Oscillatory Stress Sweep Research Articles

Calcium hydroxide-loaded nanoparticles dispersed in thermosensitive gel as a novel intracanal medicament.

Design, produce and assess the viability of a novel nanotechnological antibacterial thermo-sensible intracanal medicament This involves encapsulating calcium hydroxide (Ca(OH)2) within polylactic-co-glycolic acid (PLGA) nanoparticles (NPs) and dispersing them in a thermosensitive gel (Ca(OH)2-NPs-gel). In addition, perform in vitro and exvivo assessments to evaluate tissue irritation and penetration capacity into dentinal tubules in comparison to free Ca(OH)2. Reproducibility of Ca(OH)₂-NPs was confirmed by obtaining the average size of the NPs, their polydispersity index, zeta potential and entrapment efficiency. Moreover, rheological studies of Ca(OH)2-NPs-gel were carried out with a rheometer, studying the oscillatory stress sweep, the mean viscosity value, frequency and temperature sweeps. Tolerance was assessed using the membrane of an embryonated chicken egg. In vitro Ca(OH)2 release was studied by direct dialysis in an aqueous media monitoring the amount of Ca(OH)2 released. Six extracted human teeth were used to study the depth of penetration of fluorescently labelled Ca(OH)2-NPs-gel into the dentinal tubules and significant differences against free Ca(OH)2 were calculated using one-way anova. Ca(OH)2-NPs-gel demonstrated to be highly reproducible with an average size below 200 nm, a homogeneous NPs population, negative surface charge and high entrapment efficiency. The analysis of the thermosensitive gel allowed us to determine its rheological characteristics, showing that at 10°C gels owned a fluid-like behaviour meanwhile at 37°C they owned an elastic-like behaviour. Ca(OH)2-NPs-gel showed a prolonged drug release and the depth of penetration inside the dentinal tubules increased in the most apical areas. In addition, it was found that this drug did not produce irritation when applied to tissues such as eggs' chorialantoidonic membrane. Calcium hydroxide-loaded PLGA NPs dispersed in a thermosensitive gel may constitute a suitable alternative as an intracanal antibacterial medicament.

Shape stability control of a 3D-printed clay/biochar-based monolith to support Fe catalyst for continuous levofloxacin degradation with low metal leaching

Disposing of wet solid waste sludge has become a prominent issue because it has high environmental risks. Herein, a clay/biochar-based monolith (CBM) is prepared using a three-dimensional (3D) printing design method. The CBM is loaded with iron (Fe) and used as a catalyst (Fe/CBM) to activate peroxymonosulfate (PMS) for degrading levofloxacin (LVF) in an aqueous solution. The monolith preparation process is inexpensive, and the monolith has designable flow paths that facilitate improved mass transport and material recycling. ZnCl2, as a chemical activator, is added to the 3D-printed slurry to improve the catalytic performance of Fe/CBM. Oscillatory stress sweep and frequency sweep tests prove that incorporating ZnCl2-containing sludge in the slurry effectively increases the elastic modulus and critical stress, improving structural stability and constructability. The slurry containing 10% sludge (S10%-Zn with 1 M ZnCl2), exhibits 30% higher static yield stress than the slurry containing only clay and water (S0%). Fe/CBM is prepared by loading Fe active sites onto the surface of CBM via an impregnation method, and it leaches very low amounts of Fe (<0.2 mg/L) and other heavy metals (e.g., Cd, and Pd, etc). Fe/CBM-S10%-Zn/PMS with 10% mass fraction of sludge exhibits much higher (∼80% at 20 min) degradation efficiency than Fe/CBM-S0%/PMS (∼15% at 20 min) and Fe/CBM-S10%/PMS (∼43% at 20 min). Combining with XPS results, when ZnCl2 is added to the slurry, the content of Fe2+ in the catalytic material increases, improving the catalytic performance. The PMS activation mechanism of Fe/CBM-S10%-Zn for LVF degradation is also discussed, which is ascribed to the radical and nonradical pathways. This provides new insights on how to recycle solid waste and make high-performance clay/biochar-based monolith catalysts for the continuous wastewater treatment.

