Chapter 4 Molecules in Biomaterials and Tissue Engineering

BiomaterialsApplications Ceramics Aluminum oxide Aluminum oxide Carbon Carbon Hydroxyapatite Hydroxyapatite Dental and orthopedic Composites Carbon-carbon fibers Carbon-carbon fibers and matrices and matrices Heart valves and joint implants Metals - Atoms Aluminum, Chrome, Cobalt, Gold, Aluminum, Chrome, Cobalt, Gold, Iridium, Iron, Manganese, Molybdenum, Iridium, Iron, Manganese, Molybdenum, Nickel, Niobium, Palladium, Platinum, Nickel, Niobium, Palladium, Platinum, Tantalum, Titanium, Tungsten, Tantalum, Titanium, Tungsten, Vanadium, Zirconium Vanadium, Zirconium Metallic alloys wide variety using metallic atoms wide variety using metallic atoms Joint replacement components, fracture fixation, dental implants, fracture fixation, dental implants, pacemakers, suture wires, pacemakers, suture wires, implantable electrodes implantable electrodes Polymers Nylon Nylon Synthetic rubber Synthetic rubber Crystalline polymers Crystalline polymers Replacement of soft tissues: skin, blood vessels, cartilage, ocular lens, blood vessels, cartilage, ocular lens, sutures suturesOrthopedic Table 4.1 Classification of biomaterials in terms of their base structure and some of their most common applications.

Figure 4.1 These titanium-alloy joint replacements are an example of the many applications for metal biomaterials for implantations. (from

Figure 4.2 Polymers are made up of many monomers. This is the monomer for poly(ethylene), a common biomaterial used for medical tubing and many other applications.

Biomedical polymer Application Poly(ethylene) (PE) Low density (LDPE) Low density (LDPE) High density (HDPE) High density (HDPE) Ultra high molecular weight Ultra high molecular weight (UHMWPE) (UHMWPE) Bags, tubing Nonwoven fabric, catheter Orthopedic and facial implants Poly(methyl methacrylate) (PMMA) Intraocular lens, dentures, bone cement Poly(vinyl chloride) (PVC) Blood bags, catheters, cannulae Poly(ethylene terephthalate) (PET) Artificial vascular graft, sutures, heart valves heart valves Poly(esters) Bioresorbable sutures, surgical products, controlled drug release products, controlled drug release Poly(amides) (Nylons) Catheters, sutures Poly(urethanes) (PU) Coat implants, film, tubing Table 4.2 The clinical uses of some of the most common biomedical polymers relate to their chemical structure and physical properties.

Figure 4.3 This artificial heart valve is coated with Silizone, a biocompatible material that allows the body to accept the implant. (from

Biological system Example of application Blood Hematopoietic (production of red blood cells by) stem cells culture cells by) stem cells culture Cardiovascular Endothelialized synthetic vascular grafts (angiogenesis) grafts (angiogenesis) Regeneration of the arterial wall Compliant vascular prostheses Liver and pancreas Bioartificial pancreatic islets Bioartificial liver Musculoskeletal Cartilage reconstruction Bone reconstruction Neural Neurotransmitter-secreting cells (polymer-encapsulated) (polymer-encapsulated) Neural circuits and biosensors Peripheral nerve regeneration Skin Bioartificial skin substitutes Table 4.3 Some examples of current applications in tissue engineering. Not all of the listed applications are at the same developmental stage.

Figure 4.4 (a) TEM microscope. The electron beam passes through the sample, generating on the fluorescent screen a projected image of the sample, which can be recorded by photographic means.

Figure 4.4 (b) SEM microscope. Condenser lenses focus the electron beam on the specimen surface leading to secondary electron emission that is captured by the detector and visualized on the CRT screen. Both TEM and SEM operate in a particle free (vacuum) environment.

Figure 4.5 Principle of SEM operation. An incident beam of primary electrons displaces orbital electrons from the sample atoms resulting in secondary electron emission which is detected for image formation. Some primary electrons pass by the nucleus to become backscattered electrons.

