Microspheres are the spherical particles of micrometer range (typically 1 μm to 1000 μm) synthesized from natural and synthetic materials [1]. They are efficiently utilized in controlled delivery of many drugs. Wastage of drug due to low drug entrapment efficiency, repeatability and difficulty in scaling to large production facility are the major drawbacks of microspheres [2]. Thus, care should be taken while designing microspheres and design should be appropriate such that loading and entrapment efficiency of a particular drug can be enhanced and also drug wastage can be minimized. A sound knowledge about the effects of various variables in entrapment efficiency can help in designing the robust microsphere.

Nanospheres are small particles with a size range of 10 to 200 nm. The hydrophobic surfaces of these particles are susceptible to opsonization and clearance by the reticulo endothelial system [4]. There are various kinds of nanospheres such as gelatin nanospheres, albumin nanospheres, modified starch nanospheres, propylene dextran nanospheres, polylactic acid nanospheres, magnetic nanospheres, immune nanospheres etc. Nanospheres can be prepared by various methods but the solvent evaporation is the most widely used. Nanospheres are extensively used for the targeted delivery systems [4].

Solvent evaporation method is one of the most widely used methods due to its ease in the formulation process and also its suitability for various modifications for preparing micro and nanospherical systems with improved properties. It was first patented in 1979 by Vanderhoff et al. as a polymer emulsification process to obtain polymeric particles. This method finds its claims in several patents thereafter.

In this process, the polymer or micro or nano capsule coating agent is dispersed in a volatile solvent which is insoluble in liquid manufacturing vehicle phase. The core material or the drug to be encapsulated is then dissolved or dispersed in the coating polymer solution. Application of the stirring at certain speed causes the core material to get dispersed in the liquid manufacturing vehicle phase to obtain a desired size range of the microsphere. This technique forms an emulsion between polymer solution and continuous phase. With the application of heat or evaporation of the volatile solvent with stirring, the polymer shrinks around the core or forms the matrix microspheres as the core is also dissolved in the coating solution. This technique can incorporate both water soluble and insoluble core materials inside the matrix system [8]. The list of microspheres prepared with solvent evaporation method is given in Table 3.

It is a very clearly observed phenomenon that the release mechanism can easily be modulated by the weight of used polymer as higher molecular weight polymer shows slower release of the drugs [57]. It has been observed that at the same amount of polymer in the micro spherical system, the polymers of higher viscosity are difficult to dissolve. The main reason behind this phenomenon is the requirement of higher energy to pull the chain of polymers of higher viscosity from their matrix and formation of thicker gel layer after the hydration. The viscosity effect is mainly due to the differences in the molecular weight of the polymers. The following equation has been derived to explain the relation between the polymer molecular weight and disentanglement concentration [58]:

Cp, dis= 2700/ MW

Where,

MW= molecular weight of the polymer,

Cp, dis= polymer disentanglement concentration.

The above relation shows that when the molecular weight of the polymer increases, the total porosity of the polymers decreases which causes prolonged release of drug from the microspherical system. Take for instance HPMC polymers such as HPMC k4M, HPMC k15M and HPMC K100M having molecular weight of 95, 120 and 250 kDa in which HPMC K100M is seen to have a lower release rate than the others. This relationship also explains that at a higher MW, the polymer disentanglement concentration is very low and reaches a plateau [59]. Young and Nelson equation can be used to explain the sorption and desorption profile of different viscosity grades of HPMC polymers. These equations describe three locations of the sorbed moisture: monolayer adsorption, externally adsorbed moisture, and internally absorbed moisture. Increase in the polymer weight results in the reduction of amount of internal water absorption and increase in external adsorption. In the experiment on “Elucidating the moisture sorption and desorption profile of different viscosity grades of HPMC (K4M, K15M and K100M)”, it was observed that the lowest value of internally absorbed moisture was obtained for HPMC K100M [60]. Since viscosity of the HPMC polymers is related to the molecular weight and has a large influence on the erosion rate of the matrix tablet, the erosion rate can be adjusted by the choice of HPMC polymer viscosity or by mixing polymers of different viscosity grades [61]. A higher molecular weight HPMC polymer thus has a higher water holding capacity and the matrices of such polymeric system are less prone to erosion [62].

