ACTA MEDICO-BIOTECHNICA 2009;2(2):19-24 19 Pregledni ~lanek / Review Avtor / Author Erih Teti~kovi~, Marija Menih, Jožef Magdi~ Ustanova / Institute Department of Neurology, University Clinical Center Maribor, Maribor, Slovenia SPREMLJANJE PRETOKA V MOŽGANSKIH ARTERIJAH S TCD TCD MONITORING OF CEREBRAL BLOOD FLOW Članek prispel / Received 23.07.2008 Članek sprejet / Accepted 09.10.2009 Naslov za dopisovanje / Correspondence Prof. dr. Željko Knez Univerza v Mariboru, Fakulteta za kemijo in kemijsko tehnologijo, Smetanova ul. 17, Maribor, Slovenija Telefon: +386 222 94 461 Fax: +386 225 16 750 E-po{ta: zeljko.knez@uni-mb.si Abstract A wide variety of natural and syn- thetic polymers have applications in biomedical devices. They are usu- ally processed by melting or by using organic solvents, but these methods may affect the efficiency of incorpo- rating delicate bioactive compounds, such as drugs and proteins, during polymer processing. These short- comings can be avoided by using supercritical fluids as processing solvents or plasticizers. Supercriti- cal carbon dioxide (scCO2) has at- tracted attention for its potential as a plasticizer in polymer processing. It is used for obtaining microspheres, microcapsules, foams, membranes and polymer/drug composites. The method offers important advantages over other techniques, including the absence of harmful organic solvents, the mild processing conditions, and the ready control of particle and foam morphology simply by varying Izvleček Za pripravo biomedicinskih pripomo~- kov se uporabljajo razli~ni sinteti~ni in naravni polimeri, ki jih je obi~ajno potrebno staliti ali procesirati z upo- rabo organskih topil. Te tradicional- ne metode procesiranja lahko vplivajo na u~inkovitost vklju~evanja bioak- tivnih komponent, kot so zdravila in beljakovine, v polimerno strukturo. Tem slabostim se lahko izognemo z uporabo superkriti~nih fluidov kot to- pil ali plastifikatorjev. Kot potencialni plastifikator pri procesiranju polime- rov je zanimiv predvsem superkriti~ni ogljikov dioksid (scCO2), in sicer za pridobivanje mikrokroglic, mikro- kapsul, pen, membran in kompozi- tov polimer/droga. Metoda ima pred ostalimi tehnikami pomembne pred- nosti, saj ne vklju~uje uporabe {ko- dljivih organskih topil, procesiranje poteka pri blagih pogojih, hkrati pa omogo~a enostavno kontrolo morfo- logije delcev in pen preprosto s spre- Ključne besede: superkritični CO 2 , polimerni biomateriali, polimerne pene, mikrodelci, kompozitni materiali Key words: supercritical CO 2 , polymeric biomaterials, polymeric foams, microparticles, composite materials lena A onicesei, Mo ca Škerget, Željko Knez Unive za v Mariboru, Fakulteta za kemijo in kemijsko tehnol gijo, Maribor, Slovenija University of Maribor, Faculty of Chemistry and Chemical Engineering, Laboratory for Separation Processes, Maribor, Slovenia Superkritični CO2, čistejša alternativa tradicionalnim metodam procesiranja polimernih biomaterialov Supercritical CO2, a clean alternative to traditional methods of processing polymeric biomaterials Pregledni članek / Review 20 ACTA MEDICO-BIOTECHNICA 2009;2(2):19-24 Pregledni članek / Review INtROdUCtION Polymers are the most widely used materials in bio- medical applications. They offer advantages (highly versatile, easily obtained and processed, similar to natural compounds) which recommend them for ap- plications in all domains of medicine, such as im- plants, grafts, medical equipment, drug delivery sys- tems and tissue engineering scaffolds. MAtERIAL & MEtOdS A wide variety of natural and synthetic polymers has been investigated for drug targeting and release or resorbable or non-resorbable implants, including protein-based polymers, polysaccharides, polyesters, polyanhydrides, polyamides, silicones, acrylic poly- mers, polyorthoesters, polyurethanes, polyacetals, homopolymers, copolymers and blends [1-4]. A few examples of the synthetic polymers used as biomate- rials are presented in Figure 1. RESULtS Processing of polymeric biomaterials. The traditional methods for polymer processing in- volve either high temperatures, necessary for melting or viscosity reduction, or hazardous volatile organic solvents (VOCs) and chlorofluorocarbons (CFCs). These methods may affect the incorporation of deli- cate bioactive compounds during processing, since de- naturation may occur upon exposure to solvents, high temperature or shear stresses [5]. Therefore, extensive research is focussing on seeking new and cleaner meth- ods for processing