Effects of Early-Age rheology and printing time interval on Late-Age fracture characteristics of 3D printed concrete

• Fracture of 3D printed concrete is strongly affected by rheology and printing process. • Increasing printing time interval changes rheology and decreases fracture toughness. • Time effect is stronger on fracture characteristics of interlayers than filaments. • Dynamic oscillation sweep method reveals time-dependent storage modulus and viscosity. • Novel fracture experiment using DIC measures and visualizes process zone growth. Concrete additive manufacturing, also known as 3D printing, opens tremendous opportunities in the construction industry, architectural design, defense and space exploration. As concrete is a quasi-brittle material and prone to fracture, it is crucial to understand the effect of the layer-by-layer manufacturing process, such as the variation in printing time intervals and the introduction of printing interlayers, on the fracture behavior of 3D printed concrete. This study elucidated the effects of early-age material rheology, printing time interval and printing path on the late-age load versus crack mouth opening displacement relation, fracture process zone, fracture toughness and fracture energy of 3D printed concrete materials with notch locations at interlayers vs filaments. A dynamic oscillation stress sweep method was used to reveal the time-dependency of early-age rheological parameters. A novel fracture experiment was designed to integrate a digital image correlation system to enable closed-loop control, measurement, and visualization of crack tip parameters during the rapid fracture processes along the printing interlayers or filaments. The results revealed that the fracture characteristics of 3D printed concrete are strongly influenced by its rheology and time-dependency of rheology, which is fundamentally different from normal cast concrete. Increasing the printing time interval increases the storage modulus, yield stress and complex viscosity of 3D printed concrete materials at different shear stress levels, consequently increasing the likelihood of imperfections, and thus decreasing the maximum process zone size developed at the crack tip, fracture toughness and fracture energy of 3D printed concrete. Such effects were found to be stronger on the fracture characteristics of printing interlayers than filaments.

Rheological properties and screen printability of UV curable conductive ink for flexible and washable E-textiles

As a critical component for the realization of flexible electronics, multifunctional electronic textiles (e-textiles) still struggle to achieve controllable printing accuracy, excellent flexibility, decent washability and simple manufacturing. The printing process of conductive ink plays an important role in manufacturing e-textiles and meanwhile is also the main source of printing defects. In this work, we report the preparation of fully flexible and washable textile-based conductive circuits with screen-printing method based on novel-developed UV-curing conductive ink that contains low temperature and fast cure features. This work systematically investigated the correlation between ink formulation, rheological properties, screen printability on fabric substrates, and the electrical properties of the e-textile made thereafter. The rheological behaviors, including the thixotropic behavior and oscillatory stress sweep of the conductive inks was found depending heavily on the polymer to diluent ratio in the formulation. Subsequently, the rheological response of the inks during screen printing showed determining influence to their printability on textile, that the proper control of ink base viscosity, recovery time and storage/ loss modulus is key to ensure the uniformity of printed conductive lines and therefore the electrical conductivity of fabricated e-textiles. A formulation with 24 wt% polymer and 10.8 wt% diluent meets all these stringent requirements. The conductive lines with 1.0 mm width showed exceptionally low resistivity of 2.06 × 10−5 Ω cm Moreover, the conductive lines presented excellent bending tolerance, and there was no significant change in the sample electrical resistance during 10 cycles of washing and drying processes. It is believed that these novel findings and the promising results of the prepared product will provide the basic guideline to the ink formulation design and applications for screen-printing electronics textiles.