Want et al. has explained that initial increase in the thickness of the matrix system (e.g microspheres and tablets) is higher for the higher molecular weight HPMC grades. With the increase in water sorption (hydration of the polymer chain), the disentanglement concentration is reached above which the polymers disentangle and detach from the gelled matrix resulting in the polymer erosion [63]. The effect of the molecular weight of the polymer has also been explained with the PLGA polymers with different molecular weights: PLGA 15000, PLGA 20,000 and PLGA 30,000 and it was found that with the increase in molecular weight, the loading capacity is increased because of presence of stronger barrier properties of polymers with high molecular weight [64]. The release of the drug from the microsphere is also the function of degradation of polymers. The release of the drug can be programmed into prolonged released systems by loading the drug into microspheres made of biodegradable aliphatic polyesters. For instance, the release of Gentamicin Sulphate was faster in PLHMGA than the PLGA polymers as degradation rate of PLHMGA is more rapid than PLGA [65].

The incorporation of higher amount of surfactant is not usually a good idea for designing the controlled or sustained release microparticulate system because it has been observed that surfactant increases the rate and amount of drug release from the microparticulate system. The basic reason to explain this phenomenon is the increase in wettability and better solvent penetration inside the matrix system. Increase in surfactant concentration is also believed to increase the drugs deposited at the surface of those matrix microspherical systems [66].

The presence of the surfactants also affects the modification of encapsulated products such as insulin and gives rise to a sustained release profile from the matrices of microspheres. In a research, related to encapsulating bovine insulin with poly (lactide co-glycolide) using different concentrations of non-ionic surfactants like poloxamer 188, polysorbate 20 and sorbitan monooleate 80, it was observed that during the period of sustained release, modification of insulin to a high-molecular weight product was observed and its amount was related to the surfactant used. Among all the surfactants used, polysorbate 80 at 3% w/v concentration was observed to have good insulin loading and slow drug release [67].

The HLB values of the emulsifiers have marked effect in defining the micromeretic properties, release and encapsulation of the drug in nano and microspherical systems. It is generally observed that emulsifiers with higher HLB were found to be more effective for the encapsulation of the drug and showed better control of drug release from the matrix system [68].

Various micro and nanospherical preparation methods require the use of stirrers. This is usually more popular in the solvent evaporation or the emulsion solvent evaporation technique. Stirring rate has been used to explain the average diameter of the microspherical system.

The sensitivity of polymer erosion to the degree of agitation can influence the polymer’s ability to give reproducible and agitation-independent release in the complex hydrodynamic environment of the gastrointestinal tract compared to more rigid non-eroding matrix materials [69].When the stirring rate is higher, it causes the formation of finer dispersion of the microparticles. When these microparticles are observed from SEM (Scanning Electron Microscope) photographs, the larger microspheres formed at lower stirring rate are seen to be porous than those formed at higher stirring rate. As the porosity of systems increases, the diffusion of the drug from the matrix system becomes easier [70]. On the another study, the influence of stirring speed on the release of drug from biodegradable matrix microsphere was studied which showed that by increasing speed from 400 to 1200 rpm, there was a marked effect on mean diameter of the microsphere which decreased from 251µ to 104µ. The release rate of the drug was observed to be faster in the small size microsphere [71]. Particle size also had the influence on the release property of the drug from microspheres. This is also supported by the results of the research done by Pan Yan et al. where they found that particle size had an important influence on the release of insulin from microspheres. They found that larger particles released relatively less amount of insulin at the initial burst stage thus preventing initial burst which is considered as one of the main drawbacks of microsphere [72]. It has also been observed that drug release depends strongly on microsphere diameter of 10-μm and 20-μm particles exhibiting concave-downward release profiles while larger particles resulted in sigmoidal release profiles [73].

In all the methods of preparation of microsphere which involves evaporation of the volatile component, temperature has a very important role in defining the drug release. Generally, an abrupt increase in temperature during the manufacture or use of very high temperature causes the rapid expansion of methylene chloride from the body of the microsphere and causes the formation of hollow core and a thin permeable body [74]. It has been observed that the amount of continuous phase and the micro/nanoparticle: bulk fluid ratio affects the release behavior of drugs from the system. The solvent evaporation method is generally inconvenient for micro/nano spherical systems of water soluble drugs because there is tendency of drug loss from emulsified polymeric phase before solidification of polymer in microsphere [75]. Also, drug release is significantly faster in higher particle mass:bulk fluid volume ratio in case of non-porous and initially porous system which should be taken into account when defining the experimental conditions for drug release measurements from this type of advanced drug delivery systems [76].

