Chapter 4 Molecules in Biomaterials and Tissue Engineering
Table 4.1 Classification of biomaterials in terms of their base structure and some of their most common applications. Replacement of soft tissues: skin, blood vessels, cartilage, ocular lens, sutures Orthopedic Polymers Nylon Synthetic rubber Crystalline polymers Joint replacement components, fracture fixation, dental implants, pacemakers, suture wires, implantable electrodes Metals - Atoms Aluminum, Chrome, Cobalt, Gold, Iridium, Iron, Manganese, Molybdenum, Nickel, Niobium, Palladium, Platinum, Tantalum, Titanium, Tungsten, Vanadium, Zirconium Metallic alloys wide variety using metallic atoms Heart valves and joint implants Composites Carbon-carbon fibers and matrices Dental and orthopedic Ceramics Aluminum oxide Carbon Hydroxyapatite Applications Biomaterials
Figure 4.1 These titanium-alloy joint replacements are an example of the many applications for metal biomaterials for implantations. (from http://www.spirebiomedical.com)
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.
Table 4.2 The clinical uses of some of the most common biomedical polymers relate to their chemical structure and physical properties. Catheters, sutures Poly(amides) (Nylons) Bioresorbable sutures, surgical products, controlled drug release Poly(esters) Artificial vascular graft, sutures, heart valves Poly(ethylene terephthalate) (PET) Coat implants, film, tubing Poly(urethanes) (PU) Blood bags, catheters, cannulae Poly(vinyl chloride) (PVC) Intraocular lens, dentures, bone cement Poly(methyl methacrylate) (PMMA) Bags, tubing Nonwoven fabric, catheter Orthopedic and facial implants Poly(ethylene) (PE) Low density (LDPE) High density (HDPE) Ultra high molecular weight (UHMWPE) Application Biomedical polymer
Figure 4.3 This artificial heart valve is coated with Silizone, a biocompatible material that allows the body to accept the implant. (from http://www.sjm.com/devices/).
Table 4.3 Some examples of current applications in tissue engineering. Not all of the listed applications are at the same developmental stage. Bioartificial skin substitutes Skin Neurotransmitter-secreting cells (polymer-encapsulated) Neural circuits and biosensors Peripheral nerve regeneration Neural Cartilage reconstruction Bone reconstruction Musculoskeletal Bioartificial pancreatic islets Bioartificial liver Liver and pancreas Endothelialized synthetic vascular grafts (angiogenesis) Regeneration of the arterial wall Compliant vascular prostheses Cardiovascular Hematopoietic (production of red blood cells by) stem cells culture Blood Example of application Biological system
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 http:// www.orc.soton.ac.uk/~wsb/photos.htm ).
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 http:// www.topac.com/l ).
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 www.thermo.com/eThermo/CDA/Products/Product_Detail/1,1075,15885-158-X-1-1,00.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.
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 ( ) = < I ( t ) I ( t + )> = delay time I ( t + ) = intensity at ( t + ) I ( t ) = intensity at time t G ( ) = ACF