Effect of solvent on rheological properties of Sn–0.3Ag–0.7Cu solder paste for jet printing

Solder paste plays a significant role in the SMT (Surface Mount Technology) of electronic products. It has become an urgent task in the field of surface mounting to develop the matching flux. The purpose of this paper is to investigate the relationship of solder paste component with rheological properties and printing performance. In this study, Sn–0.3Ag–0.7Cu solder powder was applied. The effect of solvent agents on the rheological properties and jet printing performance of solder paste was investigated systematically with a combined method. The rheological methods consisting of viscosity test, thixotropic loop test, oscillation stress sweep test, creep recovery test and jet printing test. From this study, it was found that the compound alcohols solvent has an evident viscoelastic behavior which presents a superior printability. S5 solder paste with high viscosity and small loss coefficient G″/G′ exhibits an excellent printability. The largest thixotropic loop area and unrecovered stress value of solder paste can be jetted smoothly. Besides, solder paste with high creep recovery value in jet printing can obtain better shape.

The effect of solder paste particle size on the thixotropic behaviour during stencil printing

The thixotropic behaviour of different type lead-free solder pastes (Type 3: particle size: 20–45 μm; Type 4: 20–38 μm; Type 5: 10–25 μm) was investigated and compared. The viscosity decrease during the first stencil printing cycles is also described. The measurements were carried out with a rheometer using serrated parallel plates by sweeping the shear rate between 0.0015–100 s−1 with ten recordings per decade. Thixotropic behaviour of the solder pastes within the cycles of stencil printing was evaluated by including a “rest-period” between every rheological measurement to address the idle time between the printings. The “rest-period” was set to 15 s, 30 s and 60 s in this research The viscosity curve parameters η0 and η∞ were determined and were used to describe the viscosity change of solder pastes (from initial to stabilised state). Oscillatory stress sweep tests for different idle time periods were conducted in the stabilised state of the solder pastes. Significant difference was shown in the viscosity between the initial and the stabilised state of the solder paste; the η0 parameter decreased even to one thirds, and the η∞ decreased approximately to two thirds during the measurement cycles.

Comparative Rheological Characterization of Two Natural Polymers from Sesamum radiatum and Bombax buonopozense for Application in Drug Delivery

SummarySmall deformation measurements have been used to characterize the rheological behaviour of two understudied natural polymers obtained from the leaves and calyxes of Sesamum radiatum and Bombax buonopozense respectively, by extraction with water, and then precipitation with absolute ethanol. Measurements performed on 1%w/v aqueous dispersion of each polymer at ambient conditions include viscosity, yield stress, oscillatory stress sweep, oscillatory frequency sweep, creep and recovery, and then temperature ramp. The measurements were done on the same samples on the first day and fourth day after storage at ambient temperature. The polymers exhibited shear‐ thinning with the viscosity of bombax gum remaining basically same on the 1st and 4th day while sesamum gum had a significant drop in viscosity by the 4th day. Bombax gum (yield value – 5.499 Pa at shear 0.114 s−1) had more internal strength than sesamum gum (yield value – 0.338 Pa at shear 0.032 s−1). In addition, bombax gum exhibited more elasticity and extended visco‐elastic region than sesamum gum. The present study suggests that bombax gum can be used for matrices and hydrogels with improved physicomechanical strength and sustained drug release while sesamum gum may require modification to achieve similar properties.

Rheological properties and related screen-printing performance of low-temperature silver pastes for a-Si:H/c-Si heterojunction solar cells

Four different low-temperature silver pastes were utilized to make metal grids by screen printing for silicon heterojunction solar cells. The rheological behaviors of the low-temperature silver pastes were characterized by viscosity test, thixotropy test, oscillatory stress sweep test and creep-recovery test. The correlationship between the screen-printing performance and the rheological properties was investigated. It was found that the shear thinning behavior and the thixotropy behavior of the silver pastes were desirable for the screen-printing process. An obvious viscoelastic behavior of the silver paste was also helpful for improving its printability. Further, good recovery and low creep and recovery compliances could minimize the printing defects and the tendencies to bleed out during the screen-printing, and thus increase the aspect ratio of the printed grids.

