Biocompatibility synthetic poly

A scientific breakthrough in order to design USCAs on demand can be seen in the third generation (Myomap, Quantison, BiSphere and Sonavist). Compared to the more or less free bubbles of the first and second generations, the novel type of USCAs consist of encapsulated microbubbles with a shell formed by a biopolymer (like human albumin) and/or a biocompatible synthetic polymer (like copolymers of poly-lactide and polyglycolide or derivatives of polycyanoacrylate). In addition to the prolongation of the lifetime in the blood stream, these polymer-stabilized microbubbles can be manufactured to fulfill certain needs, and to interact with diagnostic ultrasound in a defined and optimal manner. 

It is worth mentioning that currently most of the works in the literature are devoted to the study of multilayers of synthetic poly electrolytes. Some iconic examples of these systems are multilayers of type (PDADMAC + PSS)n (PDADMAC poly(diallyl-dimethyl-ammonium chloride), and (PSS poly(4-sty-rene sulfonate of sodium)) where the subindex n indicates the number of bilayers in the multilayer [80-82] or (PAH - - PSS) (PAH poly(aIlylamine hydrochloride)) [83]. However, there is a growing interest in the last years on the fabrication of biocompatible systems, e.g. biomacromolecules [84, 85]. Some examples are multilayers of type (CHI - - HEP)n (being CHI Chitosan and HEP Heparin) [86] (PLL + HA)n (PLL is poly(L-lysine) and HA is hyaluronic acid) or (PLL + PGA)n (PGA is poly(glutamic acid)) [87, 88]. 

Although the initially reported tissue compatibility tests for subcutaneous implants of poly(BPA-iminocarbonate) were encouraging (41,42), it is doubtful whether this polymer will pass more stringent biocompatibility tests. In correspondence with the properties of most synthetic phenols, BPA is a known irritant and most recent results indicate that BPA is cytotoxic toward chick embryo fibroblasts in vitro (43). Thus, initial results indicate that poly(BPA-iminocarbonate) is a polymer with highly promising material properties, whose ultimate applicability as a biomaterial is questionable due to the possible toxicity of its monomeric building blocks. 

Biodegradable polymers, both synthetic and natural, have gained more attention as carriers because of their biocompatibility and biodegradability and therewith the low impact on the environment. Examples of biodegradable polymers are synthetic polymers, such as polyesters, poly(orfho-esters), polyanhydrides and polyphosphazenes, and natural polymers, like polysaccharides such as chitosan, hyaluronic acid and alginates. 

As mentioned above, the preparation of nanogels by addition reactions of functional macromolecular precursors is mainly used for biomedical applications. Thus, the choice of synthetic precursors for microgel formation is restricted to biocompatible materials. Moreover, as most applications are in drug delivery, the molecular weight of the gel precursors should be below the threshold for renal clearance, a value that depends on the molecular architecture and chemical nature of the polymer but that is usually smaller than 30kDa, which is set as the limit for linear PEG [97], Polymers that are mostly used and thus presented in more detail here are PEG, poly(glycidol) (PG), and polyethylene imine) (PEI). 

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