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1.
Green Processing of Neat Poly(lactic acid) Using Carbon Dioxide under Elevated Pressure for Preparation of Advanced Materials : A Review (2012–2022)
Stoja L. Milovanović, Ivana Lukic, Gabrijela Horvat, Zoran Novak, Sulamith Frerich, Marcus Petermann, Carlos A. García-González, 2023, review article

Abstract: This review provides a concise overview of up-to-date developments in the processing of neat poly(lactic acid) (PLA), improvement in its properties, and preparation of advanced materials using a green medium (CO2 under elevated pressure). Pressurized CO2 in the dense and supercritical state is a superior alternative medium to organic solvents, as it is easily available, fully recyclable, has easily tunable properties, and can be completely removed from the final material without post-processing steps. This review summarizes the state of the art on PLA drying, impregnation, foaming, and particle generation by the employment of dense and supercritical CO2 for the development of new materials. An analysis of the effect of processing methods on the final material properties was focused on neat PLA and PLA with an addition of natural bioactive components. It was demonstrated that CO2-assisted processes enable the control of PLA properties, reduce operating times, and require less energy compared to conventional ones. The described environmentally friendly processing techniques and the versatility of PLA were employed for the preparation of foams, aerogels, scaffolds, microparticles, and nanoparticles, as well as bioactive materials. These PLA-based materials can find application in tissue engineering, drug delivery, active food packaging, compostable packaging, wastewater treatment, or thermal insulation, among others.
Keywords: aerogels, drying, foaming, impregnation, particle generation, PLA
Published in DKUM: 06.02.2024; Views: 83; Downloads: 4
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2.
The effect of drying methods and extraction techniques on oleuropein content in olive leaves
Darija Cör Andrejč, Bojan Butinar, Željko Knez, Kaja Tomažič, Maša Knez Marevci, 2022, original scientific article

Abstract: Increased demand for olive oil has caused higher quantities of byproducts in olive processing, such as olive leaves, olive skins, and vegetation water. It is well known that olive leaves contain several phenolic compounds, including secoiridoids. Oleuropein is the major secoiridoid in olive leaves. Oleuropein has been found to exhibit antioxidative, antimicrobial, antiviral, and antiatherogenic activities. We studied the effect of extraction techniques and drying methods on oleuropein content in olive leaves of Istrska belica and Lecino cultivar. Three different procedures of drying were used: at room temperature, at 105 °C, and freeze drying. Ethanol-modified supercritical extraction with carbon dioxide, conventional methanol extraction, and ultrasonic extraction with deep eutectic solvent were performed. Antioxidant activity was determined, as well as methanolic and supercritical extracts. The presence of olive polyphenols was confirmed by the HPLC method.
Keywords: olive leaves, drying, different extraction techniques, oleuropein
Published in DKUM: 10.08.2023; Views: 193; Downloads: 18
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3.
Physicochemical properties of gold nanoparticles for skin care creams
Peter Majerič, Zorana Jović, Tilen Švarc, Žiga Jelen, Andrej Horvat, Djuro Koruga, Rebeka Rudolf, 2023, original scientific article

Abstract: Gold nanoparticles (AuNPs) have now been used in skin care creams for several years, with marketed anti-aging, moisturizing, and regenerative properties. Information on the harmful effects of these nanoparticles is lacking, a concern for the use of AuNPs as cosmetic ingredients. Testing AuNPs without the medium of a cosmetic product is a typical method for obtaining this information, which is mainly dependent on their size, shape, surface charge, and dose. As these properties depend on the surrounding medium, nanoparticles should be characterized in a skin cream without extraction from the cream’s complex medium as it may alter their physicochemical properties. The current study compares the sizes, morphology, and surface changes of produced dried AuNPs with a polyvinylpyrrolidone (PVP) stabilizer and AuNPs embedded in a cosmetic cream using a variety of characterization techniques (TEM, SEM, DLS, zeta potential, BET, UV–vis). The results show no observable differences in their shapes and sizes (spherical and irregular, average size of 28 nm) while their surface charges changed in the cream, indicating no major modification of their primary sizes, morphology, and the corresponding functional properties. They were present as individually dispersed nanoparticles and as groups or clusters of physically separated primary nanoparticles in both dry form and cream medium, showing suitable stability. Examination of AuNPs in a cosmetic cream is challenging due to the required conditions of various characterization techniques but necessary for obtaining a clear understanding of the AuNPs’ properties in cosmetic products as the surrounding medium is a critical factor for determining their beneficial or harmful effects in cosmetic products.
Keywords: gold nanoparticles, ultrasonic spray pyrolysis, freeze drying, characterization, creams
Published in DKUM: 19.04.2023; Views: 471; Downloads: 41
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4.
Preparation of Three Dimensional Structures of Polysaccharide Derivatives for Application in Regenerative Medicine : doctoral disertation
Andreja Dobaj-Štiglic, 2022, doctoral dissertation