polymeric biomaterials. One such method is the use of supercritical fluids as processing solvents or plasticizers. A supercriti- cal fluid is a substance for which both pressure and temperature are above the critical values [6]. The critical state denotes the conditions at which the phase boundary between liquid and gas ceases to exist. The special combination of gas-like viscosity and diffusivity, and liquid-like density and solvating properties, of a supercritical fluid makes it an excel- lent solvent for various applications [5]. Supercritical carbon dioxide (scCO2) is the preferred choice for these applications. It is a clean, versatile sol- vent and a promising alternative to organic solvents and chlorofluorocarbons. It is non-toxic, non-flamma- ble, chemically inert, environmentally safe and inex- pensive. Its supercritical conditions (Tc = 304.1 K, Pc = 7.38 MPa) are easily attained and it can be removed from a system by simple depressurization [5, 6]. ScCO2 is a good solvent for many low molecular weight compounds and a few polymers, but it is gen- erally a very poor solvent for high molecular weight polymers. However, its solubility in many polymers is substantial, being influenced by temperature, pres- sure and, sometimes, by weak interactions with the groups in the polymer. Dissolved CO2 causes a reduction in the viscosity of the polymers by increasing their free volume. Thus minjanjem tlaka in temperature. ScCO2 hkrati predstavlja tudi alternativo obi~ajnim postopkom sterilizacije medicin- skih pripomo~kov. Zahteve na podro~ju biomaterialov so specifi~ne in postajajo s ~asom vedno strožje, zato pred- stavlja tehnika procesiranja polimerov s scCO2 obetavno alternativo klasi~nim metodam tkivnega inženiringa ter metodam za pridobivanje nosilcev, ki omogo~ajo kontroli- rano spro{~anje zdravil.itor replikacije Pseudorabies virusa v okuženih celicah. pressure and temperature. ScCO2 may also represent a viable alternative to conventional sterilization processes for medical devices. Because the requirements of the field of biomaterials are specific and increasing with time, the technique of polymer processing with scCO2 may repre- sent a promising alternative to classical methods of ob- taining controlled delivery systems and tissue engineering scaffolds. ACTA MEDICO-BIOTECHNICA 2009;2(2):19-24 21 Pregledni članek / Review the polymers are plasticized, allowing processing at lower temperatures. The plasticization is confirmed by a decrease in the glass transition and melting tem- perature of the polymer [6]. The supercritical fluid also alters the physical properties of the polymers, such as density, diffusivity and swollen volume. ScCO2 has been used successfully in polymer synthe- sis and (as a solvent, an antisolvent or plasticizer) in polymer processing for microcellular foaming, parti- cle production, impregnation of polymers, obtaining polymer composites and solvent extraction. ScCO2 as the polymerization environment Polymerization with scCO2 has been studied for the production of polycarbonates [6] and polyesters [7-12], among other things. The main advantage of using CO2 is the reduced viscosity of the polymer during synthesis, which decreases the mass transfer resistance, leading to an increased conversion and molecular weight [6]. Porous materials In the domain of polymeric foams, scCO2 has found an application as a blowing agent for obtaining polymeric devices with controlled porosity [5, 13- 24]. The replacement of traditional blowing agents, such as CFCs, VOCs and hydrochlorofluorocarbons (HCFCs), with CO2 has proven beneficial for the biocompatibility of the final medical devices. More- over, supercritical fluids offer the possibility of con- trolling the size and distribution of the pores by sim- ple variation of the processing parameters (pressure, temperature and depressurization rate) [5]. Howev- er, despite the obvious advantages, there are some limitations. The control over the internal scaffold architecture cannot approach that offered by 3D printing techniques, and this indicates the need for further process optimization [5]. Microparticles Extensive research has focused on the use of scCO2 for obtaining particles for drug delivery applications. For this purpose, pharmaceuticals alone [22, 25-29] and in combination with polymeric supports [10, 30-36] have been processed. When compared to the traditional methods for ob- taining particles – oil-in-water (o/w), water-oil-wa- ter (w/o/w) double emulsion, hydrous