Biomaterial Scaffolds

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1 Biomaterial Scaffolds Biomaterial Properties Surface properties Bulk properties Biological properties Types of Biomaterials Biological materials Synthetic materials

2 Surface Properties The body reads the surface structure and responds. Surface superficial or skin deep.

3 Some Possibilities for Surface Structure

4 Surface Properties The surface region of a material is known to be uniquely reactive The surface of a material is inevitably different from the bulk Surfaces readily contaminate The surface structure of a material is often mobile.

5 How surfaces interact with molecules? Nonspecific interaction Specific binding Surface topology

6 Nonspecific Interaction Fundamentally attractive van der Waals force that arises from dipole dipole type interactions; Electrostatic forces resulting from charged molecules Hydration or solvation force that results from expulsion of water between the two surfaces Hydrophobic effects that non polar molecules tend to form intermolecular aggregates in an aqueous medium Repulsive steric forces that arise due to proteins on both surfaces forming spikes of up to 10 nm.

7 Surface Topology Topological features occur at different length scales: Sub cellular level (< 10 μm) Cellular level ( μm) Multi cellular level (>100 μm) Modulate protein adsorption Constrain receptor binding and related signaling pathways cell attachment, spreading, migration, and function Porosity

8 Surface Characterization Contact Angle Methods Electron Spectroscopy for Chemical Analysis (ESCA; a.k.a. X ray Photoelectron Spectroscopy (XPS)) Secondary Ion Mass Spectrometry (SIMS) Infrared Spectroscopy (IRS) Scanning Electron Microscopy (SEM) Scanning Tunneling Microscopy (STM) Atomic Force Microscopy (AFM)

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10 Contact Angle Methods Wettability

11 Young s Equation: γ = γ + γ cosθ sv sl lv γ sv where is solid vapor surface tension γ sl is solid liquid surface tension γ lv is liquid vapor surface tension Surface tension can be thought of as the energy required to create a unit area of an interface.

12 Critical Surface Tension γ c For a particular surface, liquids with a surface tension lv c will wet the surface. Conventionally, a liquid wets the surface if the contact angle is less than 10 degree. Zisman Method γ γ Surface Free Energy Surface free energy is proportional to critical surface tension. Therefore, the lower critical surface tension, the lower free energy of the surface.

13 Concerns in Contact Angle Measurement The measurement is operator dependent Surface roughness influences the results Surface heterogeneity influences the results The liquids used are easily contaminated (typically reducing their surface tension) The liquids used can reorient the surface structure The liquids used can absorb into the surface, leading to swelling The liquids used can dissolve the surface Few sample geometries can be used Information on surface structure must be inferred from the data obtained

14 Electron Spectroscopy for Chemical Analysis (ESCA) Based on photoelectric effect. X rays are focused upon a specimen. The interaction of the X rays with the atoms in the specimen causes the emission of a core level (inner shell) electron. The energy of this electron is measured and its value provides infromation about the nature and environment of the atom from which it came.

15 Secondary Ion Mass Spectrometry (SIMS) Instead of a beam of electrons, a beam of primary ions is used in SIMS. Emitted secondary ions are collected and analyzed. In ESCA, the energy of emitted electrons is measured. SIMS measures the mass of emitted ions.

16 Infrared Spectroscopy (IRS) or Fourier Transform Infrared (FTIR) Spectroscopy The infrared spectrum of a sample is collected by passing a beam of infrared light through the sample. Examination of the transmitted light reveals how much energy was absorbed at each wavelength. An absorbance spectrum shows at which IR wavelength the sample absorbs.

17 Bulk Properties The electronic and atomic structures, and almost all the physical properties, of solids depend on the nature and strength of the inter atomic bonds: Ionic Bonding Covalent Bonding Metallic Bonding Weak Bonding van der Waals and hydrogen bonding

18 Mechanical Properties Orthopedic applications Cardiovascular applications The mechanical properties of the engineered tissues should match that of the host tissue.

19 Wolff s law Bone in a healthy person is capable of adapting loads that is placed under. If loading on a particular bone increases, the bone will remodel itself over time to become stronger to resist the loading. Conversely, if the loading on a bone decreases, the bone will become weaker.

20 Stress Shielding The metal alloy implants have a much higher stiffness than the bone. This causes an effect called stress shielding, where all load is transferred through the metal and not the bone, causing the body to resorb the bone.

21 Diminished shield effect of the femoral shaft component is one of the selling argument of new models of total hip devices. But can one really produce artificial total hips with stiffness values that are almost identical with the stiffness of the skeleton around the total hip device? With Tissue Engineering, you can!

22 Another Example Compliance mismatch of the vascular graft to the host artery could lead to intimal hyperplasia at the joint site, resulting in graft failure Neointimal hyperplasia

23 Characterization of Mechanical Properties Characterization of mechanical properties of materials is important for matching biomaterial properties to the in vivo microenvironment. Elasticity typical reported parameter: Elastic modulus (slope of stress strain curve) Yield strength (the stress at which a given amount of plastic deformation occurs) Ultimate tensile strength (the stress at which the material fails) Ductility (the total plastic strain exhibited before fracture) Toughness (area under the stress strain curve until failure)

24 Viscoelasticity dynamic mechanical testing In order to match the mechanical properties of the surrounding tissue, it is important to note that the elastic moduli of biological tissues are highly nonlinear. Such nonlinearity is much harder to quantify. We will talk about mechanical properties in details later when we reach Cell and Tissue Mechanics.

25 Biological Properties Factors that influence biocompatibility Surface finish (smooth, rough, powder, and surface porosity) Implant size Surface free energy (hydrophilicity, and charged) Wear debris and degradation Immunogenicity (tendency to stimulate the immune response) Mechanical properties (compliance)