Figure 4.6 (a) An STM probe tip made of tungsten magnified 4,000 times. The tip is very small, and can be ruined on a sample, which is seen in Figure 4.6 (b). (from

Figure 4.7 This is a sample of a piezotube. There are different approaches, but all use the same method of two opposing piezoelectric materials to move the sample in each axis. (from

Figure 4.8 STM schematics. The tip of a probe scans the surface of the sample. Three dimensional movements of the sample under the tip are accomplished using a voltage- controlled piezoscanner. The tunneling current crossing from the sample to the tip is further processed leading to a topographical image.

Figure 4.9 Sketch of an SFM. A laser beam is focused on the cantilever, and reflected back to a two-segment photodetector. The difference in output from each segment is proportional to the deflection amplitude of the cantilever scanning the sample.

Figure 4.10 When an X-ray photon (a) interacts with an atomic orbital electron of the sample, a photoelectron (b) is emitted. The now unstable atom must relax to the ground state. The relaxation process can be accomplished by either of two mechanisms: (1) an outer orbital electron releases energy as fluorescent radiation (c) while occupying the place of the emitted photoelectron, or (2) the excess energy is used to unbind and emit another outer orbital electron called an Auger electron (d). These mechanisms operate for different sample depths, yielding the Auger electron emission characteristic of the outermost surface of the sample.

Figure 4.11 A typical XPS spectrum, showing photoelectron intensity as a function of binding energy. Each peak may correspond to a distinct element of the periodic table or to different orbital electrons of the same element. Some peaks may also represent Auger radiation.

Figure 4.12 Basic schematics of an XPS instrument. An X-ray beam strikes the sample surface, giving photoelectron radiation. These electrons enter the hemispherical analyzer where they are spatially dispersed due to the effects of the retarding grid and of the electrostatic field of the concentric hemispheres. Ramping voltages at the retarding grid allow kinetic energy scanning. At the other end of the analyzer electrons are detected, counted, and a spectrum of photoelectron intensity versus binding energy is displayed.

Figure 4.13 Photograph (from 1,00.html) and schematics of an ESCALAB. This iXPS instrument offers the capability of parallel imaging, which obtains positional information from dispersion characteristics of the hemispherical analyser and produces photoelecton images with spatial resolution better than 5  m. 1,00.html

Figure 4.14 Schematic diagram of a SIMS instrument. Bombardment of primary ions on the sample surface leads to secondary ion emission. A mass analyzer separates these ions in terms of their mass-to-charge ratio. The ion detector converts this ionic current into an electrical signal for further processing. The display presents the SIMS spectra, consisting of the count of ions versus their mass-to-charge ratio.

Figure 4.15 Michelson interferometer. A beamsplitter transmits half of the source radiation to the fixed mirror and the other half to the sliding mirror. A phase difference between the beams can be induced by sliding the mirror causing detection of the two beams at different times. The detector provides the interferogram, a plot of energy as a function of differences in optical paths. Beams have been slightly shifted in the drawing to allow easy following of their path.

Figure 4.16 When an incident beam traveling at an angle  in a medium of refractive index  c encounters another medium of refractive index  s, it will reflect in a direction given by  and refract in the direction given by , verifying Snell’s Law of Refraction.

Figure 4.17 Surface tension components of a three-phase system to limit the spread of a drop on top of a surface.  is the interfacial free energy for each of the phases.  is the contact angle.

Figure 4.18 The amino acid molecule. To a central carbon atom, an amino group, a carboxyl group and a hydrogen atom are bonded. R represents the rest of the molecule, which is different for each of the 20 amino acids.

= time average Figure 4.19 The autocorrelation function G(  ) is 1 when two signals have delay time  = 0, then decays to 0 for long delay time. G(  ) =  = delay time I(t +  ) = intensity at (t +  ) I(t) = intensity at time t G(  ) = ACF