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ROLE OF BIOPOLYMERS AS BIO IMPLANTS

3.3.1 BACTERIAL PLASTICS

Bio-based plastics are starch-based plastics, protein-based plastics, and cellulose-blended plastics. Blends are prepared with conventional plastics such as polyethylene (PE), PP, and polyvinyl alcohol. These are bio-based plastics which are partially biodegradable. Petroleum derived plastics will remain as broken pieces may create additional pollution. These plastics have intrinsic thermal and mechanical weaknesses and they are now discouraged for applications. Bacteria are employed to make the building blocks for biopolymers from renewable sources. To produce bio-based plastics completely resembling conventional plastics, starch, cellulose, fatty acids are used. Bacteria can consume the organic material for growth. Some of the building blocks can be produced microbial for polymerization purposes. There are many structural variations of hydroxyalkanoic acids. They are lactic acid, succinic acid, (R)-3-hydroxypropionic acid, bioethylene produced from dehydration of bioethanol, 1, 3-propanediol and cis-3, 5-cyclohexadiene-l, 2-diols from microbial transformation of benzene and other chemicals. Various bacterial plastics prepared using these materials [31].

Polymerization of hydroxyalkanoates is conducted in vivo. All other monomers are polymerized in vitro by chemical reactions, leading to the formation of PHA, poly(lactic acid) (PLA), poly(butylene succinate) (PBS), PE, poly(trimethylene terephthalate) (PTT) and poly(p-phenylene) (PPP). These plastics are bio-based. Their properties are identical to those of traditional petroleum-based plastics. PE-based bioethanol leading to bioethylene. They are exactly the same as petroleum-based polyethylene. PHA is available in many varieties based on the structural. This is resulting in variable melting temperature (Tm), glass-transition temperature (Tg) and degradation temperature as reported by Steinbuchel [32], Doi et al., (1995), Wang et al. (2009), Spyros and Marchessault [33], and Galegoa et al. (2000).

3.3.2 BIODEGRADABILITY

Enzymes, bacteria, and fungi like microorganisms are taking part in the degradation of natural and synthetic plastics [34]. The biodegradation of bacterial plastics proceeds actively under different soil conditions according to their different properties. PHA is one of the natural plastics. Microorganisms can synthesis and preserve PHA even if conditions of lower nutrients availability and can undergo degradation and can metabolize it when the carbon or energy source is limited (Williams and Peoples 1996). (R)-3-hydroxybutyric acid is a biodegraded product ofpoly(3-hydroxybutyrate) (Doi et al., 1992), extracellular degradation of poly [(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate] yields both (R)-3-hydroxybutyrate and (R)-3-hydroxyvalerate [35].

PLA is fully biodegradable under the composting condition in a large- scale operation with temperatures of 60°C and above (Pranamuda and Tokiwa, 1999). PBS can degrade by hydrolysis mechanism and termed as hydro-biodegradable. The polymer molecular weights reduction takes place by hydrolysis at the ester linkages, further degradation may occur by the actions of microorganisms [38]. Polyethylene (PE) can biodegrade by two mechanisms which are hydro-biodegradation and oxo-biodegradation [36].

PBS is biodegradable aliphatic polyesters which is one of the members of biodegradable polymers family. In order to improve the properties number of copolymerization stages are made according to Darwin et al. [37]; Jin et al. (2000, 2001). Phenyl units are introduced into the side chain of PBS, which leads to the better biodegradability of the copolyesters. Jung et al. (1999) successfully synthesized new PBS copolyesters containing alicyclic 1; 4-cyclohexanedimethanol. The applications for PLA are mainly as ther- moformed products such as drink cups, take-away food trays, containers, and planter boxes. Polystyrene and PET are partially replaced with PLA material due to the rigidity. Applications include mulch film, packaging film, bags, and ‘flushable’ hygiene products [38].

Polybutylene succinate (PBS based different blends are tried gave greater toughness. Po!y(3-hydroxybutyrate-co-3-hydroxy’aIerate) (PHBV) and poly(butylene succinate-co-adipate) (PBSA) biodegradable polymers some of them [39]. These polymers were added to a PLA/PBAT blend giving a decrease of thermal properties. There is an observation of an increase in melt flow with PBS, as it is more flexible compared to the other. There is no change in melt flow observed with PBSA or PHBV [40]. PLA degrades into water and carbon dioxide that does not cause any harm; they can be cleared out of human body, thus PLA became the most popular biomedical material in the market.