Abstract: Biocompatible polysaccharide scaffolds with controllable pore size, good mechanical properties, and no hazardous chemical crosslinkers are desirable for long-term tissue engineering applications. Despite decades of development of novel scaffolds, there are still many challenges to be solved regarding their production and optimization for specifically engineered tissues. Herein, we have fabricated several three-dimensional (3D) scaffolds using polysaccharide or polysaccharide-protein composite hydrogels or inks for 3D printing, featuring strong shear thinning behavior and adequate printability. The inks, composed of various combinations of chitosan, nanofibrillated cellulose, carboxymethyl cellulose, collagen, and citric acid, were 3D printed, freeze-dried, and dehydrothermally heat-treated to obtain dimensionally and mechanically stable scaffolds. The heat-assisted step induced the formation of covalent amide and ester bonds between the functional groups of chosen polysaccharides and protein collagen. Citric acid was chosen as a non-hazardous and „green” crosslinker to further tailor the mechanical properties and long-term stability of the scaffolds. We have investigated how the complexation conditions, charge ratio, dehydrothermal treatment, and degree of crosslinking influence the scaffolds' chemical, surface, swelling, and degradation properties in the dry and hydrated states. The compressive strength, elastic modulus, dimensional stability and shape recovery of the (crosslinked) scaffolds increased significantly with balanced charge ratio, dehydrothermal treatment, and increased concentrations of citric acid crosslinker and collagen concentrations. The prepared crosslinked scaffolds promoted (clustered) cell adhesion and showed no cytotoxic effects, as determined by cell viability assays and live/dead staining with human bone tissue-derived osteoblasts and human adipose tissue-derived mesenchymal stem cells. The water-based and non-hazardous crosslinking methods presented here can be extended to all polysaccharide- or polysaccharide-protein-based materials to develop cell-friendly scaffolds with tailored properties suitable for various tissue engineering applications.
Keywords: chitosan, carboxymethyl cellulose, nanofibrillated cellulose, citric acid, collagen, freeze drying, 3D printing, dehydrothermal treatment
Published in DKUM: 11.10.2022; Views: 489; Downloads: 121
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5.
Hollow-Fibre forward osmosis membrane module stress tests : diplomsko delo univerzitetnega študijskega programa I. stopnje
Urban Gselman, 2020, undergraduate thesis

Abstract: Humanity has recently begun to realize that resources on Earth are not infinite and that, consequently, it is imperative for us to start to use them sustainably and renewably. Aligned with this mentality, forward osmosis has been rigorously studied in the past few decades with some commercially available products already on the market. This thesis used one of those products, namely, the Aquaporin HFFO 2 membrane module, and assessed its stability in sodium metabisulfite solution and high salinity without flowing solutions under long-term storage conditions. The sodium metabisulfite preservation part was devised to upgrade the existing guidelines for prolonged storage of fouled membranes and the osmotic drying project was looking at a scenario where the modules would be exposed to a salt solution on the draw side and no flowing wastewater on the feed side. The assessments were made on pristine HFFO 2 membrane modules and with RO water prepared on site. Their performance after exposure was measured with an internal quality control procedure, producing values for water flux, reverse salt flux, normalized reverse salt flux and temperature, which were later compared with the initial values. The findings show us that the sodium metabisulfite and increased salinity have a detrimental effect on the membrane inside of the module, but not enough to be considered damaged and would need to be discarded. More valuable results will be gained if the measurements are done on fouled membranes, as the biggest factor in increasing the lifetime of a module is preventing algal and bacterial growth
Keywords: forward osmosis, Aquaporin HFFO 2, sodium metabisulfite, osmotic drying, prolonged storage.
Published in DKUM: 24.09.2020; Views: 769; Downloads: 9
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6.
Formation, characterization and application of polysaccharide aerogels
Gabrijela Horvat, 2018, doctoral dissertation

Abstract: The aim of this PhD dissertation was to describe and analyze the preparation and characterization of polysaccharide aerogels and their future pharmaceutical and medical application. For the research, we used four types of polysaccharides: pectin, alginate, xanthan and guar. We used two types of pectin, high-methoxyl and low-methoxyl pectin, because of their different gelation mechanisms. The first part of the dissertation describes the preparation and characterization of pure polysaccharide aerogels. First, we prepared pectin spherical aerogels, cross-linked with three different ions, and we investigated their final properties. Later, we developed a new method for the preparation of alginate, pectin, xanthan and guar aerogels. We used only ethanol and no other cross-linkers. Ethanol was removed in the later processes of supercritical drying, and the remaining final material was thus only porous polysaccharide. By this method, we were able to prepare pure xanthan and guar aerogels. Prior to this study, xanthan and guar aerogels were prepared only as composites. Pectin aerogels prepared by the new method have amazing properties. On the other hand, alginate aerogels show poor characteristics, and thus the methods need to be optimised. We tried different alginate viscosities, different alcohols (methanol, ethanol, 1-propanol and 1-butanol), and we investigated longer (24h) and shorter (1h) gel setting times. The second part of this dissertation describes the pharmaceutical and medical applications of prepared aerogels. The release of diclofenac sodium from spherical pectin aerogels was investigated in vitro. Calcium cross-linked aerogels were not able to retain the drug, and its release was immediate. In order to achieve controlled release of diclofenac sodium, zinc ions had to be used as cross-linkers. Later, a low water-soluble drug, nifedipine, was used as a model drug for the monolithic aerogels prepared by the new method. The release of nifedipine from pectin and alginate aerogels was highly increased, compared to the crystalline drug. This result is promising for future evaluation of these materials for increasing the bioavailability of poorly water-soluble drugs. Nifedipine release from xanthan and guar aerogels was prolonged up to two weeks. This result reveals a new perspective on such materials for their potential use in medicine as implants and local drug delivery. According to these results, we then developed a new coating material for medical-grade stainless steel from xanthan and pectin. An aerogel coating was loaded with diclofenac sodium and indomethacin, and their release profiles were investigated in vitro. Electrochemical analysis and cell tests proved the safety of such materials for use in medicine. Using aerogel coatings, the drug can be introduced locally into the body; therefore, the need for intravenous, post-operational treatment is greatly reduced.
Keywords: polysaccharides, aerogels, supercritical drying, drug carriers
Published in DKUM: 09.04.2018; Views: 1846; Downloads: 271
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7.
Effect of drying parameters on physiochemical and sensory properties of fruit powders processed by PGSS-, vacuum- and spray-drying
Urban Feguš, Uroš Žigon, Marcus Petermann, Željko Knez, 2015, original scientific article