A higher molecular weight polymer has a greater degree of entanglement and thus will reduce the molecular diffusion area and permeation of the drug across the matrix gel. When the polymer concentration increases, there will be increase in the viscosity of the polymer gel and which thus results in longer diffusional path [77]. This causes a decrease in the diffusion coefficient of the drug and reduction in the release rate of the drug. In order to achieve a constant zero order or sustained release, the polymer should be hydrated fast to form a gel layer before the contents of the matrix tablet gets dissolved. For the higher viscosity gels, they become more resistant to dilution and erosion [78]. On the other hand, the polymer forming the loose network in the matrix is seen to decrease the release rate and encapsulation efficiency as the network allows the drug particles to leach out during the microsphere production [79]. It has been observed and widely accepted that an increase in the polymer concentration increases the entrapment of the drug inside the micro and nanospherical systems. The main reason for this phenomenon can be explained with the help of following points.

The interaction between drug and polymer may be hydrophilic or hydrophobic and may result in the drug release and entrapment from micro or nanospherical system. For hydrophobic interactions, the drug is best encapsulated in polymers containing with free carboxylic acid end groups. For hydrophobic interactions the encapsulation or the entrapment efficiency may be increased by relatively hydrophobic end capped polymers [81, 82]. It is also observed that the encapsulation of microsphere of tetanus toxoid formed with PLGA increased when formulated with hydroxylpropyl cyclodextrin (g-HPCD). It is supposed that the g-HPCD increased the interaction by involving amino acid side group of the toxoid into its cavity and simultaneously interacting with PLGA through Vander Waals and hydrogen bonding forces [83]. It is also observed that the complexation between the drug and the polymer may affect in the release of the drug from the matrix system. In a study of 23 drugs with different solubility and molecular weight, matrixed with HPMC and sodium carboxy methylcellulose, it was observed that very soluble and soluble drugs such as beta-blockers and soluble vitamins (thiamine hydrochloride) released more slowly in a matrix of HPMC and sodium carboxymethylcellulose due to complex formed between cationic drug and anionic polymer [84].

Various experiments have been performed taking in consideration the observation of the stirring rate and its effect on the entrapment of the drug inside the matrix systems of microspheres. It has been observed that with the increase in the stirring rate, the mean particle size decreases and due to fast evaporation of the solvent from the matrix system during the manufacture, there is a formation of the hollow spheres. In the lower stirring rate such as 300 rpm, there is a formation of the solid and irregular type of microsphere. But as the rpm increases to 1000 or 1500, various hollow spheres begin to form in the formulation process but the microspheres are spherical in nature. This property of formation of the hollow microspheres of spherical shape can be taken as an attribute for the entrapment of the drug to transfer inside the narrow capillary space. In an experiment of formation of the microspheres of Diltiazem HCl using Eudragit RS100 and Eudragit RL 100 in which the stirring speed had been applied between 300 to 1000 rpm, solid and irregular microparticles were seen at a lower stirring rate while at 600 rpm particles size and drug entrapment efficiency were 82% and 210 μm, respectively, and at 1000 rpm spherical shape of microspheres was observed but particles coalesced to beaker wall and also decrease with decreased particle size [85]. Increase in stirring speed is thus little difficult to obtain the small microparticles owing to the loss of formulation by adhering on the walls of manufacturing vessels. Similar results were obtained in the research conducted by S. Jose et al. in the study of colon specific chitosan microspheres for chronotherapy of chronic stable angina where they observed that increasing the stirring rate from 1000 to 2000 rpm, decreases the entrapment efficiency from 89% to 86.5% [86]. In the experiment of preparation of thermoresponsive hydrogel of N-isopropylacrylamide, it was observed that the size distribution could readily be programmed by altering the stirring rate and changing the concentration of lecithin in the formulation [87].