3.3.3 BIODEGRADABLE ORTHOPEDIC IMPLANTS

The human skeleton consists of separate and fused bones supported and supplemented by ligaments, tendons, muscles, and cartilage. Bones gets arterial blood supply, venous drainage, and nerves. There is a tough fibrous layer with which particular surfaces of bones are covered. The human skeleton is changing always, it changes composition throughout lifespan. In the early stages, a fetus does not have any hard skeleton; bones are formed gradually during nine months in the womb. By birth, all bones are formed but a newborn baby has more bones than an adult. An adult human has 206 bones. A baby is bom with around 300 bones. The bones do not have pockets or space left to grow further. The strength of the bones are not the same in all direction, it shows an isotropicity. Bones are not strong and stiff if stressed from side to side.

There are several important factors to be considered in designing biodegradable orthopedic implants. Primarily material should degrade over in time, therefore the scaffolds are functioning as a temporary support and allowing space for newly generated tissue to replace the defect [41, 42]. Second, the initially implanted biomaterials or the degraded materials and the by-products, such as monomers, initiators, and residual solvents, may cause a serious inflammatory or immunogenic response in the body [43]. Finally, the material should possess sufficient strength to sustain the mechanical loads subjected to defective area during the process of healing. The material may be showing a lower mechanical strength as defects are replaced with new tissue designed with scaffolds of orthopedic implants. From the beginning, whatever is the proposed final application must be a considered as the primary concern. Scaffolds can be used as fixation as internal devices to support the defective sites. Tissue repair will be aided by scaffolds implanted by the induced cell migration and proliferation. Scaffolds are used to give out bioactive molecules and cells closers to the organ to enhance defect healing process.

Biomaterials are getting attraction in the usage as implants which help in the regeneration of orthopedic defects [41, 42]. In the United States alone, every year more than 3.1 million orthopedic surgeries are performed [44]. The currently available treatments using no degradable fixation materials have proven effective, even though tissue engineering procedures with biodegradable implants are being considered as more promising future alternative [4,45]. Biodegradable implants can be processed to give temporary support for bone fractures. They can degrade at the same rate which is used for the natural new tissue formation and their use can be eluninated the need of a second surgery [40]. The scaffold can function as a substrate for seeded cells, to give support for the tissue containing a defect and new tissue formation at the locations of injury [46,47]. New tissue formations can be made faster by the usage of drugs or molecules which are bioactive. It can be used to treat osteomyelitis like conditions [48,49].

3.3.4 SYSTEMS WHICH GIVE MECHANICAL SUPPORT

Biodegradable orthopedic are used during the healing process in the form of fixation implants like screws, staples, pins, rods, and suture reinforces the support areas weakened by bone fracture, sports injury, or osteoporosis [50-52]. It is very important that high mechanical strength and stiffness are required for biodegradable devices especially used for orthopedic applications. They are subjected to high loads when the devices are implanted. These biomaterials should show longer biodegradation tunes [17, 20]. A biodegradable screw made of poly(L-lactide) with a titanium screw in a ligament demonstrated that it could provide a promising alternative in terms of primary fixation strength [53]. A blend of polyipropylene funrarate) (PPF) and polypropylene fumaratej-diacrylate (PPF-DA) has been molded into a biodegradable fixation plates. It can be used as a bone allograft interbody fusion spacer with acceptable mechanical properties [52]. Scaffolds are providing physical support also have been used to introduce bioactive molecules at the defect site [56]. Scaffolds also can be used to control the release of bioactive molecules and thus accelerating the healing process [57]. In the case of less stable drugs, its effectiveness can be extended by encapsulating them inside a matrix [58]. Nano- or microparticles and hydrogel-based implants systems are some of the delivery systems.

BIOMATERIALS: PROPERTIES REQUIREMENTS FOR BIOMEDICAL APPLICATION

Biomaterials are inert substance or combination of substances greatly used for implantations. It may be used with a living system to support or replace functions of living tissues or organs. Biomaterials can be even metals, ceramics, natural or synthetic polymers, and composites. Biomaterials [59] can be natural or manmade, which can make the whole part of a living structure. These biomedical devices can perform, augment, or replaces a natural function. This material should have biocompatibility which decides whether the material is suitable for exposure to the body or bodily fluids. It shows its ability to perform and give a proper response in the biological environment. If these materials placed inside the body will allow the body to function normally without creating any complications then these materials are said to be biocompatible. Some of them may cause an allergic reaction to the body once it comes in contact with body fluids. Polymers are the most promising and largest class of biomaterials. It is proved by their widespread use in various medical applications. There is a large number of polymeric biomaterials developed and developments are continuing as of its popularity. The polymer can be synthesized with a wide range of properties and functionality. This becomes the key to the success of polymer-based biomaterials and the ease coupled with low cost.

