This chapter describes the major topics of this dissertation. Successful clinical use of resorbable, bioinert and bioactive implants will significantly advance medical needs. Tissue engineering strategies have recently emerged as the most advanced therapeutic option presently available in regenerative medicine. Tissue engineering includes the use of cells and biomolecules in artificial implants that compensate for body functions that have been lost or impaired as a result of disease or accidents (6; 8). Tissue engineering is based upon scaffold-guided tissue regeneration, a process that involves the seeding of porous, biodegradable scaffolds with donor cells, which then differentiate and produce tissue that mimic naturally occurring tissues. These tissue engineered constructs are then implanted into the patient to replace diseased or damaged tissues. With time, the scaffolds are resorbed and replaced by host tissues that include viable blood supplies and nerves. Current clinical applications of tissue-engineered constructs include the engineering of skin, cartilage and bone for autologous implantation (7; 13). There are some implants which are not biodegradable for example titanium.
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In this study we are using two different types of scaffolds: anodized titanium and halloysite-PCL scaffolds. Anodized titanium can be used in orthopaedic and dental implants. Halloysite-PCL scaffold can be used for drug delivery, wound healing and tissue engineering.
Titanium implants are used in dental and orthopedic application. It is very difficult to produce properties of bone in synthetic implants. These synthetic implants do not give satisfactory results; the average life of a knee, hip or ankle implant is 10 to 15 years. Accordingly, patients need to undergo repeated implant surgery. The reason for implant fail is, that implants are unable to produce a sufficiently strong cellular response and so integrate the implant into surrounding tissue (42). In this study, we examined if nanoporous titanium, manufactured by anodization, helps improve an implant's life span or not. Nano means one billionth (). Nano-titanium has a unique property due to nano pores on it; these provide improved magnetic, electric properties and better structural integrity leading to a better cellular response (42; 43; 44).
Anodization of Titanium
When a constant voltage or current is applied between the anode and cathode, an oxide layer forms on the anode surface. The main chemical reactions specifically for anodizing titanium are listed below. Figure 2.1 shows anodization set up.
Ti/Ti oxide interface
Ti ↔ Ti2+ +2e-
At the Ti oxide/electrolyte interface
2H2O ↔ 2O2-+ 4H+
2H2O ↔ O2+4H+ + 4e-
At both interfaces:
Ti2+ + 2O2- ↔ TiO2 + 2e-
Figure 2. Anodization set up (45)
Because titanium oxide has higher resistivity than the electrolyte and the metallic substrate, the applied voltage will mainly drop over the oxide film on the anode. As long as the electric field is strong enough to drive the ion conduction through the oxide, the oxide film will keep growing. This explains why the final oxide thickness is almost linearly dependent on the applied voltage.
Anodization of titanium can be done with different chemicals, for example, electrochemical anodization in H2SO4/HF solutions, phosphate electrolytes, chloride-ion-containing media, acid/ethanol mixture (46; 47; 48; 49). Titanium has corrosion resistant propertys due to its titanium oxide layer. This layer is formed automatically when titanium is exposed to the air. Still, this layer is not reactive enough, so it is unable to form a direct bonding with the bone. Due to this insufficient osseointegration, the implant life is very short. So here we are trying to improve surface properties of implants by doing anodization to make nanoporous similar to natural bone (50).
Influences of processing parameters
The resulting oxide film properties (such as degree of roughness, morphology, chemistry, ect.) after anodization varies over a wide range according to different process parameter such as applied voltage, current density, electrolyte composition, pH, and temperature. Different acids (phosphoric acid- H3PO4, sulfuric acid H2SO4, acetic acid-CH3COOH and others), neutral salts and alkaline solutions are widely-used electrolytes for the anodization of titanium. Generally, it was found that among all the electrolytes the anodic oxide thickness in H2SO4 was the highest.
Anodized oxide film
Kurze investigated wide structure and properties of anodic spark deposition (ASD). ASD is rough, porous texture with cracks on it. The diameter of the pores varied from a few hundred nanometers to a few micrometers depending on the processing parameter and is not uniform within the same anodized surface.
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After anodization, thickness of the protective oxide layer increases and it could lead to less ion release in the human body. The oxide barrier layer is considered to contribute to the improvement of corrosion resistance.
Currently, many different implant materials are under investigation, including titanium, ceramics, polymers, and biologically synthesized substances. Compared to all material, titanium is the best material because it more biocompatible, durability and corrosion resistance. Still, according to previous studies, metal such as smooth titanium failed to produce proper osseointegration (44).
Need for long lasting and better quality implants
Current implants do not help in changing bone mass that occurres due to osteoporosis and fractures. The average life of these implants is 10 to 15 years. The life time of the implant is very short for babies and young people. Bone implants replace missing bone and provide surface where bone and a vascular network can be regenerated and better osseointegration can occur. Once the implant is implanted at the injured site, protein from blood, bone marrow and other tissues for example, fibronectin and vitronectine, starts adhering to the implant surface. These proteins then control subsequent cell adhering. Protein bound to the implant surface osteoblast cells then binds to the implants. Thus it very important to have a proper surface where initial protein adsorption occurs. Here chemical and physical properties of the implant's surface determin the initial adsorption of the protein. So the goal is to produce implants with such chemical and topographic properties, that can solve current problems in orthopedic and dental implants (42; 43; 44).