Hydrogel blends with adjustable properties as patches for transdermal delivery

The effect of different preparation parameters were analyzed with respect to the rheological and pharmaceutical characteristics of hydrogel blend patches, as transdermal delivery formulation. Mixtures of pectin and gelatin were employed for the production of patches, with adjustable properties, following a two-step gelation procedure. The first gelation, a thermal one, is trigged by the presence of gelatin, whereas, the second gelation, an ionic one, is due to the formation of the typical egg box structure of pectin. In particular, the patch structural properties were assessed by oscillation stress sweep measurements which provided information concerning their viscolelastic properties. In addition, different modalities for drug loading were analyzed with respect to drug homogeneous distribution; testosterone was employed as model drug for transdermal administration. Finally, the performances of the produced transdermal patches were studied, in term of reproducibility and reliability, by determination of in vitro drug release profiles.

Mucoadhesive nanoliposomal formulation for vaginal delivery of an antifungal

Context: Ciclopirox olamine (CPO) is indicated in the treatment of vaginal fungal infections. The frequent and large dosing of available vaginal CPO creams gives rise to poor compliance amongst females. In such a situation a delivery system capable of providing sustained release of CPO is warranted and can be realized through incorporation of its liposomal formulation into a mucoadhesive gel base. The liposomal formulation would offer sustained release whereas mucoadhesive gel would prolong the contact with vaginal wall; thus avoiding frequent and large dosing.Objective: The present study aimed at investigating mucoadhesive liposomal CPO gel for vaginal use.Method: The study embarked on evaluating liposomal CPO and its Carbopol 974®P gel for stability at vaginal pH, release profile, rheological characteristics, mucoadhesive behavior and finally antifungal activity.Results: The results revealed that CPO liposomes were stable at vaginal pH; its Carbopol gel released 58.75 ± 6.4% of CPO at the end of 24 h which suggested sustained release. Rheology via viscometric, oscillatory stress sweep and oscillatory frequency sweep testing of the gel, studied at different temperatures and under different dilutions with vaginal fluid simulant testified pseudoplastic behavior of the gel. It also pointed towards the predominance of elastic behavior of the gel at all the dilutions. The gel exhibited good mucoadhesivity to sheep vaginal tissue. Furthermore, CPO entrapped in liposome too displayed antifungal activity.Conclusion: The study undertaken recommended Carbopol 974®P gel loaded with CPO liposomes as a potential delivery system for treatment of fungal vaginal infections.

Correlation between rheological properties, in vitro release, and percutaneous permeation of tetrahydropalmatine.

The aim of the present work was to investigate the influence of formulation factors including different grades of Carbopol® matrices and penetration enhancers on the percutaneous permeation of tetrahydropalmatine (THP), rheological properties, and in vitro release; and the correlation behind rheological properties, in vitro release, and percutaneous permeation. Transdermal penetration of THP through excised rabbit skin and in vitro release of THP across transparent Cellophane® were performed by vertical Franz diffusion cell. Rheological analyses were proceeded in terms of "steady flow tests", "oscillation stress sweep", and "creep recovery". The result of percutaneous penetration of THP indicated that, the emulgel prepared with Carbopol® 971P (Cp 971P) as the matrix and N-methyl-2-pyrrolidone (NMP) as the penetration enhancer had the highest cumulative permeation amount (118.19 μg/cm(2)). All the experimental data showed a good fit to the Casson model in viscosimetric studies no matter what the types of matrices or the kinds of penetration enhancers were. The release profile fitted the zero-order release kinetics model with Cp 971P as the matrix without any penetration enhancers. However, when adding penetration enhancers, in vitro release of THP presented anomalous (non-Fickian) release kinetics. Clarifying the relationship behind percutaneous permeation of THP, rheological properties, and in vitro release will provide us with profound insights and facilitate the design of specific emulgel.