water-oil-oil, water-oil-oil-oil (w/o/o/o), solid-oil-water (s/o/w), anhydrous solid-oil-oil-oil (s/o/o/o), spray drying – particle production using scCO2 as a solvent or an antisolvent offers two major advantages. The first advantage is better control of particle size, particle size distribution and morphology. This can be achieved by tuning process parameters such as the amount of dissolved CO2, temperature, pressure, nozzle diameter and depressurization rate. Control over drug or delivery system particle size is essential for good targeting and for the efficacy of the active compound [22]. The second advantage is not needing an organic solvent or to efficiently remove and recover a sol- vent. Supercritical fluids provide a clean alternative to traditional techniques that employ toxic organic solvents or elevated temperatures. This has allowed sensitive bioactive molecules, such as proteins, drugs, and nucleic acids, to be introduced during the polymer processing stage [22]. Existing methods that use scCO2 as a solvent or an antisolvent to obtain drug or polymer-drug particles Figure 1. Examples of polymeric biomaterials 22 ACTA MEDICO-BIOTECHNICA 2009;2(2):19-24 Pregledni članek / Review over, neither the polymer nor the reinforcing agent need to be soluble in scCO2. One can also control the morphology of the final product (filler distribu- tion, pore size and distribution) and obtain high loadings of reinforcing agent [23]. dISCUSSION. Supercritical carbon dioxide has attracted particular attention due to its tremendous potential as a plas- ticizer in polymer processing. Of particular interest is the use of supercritical fluids for processing poly- mers destined for biomedical applications (as micro- spheres, microcapsules, foams, membranes and poly- mer/drug composites). The method offers important advantages over other techniques in terms of the ab- sence of harmful organic solvents or, when necessary, the efficient extraction of solvents and impurities; the mild processing conditions, which do not alter the active compound; and easy control of particle and foam morphology by simply varying the pressure and temperature. Because of the highly specific and increasing requirements of the field of biomaterials, these techniques may represent a promising alterna- tive to classical methods of obtaining controlled de- livery systems and tissue engineering scaffolds. are outlined in Table 1 and include rapid expansion of supercritical solutions (RESS) [5, 22, 26, 32, 37], gas antisolvent crystallization (GAS) [22, 26, 32], supercritical antisolvent precipitation (SAS) [33, 38, 39], precipitation by compressed antisolvent (PCA) [22, 40], solution enhanced dispersion by su- percritical fluid (SEDS) [41, 42], aerosol solvent ex- traction systems (ASES) [22, 32, 43, 44], supercriti- cal assisted atomization (SAA) [22, 45] and obtain- ing particles from gas-saturated solutions (PGSS) [5, 22, 23, 28-30, 33]. Composite materials Another use recently proposed for scCO2 is as a mix- ing environment for composite materials. Polymer composites containing inorganic nanoparticles are attracting much attention in the paint, cosmetic and chemical industries and especially as biomedical applications [23]. The method involves mixing the polymer matrix and the ceramic component in the presence of scCO2. Under these conditions the poly- mer is plasticized and has lower viscosity, and, by high shear mixing, the insoluble particles can be ef- ficiently incorporated into the matrix. No additional co-solvents are required and the entire process can be carried out at near ambient temperature. More- Table 1. Comparison of supercritical processes for microparticle formation; RESS, rapid expansion of supercritical solutions; GAS, gas antisolvent crystallization; SAS, supercritical antisolvent precipitation; PCA, precipitation by com- pressed antisolvent; SEDS, solution enhanced dispersion by supercritical fluid; ASES, aerosol solvent extraction system; SAA, supercritical assisted atomization; PGSS, particles from gas-saturated solutions; SCF, supercritical fluid. RESS GAS/SAS/PCA SEDS/ASES/SAA PGSS The substrate is dissolved in the SCF; the solution is subjected to rapid expansion through a nozzle, which causes supersaturation and particle precipitation. The substrate is dissolved in an appropriate solvent; SCF (antisolvent) is added under high pressure; the depressurization causes supersaturation of the solution and solute precipitation. 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