Abstract: Aim of this experimental work was to investigate the possibility of producing fruit powders without employing drying aid and to investigate the effect of drying temperature on the final powder characteristics. Raw fruit materials (banana puree, strawberry puree and blueberry concentrate) were processed using three different drying techniques each opera- ting at different temperature conditions: vacuum-drying (–27–17 °C), spray-drying (130–160 °C) and PGSS-drying (112–152 °C). Moisture content, total colour difference, antioxidant activity and sensory characteristics of the proces- sed fruit powders were analysed. The results obtained from the experimental work indicate that investigated fruit pow- ders without or with minimal addition of maltodextrin can be produced. Additionally, it was observed that an increase in process temperature results in a higher loss of colour, reduced antioxidant activity and intensity of the flavour profile.
Keywords: particles from gas saturated solutions drying, vacuum-drying, spray-drying, sensory evaluation, fruit powders
Published in DKUM: 18.08.2017; Views: 1225; Downloads: 377
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8.
Water sorption characteristics of a new type of gelatine powder, produced by a new high-pressure micronisation process
Christian Reibe, Željko Knez, 2010, original scientific article

Abstract: A new high-pressure micronisation process for Gelatine has been developed. By applying this process to gelatine, a new type of Gelatine was produced: Dry and pure Gelatine powder of high molecular mass. This powder was investigated,regarding its sorption behaviour and compared to its feedstock material. The feedstock Gelatine was of type B 200 Bloom, 6 mesh, having a molecular mass of 150,000 g $mol^{-1}$. The produced Gelatine powder's molecular mass was around 130,000 g $mol^{-1}$, while its mean particle size was 300 μm. Their sorption isotherms were investigated at 30 °C, 40 °C and 60 °C and water activities in the range from 0.055 to 0.836. At given water activities the Equilibrium Moisture Contents (EMC) decreases with increasing temperature.The measured sorption isotherms were fit with the Brunauer- Emmet-Teller (BET)- model and the Guggenheim- Anderson- deBoer (GAB)- model. Both models described the adsorption behaviour well in their range of validity, while only the GAB-model was adequate for describing the desorption behaviour. Hysteresis effect occurred in each adsorption-desorption pair. The isosteric heat of sorption of both substances was determined for adsorption, desorption respectively. Slight differences in the sorption behaviour occurred due to processing, while quality of the gelatine was not affected.
Keywords: $CO_2$ spray drying, gelatine powder, sorption isotherms, isosteric heat of sorption, Guggenheim-Anderson-deBoer model, Brunauer-Emmet-Teller model
Published in DKUM: 10.07.2015; Views: 1670; Downloads: 65
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9.
10.
Silica aerogels as support for lipase catalyzed esterifications at sub- and supercritical conditions
Zoran Novak, Maja Leitgeb, Vlasta Krmelj, Željko Knez, 2003, original scientific article

Abstract: The enzymes (lipases from Candida rugosa and porcine pancreas) were immobilized on silica aerogels by sol-gel procedure followed by supercritical drying with CO2. Such immobilized enzymes were used as biocatalysts for esterification in supercritical CO2 and near critical propane at 40 °C and 100 bar. It was found out that the initial reaction rates in propane rose two to three times in comparison with the same reaction, catalyzed by free lipase. SC CO2 deactivated the non-immobilized lipase in reaction mixture while with the immobilized enzyme the conversion was 35%. The initial reaction rates in propane were 20 times higher than in water medium due to the properties of propane as a medium for esterification of fatty acids.
Keywords: chemical processing, biotechnology, esterification, immobilization of enzymes, supercritial CO2, propane, lipases, supercritical CO2 drying
Published in DKUM: 01.06.2012; Views: 1718; Downloads: 28
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