Structure and viscoelastic behavior of pharmaceutical biocompatible anionic microemulsions containing the antitumoral drug compound doxorubicin

Structure and viscoelastic properties of negatively charged oil-in-water (o/w) microemulsions have been investigated. Microemulsions (ME) containing soya phosphatidylcholine (SPC), eumulgin HRE 40 (EU) and sodium oleate (SO) as surfactant, cholesterol (CHO) as oil phase, and aqueous buffer with and without the antitumoral doxorubicin (DOX) have been studied. The effect of the oil phase/surfactant ratio (O/S) and the DOX incorporation on the structural and rheological properties have been studied in several compositions of ME systems. The rheological analyses were performed through the oscillation stress sweep, creep recovery test, and viscosity test. The combination of the DOX incorporation with the high O/S ratio provided a further viscoelastic structure with linear behavior. Independently of the O/S ratio the oil phase diameter increases according to a sigmoid profile, stabilizing up to 340 min. The apparent viscosity decreases a minimum value with the shear rate, but increases with both the O/S ratio and the DOX incorporation in the system. The structural and rheological properties of the studied MEs were directly dependent on the O/S ratio and can be used to improve the application of the system in the pharmaceutical field.

Rheological properties of angel food cake made with pH unfolded and refolded egg albumen

Angel food cake was made using egg albumen subjected to the pH-induced unfolded and refolded treatment. The effect of treatment on the rheological properties of angel food cake was investigated. Egg albumen solutions were prepared and pH was adjusted to 1.5, 4.5, 8.5 or 12.5 and then held 60 min to unfold the proteins. After holding, the solutions were readjusted to pH 4.5 or 8.5, and held for 45 min to partially refold the proteins. Egg white foam with sucrose was whipped and flour and the rest of sucrose were folded into the foam. The foam batter was heated in a TA Instrument AR 2000 controlled stress rheometer equipped in a parallel geometry. Samples were heated from 21 °C to 150 °C at a rate of 8.5 °C per min and then cooled down to 21 °C to bake the angel food cake. At 21 °C, oscillatory stress sweep was performed. There was no relationship between the G′ value of angel cake batter and its G′ value at 150 °C. Changes in rheological properties of batters and angel food cakes using different combinations of ingredients were studied. The pH unfolding and refolding procedure led to more rigid final products compared to the controls with egg albumen samples not subjected to pH treatment. Adding sucrose to the flour increased the starch gelation temperature up to 82 °C. Higher protein concentrations resulted in better foams in the cake batter, but the batter made with an intermediate protein concentration produced the most rigid angel food cake. Adding egg albumin did not change gelation temperature of the starch. It appears that incorporation of flour with the egg white foam, leads to about a ten times decrease in the strength of the foam, and a decrease in the gelatinization process of starch after adding sugar, are crucial in forming of an angel food cake texture.

Rheological characterisation and printing performance of Sn/Ag/Cu solder pastes

Lead-free solder paste printing process accounts for majority of the assembly defects in the electronic manufacturing industry. The study investigates rheological behaviour and stencil printing performance of the lead-free solder pastes (Sn/Ag/Cu). Oscillatory stress sweep test was carried out to study the visco-elastic behaviour of the lead-free solder pastes. The visco-elastic behaviour of the paste encompasses solid and liquid characteristic of the paste, which could be used to study the flow behaviour experienced by the pastes during the stencil printing process. From this study, it was found that the solid characteristics ( G′) is higher than the liquid characteristic ( G′′) for the pastes material. In addition, the results from the study showed that the solder paste with a large G′ = G′′ has a higher cohesiveness resulting in poor withdrawal of the paste during the stencil printing process. The phase angles ( δ) was used to correlate the quality of the dense suspensions to the formulation of solder paste materials. This study has revealed the value of having a rheological measurement for explaining and characterising solder pastes for stencil printing. As the demand for lead free pastes increases rheological measurements can assist with the formulation or development of new pastes.

