and the solvent particulate removal too becomes difficult [21]. Under these circumstances, the remaining amount of solvents or particulates available may be removed with the help of vacuum drying [22]. The orthopedic scaffolds that are used for bone replacement or repair have been fabricated by the SCPL method [23]. For avoiding the toxic nature of the solvents used with the host tissues, a highly porous biopolymer foam has been created through a method of gas foaming with the help of carbon dioxide in addition to the particulate leaching technique [24, 25]. Figure 2.1 depicts the steps involved in solvent casting and particulate leaching. For creating a homogeneous mixture, the polymer solution, prepared using an organic solvent, is mixed with a porogen particulate (i.e. NaCl). This mixture is then poured into the desired mold, after which the solvent has been made to evaporate. In order to obtain a complete porous structure, the mold has been washed by means of a particulate solution like water which dissolves easily for removing the residual particulates. Drying is carried out under vacuum in order to remove the residual solvent or particulate to get the newly formed scaffold.
Figure 2.1 Steps involved in solvent casting and particulate leaching.
2.4.2 Emulsion Freeze Drying
This method is particularly suitable for collagen‐based scaffolds. This is because the collagen has a higher sensitivity to heat denaturation and chemicals. It has been found that aortic heart valve cells can be grown by creating these porous collagen scaffolds [26]. For entrapping the ice crystals, these collagen type 1 fibers were frozen at −30 °C. The required porous materials can be obtained by removing the crystals using a process known as lyophilization. The prominent parameters that affect the pore size include temperature, pH, and solution concentration. Emulsion freeze drying method is used for producing scaffolds having heat‐sensitive bioactive molecules with pore sizes in the range 20–200 μm and with porosity greater than 90% [27, 28].
2.4.3 Electrospinning
Electrospinning is another technique which is used for the fabrication of cardiovascular scaffolds [29]. Figure 2.2 depicts the schematic representation of the electrospinning process. An electric field is applied as soon as the solution containing a conductive polymer is ejected from the needle and collected on a target. When the solvent dries out, a fibrous network exactly similar to the structure of our body's natural ECM is formed [30]. By adjusting the processing parameters (such as voltage, working distance, flow rate, and temperature) or the polymer solution conditions (such as conductivity, viscosity, and concentration), the structure as well as the diameter of the fiber can be altered [31]. Taking advantage of the synthetic polymers' mechanical strength and natural polymers' biocompatibility, serious efforts have been taken to hybridize these structures. Successful and interesting advancements have been made in the field of heart valve tissue engineering [32]. Under these circumstances, electrospinning technique has been used to develop a biohybrid scaffold, which consists of non–cross‐linked decellularized bovine pericardium ECM coated with an adhesive layer of polycaprolactone (PCL)–chitosan nanofibers [33]. In order to enhance the fiber–polymer interactions, and due to the hydrophobic nature of PCL, it has been blended with a mixture of several materials (dextran, cellulose acetate, polyhydroxybutyrate, and chitosan nanofibers) [34]. Dip coating technique has been used to improve the mechanical properties of the decellularized scaffolds, without the use of cross‐linkers [35]. Even though these dip‐coated scaffolds resulted in having better mechanical properties than electrospinning, the structural integrity of the ECM nanofibers were disrupted with the use of organic solvents. Thus, electrospinning forms the most appropriate choice for processing.
Figure 2.2 Fabrication of electrospun nanofibers under high voltage.
2.4.4 Blow Film Extrusion
Blow film extrusion is used for the manufacture of polymeric films. This method involves the extrusion of molten polymeric material, which takes the shape of a die into which it is blown, while air blown into it forms a thin film. Due to their significant advantages such as biodegradability, low cost for processing, and nontoxicity, these blown films are finding their applications in the pharmaceutical industry. Chitosan [36], polysaccharides [37], cellulose [38], and their derivatives are the most commonly used biobased polymers in film industries. Increased attention has been gained for the preparation of blown films made from starch‐based materials [39]. Chitosan‐ and starch‐based thermoplastic films have been found to be suitable for applications in pharmaceutical and food processing industries. By increasing the percentage of chitosan in the film, properties such as water vapor and oxygen barrier tend to increase, while the surface hydrophilicity is reduced. Hence, by controlling the amount of chitosan in the film, the rate of degradation of the film, and consequently, its shelf life can be tuned [39].
2.4.5 3D Printing
In 3D printing technique, a preprogrammed printer head moves over the target surface and ejects a fixed quantity of molten polymer on the target in order to create the final desired shape. For attaining the final 3D structure, the movable printer head deposits the polymer material in a layer‐by‐layer fashion. This process is also known as Rapid Prototyping and has prominent applications in biomedical fields such as dental implants [40], orthopedic prosthetics [41, 42], 3D surgical and medical models, and also hearing aids. 3D bioprinting, a new technique, is nothing but a variation of traditional 3D printing in which 3D biofunctional structures are engineered. It involves the deposition of living cells onto a gel medium [43]. Some studies have used nanomaterials in conjunction with biohybrid scaffolds to create additional functions [44]. Some examples show that, new bone growth could be induced by adding magnesia nanoparticles to PCL–chitosan nanofibers through modulation of signal transduction and cell proliferation [44]. In another example, it has been found that this cell proliferation could also be induced by means of magnetic heating when magnetic nanoparticles are added to PCL–chitosan nanofibers [45].
In the field of tissue engineering, electrospinning and 3D printing are the most common techniques by which the scaffolds are fabricated. In addition to these techniques, common biomedical supplies are prepared by conventional techniques such as compression molding or injection molding. In recent years, in order to combine the advantages of both, a combination of advanced methods and conventional techniques is used to reduce the overall processing time and cost. Examples include particulate leaching and solvent casting [46].
2.5 Fillers and Reinforcements Used in the Preparation of Biobased Composites
Generally, there are three different methods by which biobased composites are obtained. They are as follows:
1 The reinforcement of non‐biobased polymers with biobased fillers/fibers
2 The reinforcement of biobased polymers with non‐biobased fillers/fibers
3 The reinforcement of biobased polymers with biobased fillers/fibers.
2.5.1 Biobased Fillers/Reinforcements with Non‐biobased Polymers
Fillers/additives
