by factors like age, gender, and health.
• Two types of bone tissue: cortical (dense outer layer) and cancellous (porous inner layer).

Cartilage

• Specialized connective tissue with high collagen content but no mineral content.
• A major component of the musculoskeletal system, plays a key role in joint movement.
• Properties: low coefficient of friction, can withstand high compressive loads.
• Inhomogeneous, viscoelastic, and anisotropic mechanical properties.
• Limited healing capacity due to lack of blood vessels.

Ligament

• Fibrous band of soft tissue joining two bones in a joint.
• Functions include resisting external loads, guiding relative movements of the bones, and controlling the range of joint motion.

Tendon

• Strong fibrous cord of soft tissue connecting muscle to bone.
• Functions: transmitting muscle force to bone and storing elastic energy.
• Contains blood vessels and can heal after injury.

Muscle

• Activatable soft tissue responsible for generating and exerting force for movement.
• Shortening and force production are known as contraction.
• Skeletal muscles are composed of cells (intracellular space) and the surrounding tissue (extracellular space). Muscle fibers are the activatable cells of muscle tissue.
• The intracellular space is responsible for active mechanical properties, while the extracellular space governs passive mechanical properties (anisotropic, nonlinear, viscoelastic, constant volume).

Historical Highlights

• 384-322 B.C.: Aristotle's contributions to understanding muscles as force and movement producers.
• 129-201: Galen's identification of muscles as organs of voluntary movement.
• 1543: Vesalius's discovery of the contractile power residing in muscle substance and identification of its structural components.
• 1663: Swammerdam's demonstration of constant muscular volume during contractions.
• 1663: Stensen precise descriptions of muscular structures and that contractions can occur without volume changes.
• 1682: van Leeuwenhoek's discovery of skeletal muscle cross-striation using light microscopy.
• 1939: Engelhardt research in muscle biochemistry associating actin and myosin with myofilaments activity.
• 1954: Huxley's and Huxley's theory of sliding filaments and the cross-bridge theory.

Morphology

• Diagram of muscle components detailing muscle belly, epimysium, perimysium, endomysium, group of muscle fibers, myofibril, sarcomere, myosin, actin, and cross bridges

Structural Units

• The different component parts of skeletal muscle : epimysium, fascicle, perimysium, muscle fiber, sarcolemma, endomysium, myofibrils, sarcomere and myofilaments.

Thick Filament: Myosin Molecule

• Myosin molecules contain a long tail region (light meromyosin) and a globular head at the end (heavy meromyosin).
• The globular heads include an actin binding site and an ATP enzymatic site for energy for muscle contraction.

Thick Filament: Cross-Bridges

• Myosin heads extend from the thick filaments.
• Contain an actin binding site and an enzymatic site for ATP hydrolysis.
• Essential for muscle contraction.
• Specific spacing of the cross-bridges and a 180º orientation pattern.

Thin Filament: Actin Molecule

• Two strands of actin molecules form the backbone of the filament.
• Each actin molecule roughly a diameter of 5-6 nm.
• The strands are arranged in somewhat random manner.

Thin Filament: Troponin and Tropomyosin

• Tropomyosin protein lies within groove of actin chains.
• Troponin molecules at 38.5 nm intervals within the thin filament.
• Composed of three subunits; C (calcium ion binding sites), T (contacts tropomyosin), I (blocks cross-bridge sites in the resting state).

Titin Filaments

• Huge protein spanning the Z-disc to M-line within the sarcomere.
• Acts as a spring, the main mechanical element of the intracellular cytoskeleton.

Sliding Filament Theory

• Describes the mechanism of muscle contraction.
• Myosin heads move along actin filaments, pulling them closer together.
• Length of the filaments doesn’t change but the filaments slide.
• The force of contraction is proportional to the overlap of actin and myosin.
• Force is related to the # of cross-bridges that can form.

Sliding Filament Theory: Huxley Models

• Hugh Huxley and Jean Hanson proposed a model of the filament arrangement in a muscle.
• In the same year, Andrew Huxley and R. Niedergerke described an identical model for the configuration of muscle proteins.
• This proposes that when the muscle shortens or lengthens, the two types of filaments slide past one another as a result.

Isometric Conditions

• Sarcomere activity is fixed.
• One end is connected to a mechanical ground.
• The other end interacts with a fixed force transducer.

Isometric Conditions: Optimal Sarcomere Length

• Optimal sarcomere length (Isao) is where filament overlap is optimum for force production.
• Equation Isao = I2 + 2 x Ithin + Ifree

Isometric Conditions: Sarcomere I-F Relationship

• Force reduction occurs to zero when overlap is diminished.

Dynamic Conditions

• Force-velocity relationship determined by isokinetic activity for a sarcomere:
• One end fixed, connected to a mechanical ground. The other end attached to a force transducer which can move at a constant speed.

Hill's Muscle Model

• Describes active muscles as an accumulation of three key elements:
  • Parallel elastic elements
  • Contractile element
  • Series elastic elements

Hill's Model: Contractile Element

• Identifiable with the sliding filament theory - generation of active tension.
• Freely extensible at rest and shortens under activation but cannot perform instantaneous length changes.

Hill's Model: Series Elastic Element

• Intrinsic elasticity of actin and myosin molecules and cross-bridges.
• Non-uniformity in the sarcomeres and myofibril activation plays a role

Hill's Model: Parallel Elastic Element

• Intramuscular connective tissue, and cell membranes.
• Collagenous sheets contribute to the parallel elasticity.

Notes on Hill's Model: Limitation

• Assumption of uniform sarcomere length is a simplification.

