580 L Yu et al.Prog.Polym.Sci 31 (2006)576-602 alone.Phase separation in some was depc ntent made e additio 2.2.Aqueous blends of rubber possible.The addition of rubber was. however.limited by phase separation,the appear. Many natural polymers cannot be melt processed. ance of which depended on the glycerol content either because they degrade on or before melting Scanning electron microscopy (SEM)showed good (softening)or because they are designed to incorpo- rate substances that do not stand high temperature h starch ma h drugs,etc.).Fo form withou any kind of nu ion.More are usually biocom atible and the presence of the non-rubher constituents of the non-cytotoxic due to their similarity with living latex was not only responsible for insuring the tissues.Biopolymers are an important source of latex stability,but also for improving the compati- material with a high chemical versatility and with bility between the thermoplastic starch and the high potential to be used in a range of biomedical natural rubbe the hoth the r phases Finally,glycerol seemed to applications [1,19.A great variety of materials contribute derived ces ha of om natural sour al and st3h、 abundant biopolymers on earth.S Since of the variety of biofibe ers such as lig of civilization,the excellent chemical and physical losic natural fibers [20.21].Starch-based polymers properties of these materials have made them a present enormous potential for wide used in the useful component in many applications.However. biomedical field,as these natural polymers are blends of starch and protein have not showi totally biodegradable and inexpensive when com to other bio vailabl he to d egradable polymers av od and rc 20 protein/starch interaction during ties that make them suitable for in established a kinetic model for starch gelatiniza wide array of biomedical applications,ranging from tion and the effect of starch/protein interactions bone replacement to engineering of tissue scaffolds Matveev et al.[12]studied the effect of water on the and drug-delivery systems. glass transition of protein,polysaccharides and Starch-based thermoplastic hydrogels for use as blend considering inter-macromolecular hydrogen one cement ts or drug-delivery carriers hav an et develope ugh b d th bicate 26- Thermoplastic polysaccharides such as cellulose radical 2 5.actetate were polymerization with methyl methacrylate and/or an acrylic acid monomer.The polymerization was simultaneously onto polysaccharides,hydroxy-func- initiated by a redox system consisting of benzoyl tionalized plasticize and also peroxide and 4-dimethlyaminobenzyl alcohol at lov hydroxy-functional fillers.Organosolv lignin,cellu odegradable characte we dified。 polymers poly( ma. sult of me ko ramic con nd h lapatite [32 the excellent compatibility of nolv(. aprolactone)alone. Phase separation was observed in some compositions, which was dependent on rubber and plasticizer content (glycerol). Increasing the plasti￾cizer content made the addition of higher amounts of rubber possible. The addition of rubber was, however, limited by phase separation, the appear￾ance of which depended on the glycerol content. Scanning electron microscopy (SEM) showed good dispersion of the natural rubber in the continuous phase of the thermoplastic starch matrix. The process employed in this investigation called upon the use of both starch and latex in their natural form, without any kind of purification. Moreover, the presence of the non-rubber constituents of the latex was not only responsible for insuring the latex stability, but also for improving the compati￾bility between the thermoplastic starch and the natural rubber phases. Finally, glycerol seemed to contribute to both the plasticization of the starch and to the improvement of the starch/rubber interface. Cellulose and starch are some of the most abundant biopolymers on earth. Since the origin of civilization, the excellent chemical and physical properties of these materials have made them a useful component in many applications. However, blends of starch and protein have not shown significant promise in biodegradable materials applications to date. The system of starch/protein, however, has been studied in food field [12,16]. Kokini et al. [16] studied starch conversion and protein/starch interaction during processing, and established a kinetic model for starch gelatiniza￾tion and the effect of starch/protein interactions. Matveev et al. [12] studied the effect of water on the glass transition of protein, polysaccharides and blends, considering inter-macromolecular hydrogen and dipole–dipole interactions. Warth et al. [17] used the technology of reactive extrusion to develop starch/cellulose acetate blends. Thermoplastic polysaccharides such as cellulose- 2,5-actetate were produced by means of reactive processing technology that grafted cyclic lactones simultaneously onto polysaccharides, hydroxy-func￾tionalized plasticizer, and optionally also onto hydroxy-functional fillers. Organosolv lignin, cellu￾lose, starch and chitin were added for reinforcement of the polymer blends. Compatibility between oligolactone-modified cellulose acetate and fillers were markedly improved when fillers were added during the reactive extrusion process. As a result of the excellent compatibility of poly(e-caprolactone) with numerous polymers, it has been possible to prepare a wide range of new polymer blends. 2.2. Aqueous blends Many natural polymers cannot be melt processed, either because they degrade on or before melting (softening) or because they are designed to incorpo￾rate substances that do not stand high temperature (proteins, drugs, etc.). For these examples, aqueous blending is the preferred technology, particularly in biomedical applications. Natural polymers are usually biocompatible and non-cytotoxic due to their similarity with living tissues. Biopolymers are an important source of material with a high chemical versatility and with high potential to be used in a range of biomedical applications [18,19]. A great variety of materials derived from natural sources have been studied and proposed for different biomedical uses, namely polysaccharides (starch, alginate, chitin/chitosan) or protein (soy, collagen, fibrin gel) and, as rein￾forcement, a variety of biofibers such as lignocellu￾losic natural fibers [20,21]. Starch-based polymers present enormous potential for wide used in the biomedical field, as these natural polymers are totally biodegradable and inexpensive when com￾pared to other biodegradable polymers available [22–25]. Aqueous blends of soluble starch and cellulose acetate have been studied intensively [26–29] because these blends have a range of properties that make them suitable for use in a wide array of biomedical applications, ranging from bone replacement to engineering of tissue scaffolds and drug-delivery systems. Starch-based thermoplastic hydrogels for use as bone cements or drug-delivery carriers have been developed through blending starch with cellulose acetate [26–28]. Pereira et al. [26] reported on biodegradable hydrogels, based on cornstarch/ cellulose acetate blends, produced by free-radical polymerization with methyl methacrylate and/or an acrylic acid monomer. The polymerization was initiated by a redox system consisting of benzoyl peroxide and 4-dimethlyaminobenzyl alcohol at low temperature. Utilizing the biodegradable character of starch-based blends, with the biostability of the acrylic polymers poly(methyl methylacrylate (PMMA) and poly(acrylic acid) [30,31] used as the matrix of these systems, and the incorporation of the well-known ceramic compound hydroxylapatite [32], Espigares et al. [27] developed partially biodegradable ARTICLE IN PRESS 580 L. Yu et al. / Prog. Polym. Sci. 31 (2006) 576–602