Components of implants are getting the direct interaction or contact with the human body. Therefore, the safety level of these materials used in biomedical applications is very high. It must be non-toxic, biodegradable, and biocompatible and meet the specifications given in the standards. Three basic properties are required of any biocompatible materials. They are superhydrophobicity, adhesion, and self-healing. These properties make it fit for biomedical applications. To meet all the requirements a large number of research works are going in order to develop materials satisfying all requirements.

3.4.1 SUPER HYDROPHOBICITY

Materials with superhydrophobic surfaces are very difficult to wet. This property exhibited by many plants and insects. It can reduce blood coagulation and unfavorable platelet adhesion if there is superhydrophobicity for these biomaterials. Then they can be making use in biomedical applications [9]. There are many biomaterials showing such properties are produced [60].

3.4.2 ADHESION

Plants and animals are showing this phenomenon of selective adhesion, which is required for those organisms for their survival. These organisms attach temporary or permanently to their surrounding substrates. Adhesion abilities are also important for bacteria, animals, and plants. Polymeric materials with these properties are created and using in biomedical applications by Bassas-Galia et al. [61].

3.4.3 SELF-HEALING

Self-healing is known as the human body repairs the damaged tissues by itself whenever there is an injury. This happens when the portion of the injury is smaller in size. When there is an injury or damage which is beyond self-healing, there is an introduction of alternative material like the use of an implant. Implants may be subjected to various loads; wear and aging which causes failure and needed to replace. Researchers are trying to produce materials that can heal or repair by itself the first types of self- healing biomaterials are composites that irreversibly repaired. Second generation self-healing materials can reversibly restore the damaged matrix. To get unproved mechanical and physical properties polymeric self-healing materials, nanoparticles are used. Compared to metals and ceramics, polymers show lower strength and modulus which can be improved by various techniques.

Polymer-based composites have been widely used due to their advantages and ability to handle the loads. Self-healing materials [62, 63] should have the characteristics ability to repair the damaged portion many times. These materials should have the capability to repair the defects in the substrates of any size and also should have reduced maintenance costs. When comparison with the normal substrates, it must exhibit superior quality and should be more economical compared with the conventional materials.

Microcapsules were added to PLA to form a composite material with self-healing property. The microcapsules are filled with additives for healing. It can function when cracked and releasing the self-healing additives to fractured areas. This can also function as nucleating agents to improve the PLA composite’s temperature resistance. Self-healing microcapsules can be created by encapsulating the dicyclopentadiene and Grubbs catalyst. It is then released into damage volumes and undergoing polymerization by the chemical reaction of the catalyst. This technique helps in the recovery of the polymer composite’s toughness towards facture.

3.4.4 BULK PROPERTIES

The modulus of elasticity of the implant material should be comparable to the bone. It must be showing a uniform distribution of stress at implant so that the relative movement at implant-bone interfaces to be the minimum. To prevent fractures and improve functional stability an implant material should have high tensile and compressive strength. Stress transfer will be improved from the implant to bone if the interfacial shear strength is increased, so that implant is at lower stresses. Yield strength and fatigue strength required for an implant material. Higher yield strength and fatigue strength is essential to prevent sudden fracture of the implant with cyclic loading. Ductility is another bulk property required for making the contours and shapes for an implant. If hardness and touglmess increase the wear of implant, material decreases which also prevents the premature fracture of the implants.

3.4.5 SURFACE PROPERTIES

For any implants surface tension and surface energy essentially affects the wettability of implant. This causes wetting of fluid blood and affects the cleanliness of implant surface. Osteoblasts are cells from bone can cause improved adhesion on implant surface. Surface energy can change adsorption of proteins [64]. If the surface roughness of implants increases the surface, area increases and improves the cell attachment to the bone. An implant surface can classify on different criteria, such as roughness, texture orientation, and irregularities [65, 66]. Wennerberg et al. have subdivided implant surfaces as per the surface roughness which can be of three categories: minimally rough (0.5-1 m), intermediately rough (1-2 m), and rough (2-3 m). The implant surface can also be classified according to their surface texture as concave and convex texture and orientation of surface irregularities as anisotropic and isotropic surfaces.

3.4.6 BIOCOMPATIBILITY

The implant material shows the proper response in a biological environment is referred as biocompatible. It also referred as the corrosion resistance and cytotoxicity of the products. Corrosion resistance [67-69] basically means the release of metallic ions from metal surface to the surrounding environment. There are many types of corrosion crevice corrosion, pitting corrosion, galvanic corrosion, electrochemical corrosion, etc. Clinical significance of corrosion is that the implant made up of bio-material should have the corrosion resistance. Corrosion can results into rough surface, weak restoration, release of elements from the metal or alloy, toxic reactions, etc. There can be allergic reactions in patients due to corrosion.

 
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