Rheological characterisation of solder pastes and isotropic conductive adhesives used for flip-chip assembly

Solder pastes and isotropic conductive adhesives (ICAs) are widely used as a principal bonding medium in the electronic industry. This study investigates the rheological behaviour of the pastes (solder paste and isotropic conductive adhesives) used for flip-chip assembly. Oscillatory stress sweep test are performed to evaluate solid characteristic and cohesiveness of the lead-free solder pastes and isotropic conductive adhesive paste materials. The results show that the G′ (storage modulus) is higher than G″ (loss modulus) for the pastes material indicating a solid like behaviour. It result shows that the linear visco-elastic region for the pastes lies in a very small stress range, below 10 Pa. In addition, the stress at which the value of storage modulus is equal to that of loss modulus can be used as an indicator of the paste cohesiveness. The measured cross-over stress at G′ = G″ shows that the solder paste has higher stress at G′ = G″ compared to conductive adhesives. Creep-recovery test method is used to study the slump behaviour in the paste materials. The conductive adhesive paste shows a good recovery when compared to the solder pastes.

An investigation into the rheological properties of different lead‐free solder pastes for surface mount applications

PurposeThe purpose of this paper is to investigate the rheological behaviour of three different lead‐free solder pastes used for surface mount applications in the electronic industry.Design/methodology/approachThis study concerns the rheological measurements of solder paste samples and is made up of three parts. The first part deals with the measurement of rhelogical properties with three different measuring geometries, the second part looks into the effect of frequencies on oscillatory stress sweep measurements and the final part reports on the characterisation and comparison of three different types of Pb‐free solder pastes.FindingsAmong the three geometries, the serrated parallel plate was found effective in minimising the wall‐slip effect. From the oscillatory stress‐sweep data with different frequencies; it was observed that the linear visco‐elastic region is independent of frequency for all the solder paste samples. To understand the shear thinning behaviour of solder paste, the well known Cross and Carreau models were fitted to the viscosity data. Moreover, creep‐recovery and dynamic frequency‐sweep tests were also carried out without destroying the sample's structure and have yielded useful information on the pastes behaviour.Research limitations/implicationsMore extensive research is needed to fully characterise the wall‐slip behaviour during the rheological measurements of solder pastes.Practical implicationsThe rheological test results presented in this paper will be of important value for research and development, quality control and facilitation of the manufacturing of solder pastes and flux mediums.Originality/valueThis paper shows how wall‐slip effects can be effectively avoided during rheological measurements of solder pastes. The paper also outlines how different rheological test methods can be used to characterise solder paste behaviours.

Rheological and adhesive properties of new thermoresponsive hyperbranched poly[ethylene oxide-b-propylene oxide-b-ethylene oxide

Hydrogels are promising platforms for drug delivery, primarily due to the fact that many desirable physicochemical properties can be simultaneously conferred to such systems. In this respect thermoresponsive hydrogels are of interest. The adhesive and rheological properties of new hydrophilic thermoresponsive polymers are based on hyperbranched block copolymers of ethylene oxide and propylene oxide, EG35 and EG 40. EG 40 is constituted by the polycondensation of dihydroxytelechelic POE-PPO-POE triblocks linked together via carbanate, urea, allophanate and biuret bridges. The structure of EG35 is similar to EG40 if one accepts that EG35 exhibits acidic pendant groups. Oscillation stress sweep and creep recovery tests showed that EG35 and EG40 hydrogels were characterized by a remarkable elasticity. Temperature sweep tests and studies of the variation of viscosity at increasing temperatures have also proved the thermogelling character of these hydrogels. The gelling temperature was in the range of 30-37 °C. Furthermore, the adhesiveness of these hydrogels was shown to depend simultaneously on the intensity of the interfacial interactions developed between the hydrogel and the adhesion substrate, and the mechanical behavior of the hydrogel under mechanical solicitation. The mucoadhesion of the two hydrogels depended strongly on the temperature. Very interestingly, at 37 °C, thus over the gelling temperature, they exhibited considerable mucoadhesivity on rat intestinal mucosa compared to Carbopol 974P hydrogels, which were used as a positive control for mucoadhesion. These data suggested that EG35 and EG40 could be promising polymers for pharmaceutical applications, and particularly EG35, which had a considerable adhesiveness and a remarkable elasticity compared to Carbopol 974P.

