Material Properties of the Human Body

UMass Amherst – Lecture Notes on Material Properties of the Human Body

Key Points to Understand

  • Biological Tissue Composition
    • Composition includes various cell types and the extracellular matrix (ECM).
  • Factors Influencing Tissue/Material Function
    • Understanding factors that influence the interaction and bonding between cells and materials.
  • Biomaterial Design Inspiration
    • Investigating how native tissue characteristics can guide the design of biomaterials.
  • Cell Interaction with Environment
    • Examination of how cells sense and interact with their environment, and the consequences of these interactions.

Functional Units of Biology

  • Tissues serve as the original biomaterials from which biological functions arise, featuring:
    • Cell Types
    • Extracellular Matrix Types

Main Types of Tissue

  • There are four main tissue types in biological organisms, each with specific structural compositions:
    • Connective Tissue
    • Epithelial Tissue
    • Nervous Tissue
    • Muscle Tissue

Extracellular Matrix (ECM)

  • The ECM is a complex network composed of:
    • Fibrous Proteins
    • Polysaccharides
  • The ECM is a hydrated environment that cells produce and occupy, primarily consisting of collagen I fibers.
  • Polysaccharides play a critical role in retaining water and cytokines, thus influencing tissue hydration and cellular communication.

ECM Composition Variability by Tissue Type

  • Different tissues have distinct ECM compositions including:
    • Articular Cartilage
    • Proteins: Col2, COMP, Decorin, etc.
    • Tendon
    • Proteins: Col1, COMP, etc.
    • Bone
    • Skeletal Muscle
    • Ligament

Functions of ECM

  • The ECM performs various critical functions, including:
    • Adhesive Substrate
    • Directing migratory cells through substrate tracks.
    • Establishing concentration gradients for haptotactic migration.
    • Providing structural support and defining tissue boundaries.
    • Presenting and sequestering growth factors, aiding in signaling and cellular responses.
    • Mediating mechanical signals and responses that influence cell differentiation.
  • Cell-ECM Interaction
    • Cells remodel the ECM, influencing their own survival, proliferation, and migration.

Cell/Tissue-Biomaterial Interactions

  • Interactions significantly affect:
    • Cell Survival
    • Cell Shape
    • Cell Migration Ability/Directions
  • Tissue-derived cells commonly require adhesion to solid surfaces.
  • Understanding and fabricating requisite signals to elicit intricate cellular behaviors remain key challenges.

Determinants of Form and Function in Tissues

  • Interaction between ECM composition and cell functionality creates a dynamic reciprocity.
  • Turnover and Remodeling
    • Cells produce enzymes that degrade the ECM, leading to continuous remodeling that can induce dysfunction if ECM properties are altered.

ECM Receptors

  • Major receptors binding to the ECM include:
    • Integrins
    • CD44
    • Syndecan
    • Other ECM components such as collagen, laminin, fibronectin, etc.

Adhesive Proteins in ECM

  • Critical adhesive proteins include:
    • Elastin: High elasticity, allowing stretch.
    • Collagens: The most abundant protein, primarily fibrous.
    • Laminin: Supports adhesion and crosslinking.
    • Fibronectin: Insoluble matrix-bound protein aiding in adhesion.

Adhesive Ligands and Their Functions

  • Common sequences in ECM proteins and their location include:
    • RGDS - Ubiquitous in ECM proteins
    • YIGSR, IKVAV, LGTIPG - Found in Laminin
    • PDSGR, REDV - Fibronectin
    • DGEA, VGVAPG - Associated with Collagen and Elastin.

Structure-Function Relationship in ECM

  • Structure Adaptation
    • The ECM’s structure is specifically adapted to fulfill particular functions.
    • Changes in structural configurations can lead to functional alterations.

Factors Influencing Tissue Patterning

  • Tissue patterning is influenced by:
    • Neighboring Cells
    • Matrix Composition
    • Diffusible Cues (e.g., cytokines)
    • Environmental Conditions (like forces, temperature, ionic composition)
    • Availability of Essential Nutrients and Oxygen
  • Unique chemical, mechanical, and electrical properties develop in each tissue, influenced by these factors.

Vascular Perfusion Requirements in Tissues

  • Oxygen diffusion limit is approximately 100-200 μm.
  • Thick tissues necessitate perfusion for nutrient distribution, making it a critical constraint in artificial tissue engineering.

Interstitial Space and Fluid Flow

  • Interstitial flow plays a pivotal role in nutrient delivery, albeit operating at a slower rate and in a more complex 3D structure.

Mechanical Properties of Various Tissues

  • Mechanical properties characterized by stiffness (expressed in kPa) vary widely across different tissues:
    • Examples include skeletal muscle, myocardium, artery, skin, and cartilage among others with stiffness ratings ranging from 10−110^{-1} to 10610^{6} kPa.

Mechanotransduction and Loading Conditions

  • Mechanotransduction refers to cells' ability to sense mechanical stresses and transduce that into cellular responses.
  • Types of mechanical loading include:
    • Tensile Force
    • Compressive Forces
    • Shear Forces
  • Each type of loading informs performance predictions for implants and cellular responses.

Summary of the Tissue Microenvironment

  • The tissue microenvironment encompasses:
    • Many cell types
    • Various chemical and electrical signs
    • Mechanical cues and patterns
    • Vascular and interstitial fluid flows
  • Tissues profoundly consist of multiple cell types embedded within a hydrated ECM of protein fibers and sugars, all of which play critical roles in cellular function and behavior.

Final Summary

  • Tissues comprise various cell types immersed in an ECM of proteins and glycosaminoglycans, shaping their function.
  • The interplay of chemical, mechanical, and structural cues affects cellular behavior, migration, and gene expression.
  • Tissue properties diverge significantly between health and disease, presenting opportunities for biomaterial design leveraging natural tissue characteristics. Cells utilize integrin receptors to sense mechanical loads and fluid dynamics to facilitate vital biological processes.