Osteoporosis and implants
Osteoporosis is a bone disorder resulting in reduced bone density and bone strength. Osteoporosis can be asymptomatic. Symptoms and problems of osteoporosis include fractures, pain, and deformity. Osteoporosis is usually found in the aged population, especially women, who are very prone to this disorder. Implants that provide better structural integrity by providing quiker and better cellular response are very useful for repairing a fracture in osteoporosis patients (42; 44). Anodized titanium may prove useful in generating faster and better cellular response in osteoporosis and aged people.
Electrospinning PCL-Halloysite Scaffolds
Figure 2. Electrospinning set up (51)
Electrospinning is a technique by which scaffolds of micro and nano scale polymer fibers can be fabricated by subjecting a polymer solution to an electric field. The electrospinning set up is shown in figure 2.2. The electrospining set consists of a syringe pump, a high voltage source and a collector. Surface tension holds a polymer solution at the tip of the needle. Higher voltage in the needle causes induction of charges in the polymer solution. The polymer jet is formed when the charge repulsion wthin the solution overcomes the surface tension (52; 53; 54). As the jet leaves the tip, it thins considerably and it may experience a fluid instability resulting in bending of the jet and even whip-like motion .Since the charge on this jet allows its path to be guided by an electric field (55). During the polymer jet tavel path, the evaporation of the solvent occurs, and the fibers are collected on a collector plate. Solutions with higher conductivity have more tendencies for jet formation (56; 57; 58). In this study we are trying to electro spin halloysite-PCL scaffold and drug loaded halloysite-PCL scaffold.
Parameters affecting electrospinning
The diameters of electrospun fibers may be controlled by varying the parameters of the electrospinning process, which include electric field voltage, distance between the capillary tip and the collection screen, solution feeding rate, and solution parameters (e.g. concentration, solvent, surface tension, viscosity, polymer molecular weight) (59; 60). It has been shown that fiber diameter varies proportionally with voltage and solution feeding rate (60). The temperature, humidity, and air flow within the electrospinning chamber also have some effect on the fiber morphology (59; 61)
Drug delivery, is one of the most promising biomedical applications of nanotechnology. The use of nanomaterial's as nanocarriers for improving the delivery methods has been advantageous technically and viable economically. These nanocarriers, formed by the process of nanoencapsulation, are of two types: named as nanospheres and nanocapsules. Halloysite is an example of nanocapsules (62).
Halloysite is a nanotubular clay particle. It is mined from natural deposits in different countries. Utah has the largest deposit in the USA. Halloysite is a two-layered aluminosilicate. It has a hollow tubular structure in the submicron range. Halloysite is chemically similar to kaolinite (62; 63; 64; 65). It is formed from kaolinite over millions of years due to the action of weathering and hydrothermal processes. Layers are rolled into nanotubes due to the strain caused by lattice mismatch between adjacent silicone dioxide and aluminum oxide sheets (62; 63; 64).
Chemical Composition and Structure of halloysite
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Halloysite occurs in nature as a hydrated mineral. It is also called halloysite, due to its thickness, which is close to 10 A. Heating halloysite (10A) can easily and irreversibly dehydrate to form halloysite (7A). The chemical formula of halloysite is Al2Si2O5(OH)4.2H20, which is similar to kaolinite, except for the presence of an additional water monolayer between the adjacent layers. Figure 2.3 shows the halloysite nanotubes. The outside diameters of the halloysite vary from 40 to 190 nm. The average outside diameter is 70 nm. The diameters of the internal lumen vary from 15 to 100 nm. The average inner diameter is 40 nm. The lengths of halloysite vary from 1 to 20 µm (64; 66; 67; 68). The morphology of halloysite nanotubes vary depending on their geological occurrences and crystallization conditions. Chemical compositions of different morphologies are also different (67).
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Figure 2. Halloysite nanotubes (69)
Physical and Chemical Properties of halloysite
Halloysite minerals have relatively high specific surface areas. Surface areas vary from 50 to l40 m2g'1 with deposit type. Halloysites are abundant with narrow cylindrical pore. Due to this reason they can absorb a relatively big class of compounds. They include inorganic and organic salts as well as polymers and biologically active agents (64; 67; 70). Absorption of small organic molecules and salts primarily takes place by their intercalation into an interlayer space (71; 70; 72), while big polymer molecules like proteins and drugs are bound to its outer and inner faces. Intercalation does not occur with bigger molecules like proteins, polymers and other macromolecules due to their large molecular size (63; 67). The organic compounds that have small molecular sizes and hydrophilic functional groups such as OH and NH2 were observed to form an intercalation complex with the halloysite nanotube. This is due to the formation of hydrogen bonds between them and alumina or silica layers (65; 71).The adsorptive and ion exchange properties of halloysite nanotubes are greatly affected by their surface charges. Surface charges are pH-dependent. Halloysite has a higher negative surface charge compared with kaolinite showing the predominance of silica properties at the outer surface (62).
The capillary force helps in better adsorption of several materials. Capillary force is calculated by using the following formula,
h = (2y cosθ) / (pgr)
where, y is the liquid-air surface tension (J/m2 or N/m), p is the density of liquid (kg/m3), g is acceleration due to gravity (m/s2), θ is the contact angle, r is radius of the tube (m) (73).
For a water-filled tube in air at sea level, Y is 0.0728 J/m2 at 20 0C, θ is 20' (0.35 rad), p is 1000 kg/m3 , g is 9.8 m/s2, therefore, the height of the water column is given by:
h = (1.4 x 10 -5)/r
For halloysite nanotubes of average inner radius of 7nm the capillary force in terms of the height of the water column is h - 2000 in. Hence it is understandable that this much higher capillary force helps the halloysite in the quick adsorption of several materials (73; 74).