Length-Force Relationship of Skeletal Muscle

• Force exerted by a muscle is dependant on the length of the sarcomere.
• GM (gastrocnemius): a muscle length-force graph showing the length-force characteristics.
• EDL (extensor digitorium longus): a muscle length-force graph showing the length-force characteristics.
• Different muscles can exhibit varying length-force characteristics.
• Isometric muscle length-passive force can be subtracted from muscle length-total force to obtain muscle length-active force characteristics.

Biomaterials

• Nonviable material used in medical devices for interacting with biological systems.
• A material intended to interface with biological systems to evaluate, treat, augment or replace any tissue, organ or function.

Biocompatibility

• Ability of a material to perform with an appropriate host response in a specific application.

Other Important Definitions

• Biological response: host response towards a material.
• Biodegradation: material breakdown in a biological system.
• Bioactive material: biomaterial intended to cause or modulate biological activity.
• Bioactivity: wanted (positive) reaction from tissues.

Classes of Biomaterials

• Classification is primarily based on chemical characteristics, atomic structures, and properties.
• Basic material types are:
  • Polymers
  • Metals
  • Ceramics
• Combination of basic material types:
  • Composites
• Nanoscale forms of basic materials
• Naturally occurring form of basic materials (natural materials)

Basic Types of Biomaterials

• Metals: advantages are high strength, toughness, fatigue resistance, and ductility. Examples are platinum alloys, titanium, cobalt, chromium, nickel, silver, gold, and stainless steel. Challenges are high modulus (stiffness) and possibility of corrosion.
• Polymers: advantages are high resilience, light weight, good corrosion resistance, and easy fabrication. Examples include nylon, silicone, polyester, PTFE, polyamides, and polyurethanes. Challenges are potential for thrombogenicity.
• Ceramics: High advantages are strength and corrosion resistance, resistance to wear, and low density. Examples are hydroxypatite, tricalcium phosphate, and calcium phosphate. Challenges are brittle, low resilience, and resistance to fatigue.

Biomaterials Applications

• Bone plates, bone cement, artificial ligaments, tendons, dental implants, blood vessel prostheses, heart valves, skin repair devices, cochlear replacements, contact lenses, and breast implants.

Key Applications of Synthetic Materials and Modified Natural Materials in Medicine

• Table providing various applications, biomaterials used, and number/year-global sales figures for skeletal, cardiovascular, and other organ applications by device.

Path from Biomaterials to Device and Clinical Application

• Research into biomaterial chemistry.
• Engineering to develop a device.
• Preclinical and clinical testing
• Approvals from regulatory agencies.
• Commercialization and clinical application

Biomaterials Challenges

• Toxicology, biocompatibility, inflammation and healing, specific anatomic location and mechanical performance requirements, and industrial involvement.
• Ethical concerns regarding animal and human subjects and regulatory issues.
• Importance of an interdisciplinary approach to overcoming these challenges

History of Biomaterials

• The Romans, Chinese, and Aztecs used gold in dentistry over 2000 years ago.
• Use of glass eyes, wooden teeth.
• Significant developments like aseptic surgical technique (1860s), glass contact lenses, plates and screws for fracture fixation (1886-1887), vanadium steel, and portable pacemakers (1912,1930-31), vitallium for dental implants (1937-38), total hip replacement (1949), intraocular lens (1949), biomechanically successful total hip implants (1951), osteointegration of metal implants, prosthetic vascular graft, portable pacemakers (1952-1959), and mitral valve replacement (1960)

Biomaterials: Generations

• Categorization of biomaterials into 1st, 2nd, 3rd and 4th generations based on their relationship with cells and their capacity for interactions, response, and degradation..

Polymers

• Organic materials with large molecular chains composed of repeating units.
• Classification into synthetic (polymers derived from petroleum products, e.g., polyethylene glycol, polyvinyl chloride, polycarbonate) and natural (polymers present in plant and animal sources, e.g., collagen, cellulose).
• Common properties include low density, ease of forming shapes, corrosion resistance, and a propensity to soften with an increase in temperature.

Polymeric Nanocarriers

• Classification includes nanospheres, nanocapsules, polymersomes, micelles, and dendrimers distinguished by their size, structures, and morphologies.
• Their physical and chemical properties provide potential for use in drug delivery.

Lipid Nanoparticles

• Attractive due to ease of manufacturing, reduced immune response, high cargo capacity, flexibility, and biodegradability.
• Used for drug and gene delivery, topical drugs, and vaccination formulations.
• Commonly classified into liposomes, solid lipid nanoparticles (SLNs), and nanostructured lipid nanoparticles (NLNs)

References/Textbooks

• Several textbooks and journals listed, and referenced with author, title, and publisher/publication information.

Classification of Nanostructures

• Categorization of nanomaterials based on dimensions (0-D, 1-D, 2-D, and 3-D).

Nanotechnology Applications

• Applications of nanotechnology in information technology, medicine, energy, and consumer goods.

Nanotechnology in Healthcare

• Nanotechnology's use in targeted drug delivery.
• Nanotechnology's use in detection and diagnostics.

Nanoparticles

• Different types of metallic and metal oxide nanoparticles.
• Different types of polymer nanoparticles.
• Different types of biocompatible and biodegradable lipid nanoparticles

Classification of Composites

• Categorization of materials based on their geometry. (particle-reinforced, fiber-reinforced, structural)

Fiber Reinforced Composites

• Improved stiffness and strength for load-bearing components like the stem of a hip prosthesis.

Bioactive and Biodegradable Composites

• Material capable of forming a chemical/physiological bond with tissue in the human body.

This quiz covers the fundamental concepts of mechanics, focusing specifically on biomechanics. Questions include the relationship between mechanics and mathematics and notable figures in the field. Test your knowledge on the effects of forces and the definitions within biomechanics.