Composites

Composites and Textiles Lecture 6

UMass Amherst

Composites: The What and Why

  • Composites address the limitations of single component materials.
  • Single component materials may not provide the required strength.
  • Need for transition zones to mimic biological structures.

Composites for Reinforcement

  • Definition: Composite materials consist of two or more constituents separated by a distinct interface.
  • Characteristics:
    • Contain a discontinuous phase (filler) embedded in a continuous phase (matrix).
    • Example of Reinforced Concrete:
    • The discontinuous phase offers reinforcement, improving material properties.

Benefits of Reinforced Composites

  • The discontinuous phase generally possesses superior strength or hardness that enhances:
    • Stiffness
    • Strength
    • Toughness
  • Fillers can enhance bioactivity by mimicking tissue properties.
  • Properties are influenced by:
    • Constituent materials
    • Distribution and content of materials
    • Interfacial interactions
  • Note: Poor interfacing can negate the advantages provided by a composite material.

Types of Composites

  • Classification based on the geometry of the reinforcing material(s):
    • Fibrous Composites: Long or short fibers.
    • Particulate Composites: Particles serving as the filler.
    • Laminates: Stacked sheets (laminae).

Composite Materials on the Market

  • Examples include:
    • Middle Ear Implants: Hydroxyapatite (HA) particles in Polyethylene (PE).
    • Artificial Cartilage: Comprises Kevlar and Polyvinyl Alcohol (PVA).
    • Orthopedic Implants: Use carbon fiber with Polyetheretherketone (PEEK).
  • Reference: Xu et al., 2017, most biomedical composites contain polymer matrices.

Composites to Mimic Tissue Interfaces

  • The composite is defined by the interface region where fibers from both sides are embedded within a polymer matrix.
  • Example: Tendon Extracellular Matrix (ECM) with synthetic polymer (Chang et al., Sci Adv, 2020).

Fiber Anisotropy Improves Strength

  • Fiber orientation significantly influences the biaxial stress response.
  • Controlling the fiber directionality aids in recreating the structure and mechanics of soft tissues, such as tendons.

Fiber Forming Polymer Requirements

  1. Medium to high molecular weight (MW) ranging from 20 kD to 250 kD.
  2. Linear polymer structure is required.
  3. Low intermolecular bonding is paramount for flexibility.
  4. Ordered or crystalline structures enhance mechanical properties:
    • Some entanglement is beneficial, but excessive branching inhibits chain sliding.
    • Greater crystallinity can lead to improved fiber integrity.

Strain-Induced Crystallization

  • Drawing fibers promotes crystallinity; however, low MW or amorphous polymers may break under excessive elongation.
  • Reference: Zhang et al., 2013.

Fiber Forming Methods

  • Types of spinning methods discussed:
    • Melt spinning:
    • Involves melting polymer resin and extruding it.
    • Best suited for temperature-stable thermoplastics.
    • Shape dictated by extrusion openings.
    • Wet/Gel spinning:
    • Utilized for polymers that degrade at high temperatures.
    • Polymers are dissolved in a solvent and extruded into a non-solvent (coagulation process).
    • Fibers can be grooved as a result of solvent evaporation.
    • Electrospinning:
    • Produces fibers greater than 100 nm by exposing polymer solutions or melts to high voltage.
    • This technique results in smaller fiber sizes due to drawing forces.
    • Bicomponent spinning:
    • Involves spinning multiple polymer components simultaneously.
    • Capable of achieving small dimensions and unique structures; one component may be degradable.

More Uses for Fibers: Medical Textiles

  • Characteristics sought after for medical textiles:
    • Strong and flexible fibers
    • Porous materials with excellent fatigue properties
    • High surface area for cell adherence
  • Production methods include weaving, knitting, or braiding.
  • Advantages explored in terms of application effectiveness.

Textile Structures

  • Strong and dimensionally stable yet may be less flexible and difficult to handle.
    • High permeability and flexibility for suturability but may dilate post-implantation.
    • High longitudinal tensile strength but potential instability under torsion.
  • Types of textile structures:
    • Wovens
    • Knits
    • Braids

Strain Response of Biotextiles

  • Analyzed different forms of textiles:
    • Stress-strain relationships for single fibers, woven, and knitwear.
  • Reference: Maziz et al., 2017.

Example Biotextiles

  • Applications include:
    • Wound dressings
    • Ligament replacement
    • Gore-Tex synthetic ligament applications.

New Applications: Artificial Tissues

  • Textiles' porous structure allows for cellular growth.
  • Example: A 3D woven PCL construct seeded with human stem cells illustrates total joint resurfacing (Moutos et al., 2016).

Cardiovascular Applications

  • Various textile designs in cardiovascular surgery implementations:
    • Woven straight grafts
    • Knitted bifurcated grafts
    • Knitted removable external spiral support grafts.
  • Market examples:
    • DeBakey Soft WovenⓇ, Twill WeaveⓇ, Barone Microvelour weft knitⓇ.

Case Study: Coronary Artery Bypass

  • Procedure overview: redirects blood flow from clogged coronary arteries.
  • Common causes include:
    • Cholesterol or plaque buildup (atherosclerosis).
  • Risk factors identified:
    • High cholesterol
    • Obesity
    • Smoking
    • Hypertension

Health Disparities in CAB-Graft

  • Noted disparities among demographics in health outcomes:
    • In 2015-2016, non-Hispanic black adults (20 and over) had the highest hypertension rates.
  • Statistically adjusted data:
    • Hispanic, non-Hispanic black, non-Hispanic white, and non-Hispanic Asian adults aged 20 and over showed varied prevalence of obesity and hypertension.

Health Disparities in CAB-Graft (continued)

  • Heart disease percentage breakdown for demographics in 2017:

    • Non-Hispanic white: 11.5%
    • Non-Hispanic black: 9.5%
    • Hispanic: 7.4%
    • Non-Hispanic Asian: 6.0%

  • Trends over time (1999 to 2017) indicate a significant difference between groups.

  • Acknowledged factors contributing to disparities in care and outcomes.

  • Surgeons' centers of treatment impact the outcomes, particularly among minority patients.

  • An estimated 30% of disparity attributed to socioeconomic factors (Leigh et al., 2016; Collins et al., 2010).

  • Reported outcomes:

    • Female patients experienced higher mortality rates compared to males.
    • Higher mortality and readmission rates were noted in black patients post-CABG compared to other races.
    • Race identified as a predictor of drug-eluting stent thrombosis.