**3. Classification of nanomaterials**

A plenty of experimental methods are available for the fabrication of nanosized structures, but each of them can be put into one of the two main classes. Nanomaterials and nanostructures can be fabricated by removing some part of a bulk material to sculpture the desired shape that can be constructed from much smaller parts. These two main classes are termed as 'top-down' and 'bottom-up' approaches [9, 10].

### **3.1 Top-down methods**

Top-down manufacturing method deals with mechanical operations, for example, cutting, moulding and carving. Because of size limitations, we will get highly specialised nanostructures. Some manufacturing methods involve laserbased ones like ablation, deposition and milling; some of them use hydrothermal techniques like liquid- or gas-phase deposition. Electromechanical and electrochemical methods are also widely used (electroplating, etching, etc.).

Most top-down nanofabrication methods are for surface patterning. By patterning local surface regions of a solid substrate with nanoscale features, the substrate has the ability to recognise specific nanostructures. For instance, when the substrate is placed in a solution, millions of nanostructures can self-assemble in parallel. Some patterning methods are developed to write nanoscale features, others are to replicate.

#### **3.2 Bottom-up approach**

Bottom-up methods represent a wide range of component-construction techniques. The building blocks are probably simple molecules held together by their covalent bonds. The resulted macroscale components are strong enough and stable. Many different techniques like atomic force microscopy (AFM), liquid- and gasphase techniques, consisting sol-gel processes, and inverse micelles are involved. One of the most important characteristics is the molecular self-assembly.

#### **3.3 Deposition techniques**

Thin-film deposition involves processing above the substrate surface (typically a silicon wafer with a thickness of 300–700 μm), where different materials are

*Prologue: Thin-Film Synthesis and Application for Medical and Biological Use DOI: http://dx.doi.org/10.5772/intechopen.84968*

added to substrates in form of simple or structured layers (thin films or composed aggregates with discrete spacers later to be removed). Deposition techniques fall into two categories, depending on whether the process is primarily chemical or physical.

**Chemical deposition** means deposition of layers via chemical reactions. The deposition rate on the substrate is governed by the properties of materials like pressure, temperature, etc. Based on the precursor phase, deposition processes can be classified as plating, spin coating, low-pressure deposition, plasma-enhanced deposition or layer-by-layer (atomic) deposition.

In the process of **physical deposition**, the material (solid, liquid or vapour) is physically transferred to the substrate (heated). These processes are thermal deposition, sputtering, ion plating, etc. The main operating factors are the substrate structure and temperature. The rate of deposition describes the volume/mass of the deposited material in unit time.

These layers are deposited and subsequently patterned using photolithographic (induced by laser or electron beam) techniques and then etched or washed away to release the final structure. We should also mention the electrophoretic deposition (EPD) that is a wet electrolytic deposition technology for thin films. EPD employs the mechanism of electrophoresis. Electric field is applied between two electrodes and charged particles dispersed or suspended in a liquid medium moving towards the oppositely charged electrode (electrophoresis), followed by the accumulation of particles on the deposition electrode in an ordered manner, producing a relatively compact and homogeneous film.

## **4. Some advantages and limitation of thin films: Drug delivery**

Thin-film drug delivery stands as an alternative method to the traditional pills and capsules. Thin films and strips of a few centimetres in size might be subjected to oral or under tongue administrations. As the strip dissolves, the drug enters the bloodstream (buccally or sublingually). These drug delivery options allow the bypass of the metabolic pathway; therefore, this medication method is more bioavailable.

Polymeric thin films can also be beneficial for bedridden and non-cooperative patients. Thin films are useful in cases where rapid onset of action is required, such as in motion sickness, sudden episodes of allergic attack or coughing, bronchitis or asthma.

The use of thin films is sometimes limited due to low drug loading capacity compared to a less potent drug given at high dose. Thin films are usually very hygroscopic in nature, and sometimes it is difficult to obtain them in high degree of accuracy.

The most important polymeric films used in pharmaceutical and medical applications are hydroxypropyl methylcellulose (HPMC), carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), poly(vinyl pyrrolidone) (PVP), poly(ethylene oxide) (PEO), pullulan, pectin, chitosan, carrageenan and gelatin.