It has been observed that surfactant affects both the drug release and drug entrapment in the microspherical system. Increase in surfactant concentration leads to decrease in encapsulation efficiency of microspheres due to the fact that increase in surfactant concentration leads to stabilization of small droplets and results in smaller microspheres. Loss of drug from surface of small microspheres is more than that of larger microspheres during washing [88]. In one study of variability of PLGA nanoparticles quality by Placket- Burman design, it was observed that the higher concentration of the emulsifier increased the partition of drug from internal to external phase due to increased solubility of the drug in the external phase [89]. Statistical analysis of the results of the study showed that the increase in emulsifier concentration caused a decrease in the entrapment efficiency.

Benergee et al. observed that entrapment can be increased in interprenating polymer network (IPN) based formulation by the incorporation of higher amount of cross linking agents like gluteraldehyde, therefore, for the slightly soluble drugs, incorporation of optimized amount of cross-linker can improve the entrapment [90]. This has also been observed in the case of mucoadhesive microspheres. Patel et al. investigated in the effect of cross linker on drug-loading efficiency of glipizide mucoadhesive microspheres where a significant effect on mucoadhesion and drug entrapment efficiency of microspheres was observed. This could be explained as when the amount of glutaraldehyde increases, it tends to favor the cross-linking and consequently forming spherical free-flowing microspheres with an increase in drug loading efficiency [91].

Temperature has the significant role in affecting the morphology, entrapment and release rate of the drug from the microspherical systems. It has been observed that at higher temperature, the microspheres have hollow core and porous wall owing to the fact that the volatile components such as methylene chloride used in the formulation of the microspheres evaporate rapidly at a higher temperature. It has also been observed that the core size and wall thickness are depended on the temperature ramp. A rapid rise in temperature results in a thin wall and a large hollow core, whereas a stepwise temperature rise (15 to 25, then to 40^oC) results in a reduced core size [92].

It is widely observed phenomenon that the particle size of the microspheres increases as the ratio of polymer increases in the microspherical system because of increase in the viscosity of the polymer which results in formation of larger emulsion droplets and thus consecutively larger size of the microspheres. The explanations were well observed in an experiment by Patel et al. where the particle size of aspirin loaded microspheres was analyzed by optical microscopy. The average particle size of microspheres was found to be in the range of 328 to 990 μm. The polymer solution of increasing concentration showed greater size in SEM [93]. The polymer concentration has also been observed to have effect on the size of the nanospheres. Lung-Hsin Hung et al. studied a new method for nanosphere synthesis by droplet microfluidic solvent evaporation technique where they observed that increase in initial PLGA concentration increased the diameter of the particles and also increased the particle volume. An increase in the concentration of PLGA from 0.5% to 5% increases the volume by 20 folds from 0.52 to 9.2 pl. This concludes that on increasing the concentration of PLGA, it decreases the density and hence results in larger matrix size and cargo capacity [94].

Stirring speed also affects the percentage yield and mean microparticle size because of the turbulence developed. In higher rate of the stirrer (1500 rpm), there was the production of froth and adhesion in the walls of the container which consequently decrease the mean particle size of the microsphere. An optimum spherical shape and state free from aggregation was obtained to the stirring speed of 1000 rpm. An increase in particle size at low stirring speed (500 rpm) can be explained on the basis of tendency of the globules to aggregate and coalesce [97].

Target drug release with the help of encapsulation techniques such as microspheres is an important future directive in the formulation works. Microspheres thus have to maintain an acceptable size for the drug permeation and targeting to the desired location. The explanations are also supported by experiment by Gülen Melike Demir et al. where on increasing the stirring rate from 8,000 to 20,000 rpm, the particle size decreased from 72.54 µm to 13.67 µm and also increased the zeta potential 0.135 to 0.245 Mv [98].

It has been observed that the viscosity of the dispersed and the continuous phase greatly affects the microsphere size. In an experiment, microspheres were prepared using a hydrocarbon-perfluorocarbon solvent extraction process. It was observed that smaller microspheres were produced at the same mixing intensity when the viscosity of the dispersed phase was decreased. Increased continuous phase viscosity reduced the coalescence of the droplets and hence smaller microspheres were produced. The mean microsphere size was first found to decrease as the volume ratio of the dispersed phase to the continuous phase increased but upon further increase, the mean microsphere size was found to increase [99].