Formation and characterization of three-component-sorbitan monoester surfactant, oil and water-creams

The effect of molecular structure of four sorbitan monoester surfactants (sorbitan monolaurate, sorbitan monopalmitate, sorbitan monostearate and sorbitan monooleate) on the formation of simple three-component creams is presented. Interfacial properties of the surfactants were determined using a du Nouy tensiometer and rheological properties of selected creams with oscillation stress sweep, creep recovery and viscosity tests. Depending on the composition, sorbitan monolaurate and sorbitan monooleate formed both o/w creams and w/o creams, while sorbitan monopalmitate and sorbitan monostearate formed only o/w creams. Sorbitan monostearate and sorbitan monopalmitate had the smallest cmc and A cmc values and they were the most effective surfactants in lowering the interfacial tension. These surfactants formed the most stable and elastic creams with clear linear viscoelastic regions and small compliance values. Sorbitan monolaurate and sorbitan monooleate formed viscous creams without elastic properties.

Rheological properties of three component creams containing sorbitan monoesters as surfactants

Creams from three components, surfactant, purified water and oil, were prepared. Comparable molar fractions of components were used in order to better understand the structural properties of the components used. The surfactants were sorbitan monoesters, sorbitan monolaurate, monopalmitate, monostearate and monooleate, which differed from each other in the length or structure of the hydrocarbon chain. The oils used were isopropylpalmitate and myristate, and they differed from each other in the length of the fatty acid chain. Rheological properties, droplet size distributions and types (either o/w or w/o) of the creams were studied. The rheological tests used were oscillation stress sweep test, creep recovery test and viscosity test. The modelling of the creep phase was based on the creep recovery test. Sorbitan monolaurate and monostearate formed w/o creams, sorbitan monopalmitate and monooleate o/w creams. It appeared that the double-bonded structure of the surfactant made the cream less elastic. Elasticity was increased due to lenghtening of the alkyl chain of the surfactant and increased amount of surfactant. Also the lenghtening of the fatty acid chain of the oil made the creams more elastic. The results of the rheological tests and droplet size distributions correlated well each other. According to the modelling of the creep phase, creams could be represented either with the Burger model or with the Maxwell model.

Semisolid SLN™ dispersions for topical application: influence of formulation and production parameters on viscoelastic properties

Aqueous solid lipid nanoparticle (SLN) dispersions with a high lipid content up to 35% and viscous to semisolid consistency were produced by a high pressure homogenization process. Despite their high lipid content and viscosity these dispersions preserved their colloidal size range. The SLN dispersions were compared to nanoemulsions and microparticle dispersions with regard to particle size, viscoelastic properties and formation of a semisolid gel structure. Viscoelastic measurements including oscillation stress sweep tests and oscillation frequency sweep tests demonstrated that the existence of a solid particle matrix with a particle size in the nanometer range is a prerequisite to form a semisolid dispersion having the appropriate consistency for topical application. Striking differences were observed between solid lipid micro- and nanodispersions of the same composition. Particle size reduction resulted in an 80-fold increase of the elastic modulus. Particle size distribution, the physical state of the dispersed lipid phase and the emulsifier concentration have been identified as further key factors for the viscoelastic properties and gel structure of the lipid nanodispersions. By conducting oscillation measurements it was possible to relate the stability of lipid dispersions to specific rheological parameters therefore providing a sensitive tool in stability assessment. Changing the production process from a 40 ml batch to a 2 l batch turned out to have an influence on the colloidal structures of semisolid SLN dispersions. Consistency increased but particle size and ratio of elastic to viscous properties stayed in the same range.