Hyperbranched Polymers for Biomedical Applications

Polymers were existent in nature since the beginning of life. It existed in the form of DNA, RNA, proteins, and polysaccharides and played a pivotal role in the evolution of the biological world. Cellulose and starch were the most common naturally occurring polymers, both having glucose as the monomeric unit conjoined through the glycosidic linkage [1]. The next most common natural polymer was natural rubber, extracted from the latex of Hevea Brasiliensis (rubber tree) [2]. However, during the 1800s, the growing demand for new materials initiated the chemical modifications of the natural polymers to develop semi-synthetic polymers. Vulcanization of rubber, invented by Charles Goodyear in the nineteenth century [3] so far has been the best example of such modification. In the early twentieth century, synthetic polymers came into existence with the synthesis of Bakelite by Leo Baekeland [4]. He developed the said resin from synthetic resin with high hardness. It was soon followed by the production of Rayon, a synthetic fiber from cellulose. Following these, a series of important synthetic polymers like nylon [5], styrene butadiene rubber, acrylic polymers, neoprene rubber, and many others were synthesized under commercial scale during the period of World War II to meet the increasing demand [6].

Following the extensive works on dendrimers which are structurally perfect but tedious to prepare, the need for the development of structurally imperfect hyperbranched (hb) polymers has gained momentum. A dendrimer is constituted of terminal units (at the globular surface) and dendritic units (inside the macromolecular framework). Whereas a hb polymer is constituted of terminal units (at the irregular surface), linear units and dendritic units (both of which are distributed randomly inside the macromolecular framework). These structural variations in dendrimers and hb polymers arise from the difference in synthesis strategies and mechanism of their formation

With the growing interest and demand in the realm of hyperbranched (hb) polymers, lot of synthesis approaches have already been explored some of which are detailed earlier in the Chap. 2. Both step-growth and chain-growth approaches are widely followed in the synthesis of hb polymers. Step-growth approaches mainly include AB _x polycondensation and double monomer (symmetric/asymmetric pairs) methodologies. Whereas chain-growth approaches include radical copolymerization, surface grafting, and other controlled polymerization techniques. Interestingly, both self-condensing vinyl polymerization (SCVP) and self-condensing ring opening polymerization follow step-growth as well as chain-growth routes.

The field of hyperbranched polymers have been explored widely and have been a great topic for researchers in the last two decades because these compounds possess new, remarkable characteristics that strongly influence material properties and have opened new application fields (Inoue in Prog Polym Sci 25:453–571, 2000 [1]). Therefore, simultaneously along with the synthesis of various kinds of hyperbranched polymers from different combinations of monomers, special emphasis has been taken to establish its structure-property relationship by proper characterization and understanding of the structure of the hyperbranched polymers and its effect on its physical and chemical properties.

Alike dendrimers, hyperbranched polymers also play a significant role in various biomedical applications. An ideal delivery vehicle in biological applications should possess the following characteristics: excellent biocompatibility and biodegradability, it must have the ability to form a stable complex with the external agent, transport the external agent into the specified targeting site, and then release them in a controlled manner, while the physiological properties of the agent is kept intact. Polymer-based biomaterials are thus quite preferred as delivery vehicles over other small molecules as it meets all the above-mentioned criteria. Again among all the polymers, hyperbranched polymers are considered as an ideal matrix for drug and gene delivery vehicles because of its tailorable architecture and availability of plenty of functional groups.

Medical imaging is the process of creating visual images of the interior of a living body for clinical analysis and medical treatment, as well as to provide visual representation of the function of some organs or tissues. Medical imaging is done to reveal internal structures hidden by the skin and bones, so as to diagnose the disease accurately and treat it properly. Through medical imaging, a database of normal anatomy and physiology is also established to make it possible to mark the abnormalities.

Hyperbranched polymers have a three-dimensional structure with high functionality, high reactivity due to the presence of a large number of free terminal groups, and they exhibit enhanced absorption capacity of biomolecules on a polymeric biomaterial. More advantage with these architectural polymers is that they can be altered structurally as well as by incorporation of functional groups can be improved for better cell attachment. Hyperbranched polymers are quite capable of forming porous hydrogels or films as scaffolds, and are promising material to support adhesion and rapid reproduction of cells. Thus, hyperbranched polymers, due to their unique structures and special properties, have proved to be of high potential in various applications in tissue engineering fields.

So it is seen that, over the past few years there was a significant progress in the synthesis and development of biocompatible and biodegradable hb polymers for more versatile and sophisticated biomedical uses. Due to tuneable architecture (long and short branches) and functionality, the hb polymers have already shown a great potential of being an “indispensible” in modern biotechnology and biomedical fields.