Water-filled Cracks Research Articles

Seismic discontinuity in the Martian crust possibly caused by water-filled cracks

Recent seismic data acquired by the InSight lander have revealed seismic discontinuities in the Martian crust that have been interpreted as sharp transitions in porosity or chemical composition. Here we propose an alternative model in which the transition from dry cracks to water-filled cracks could explain the observed seismic discontinuity in the Martian crust. Our model can explain sharp increases in seismic velocity and Vp/Vs at ∼10 km depth with no associated changes in porosity or chemical composition. The present model suggests the local existence of liquid water in the Martian crust, which could potentially serve as a subsurface habitat for life.

INFLUENCE OF SURFACE CRACKS ON THE STABILITY OF CRACKED SOIL SLOPE

The slope stability is a major concern to geotechnical engineers. Traditional methods of slope stability analysis have potentially ignored the influence of surface cracks. It is also known that seasonal rainfall and seepage through crack are closely related with slope failure. First, surface cracks provide special flow channels which increase the soil permeability and decrease the soil strength. Second, water-filled cracks apply an additional active force on the slope. Finally, cracks can create a part of the critical failure surface that has no shear strength. The objective of this paper is to investigate the influence of existing cracks on the stability of a cracked soil slope in different state. The effects of crack depth, slope angle and water-filled cracks on the stability of the cracked slope are explored. The analysis was conducted using the computer modelling programs Optum G2 to analysis of slope factor of safety. The results show that with increasing of slope angle the factor of safety decreases and this problem is significant in the slope with water filled cracks. Also, Factor of safety for all of slope angles in Dry and water filled cracks states with increasing the crack depth, decrease significantly.

Evolution of the hydrogeological structure and disaster-generating mechanisms of landslides in loess slopes of the southern Jingyang Plateau, Shaanxi, China

Landslides in loess slopes are mainly triggered by heterogeneity in hydrogeology. The southern Jingyang Plateau (China) was selected to characterize the geological structure and groundwater synergetic mechanisms that trigger landslides through groundwater–soil interactions. The study investigated hydrogeological and irrigation factors, the groundwater-perching effect of paleosols, the groundwater-conducting effect of fissures, and the damming effect of slide mass accumulation. A two-dimensional steady unsaturated numerical seepage model of a vertical section was established to simulate landslide evolution induced by irrigation and fissures. Long-term irrigation causes springs and weak zones to develop when the water table rises above the slope foot. Perched groundwater can lead superfluous water to migrate to the slope edge. The downward expansion of fissures and water migration can mutually cause extension of weak zones beneath fissures. A slip zone may appear when the weak zone connects to weaknesses at the slope foot, which forms a larger foot along the slope. Landslides occur frequently under the combined action of hydrostatic pressure from water-filled cracks, dynamic water pressure from a rising water table, and depressed sliding resistance caused by water enrichment in soil. After sliding, the slide mass will accumulate at the slope foot, blocking the original groundwater drainage point, so the water level may rise again, leading to subsequent landslides developing with new fissures and continued irrigation. Investigations in areas irrigated by groundwater wells showed that pumping can lower the water table in the soil and eliminate weak zones at the slope foot, effectively reducing the occurrence of landslides.

Characterization of seasonal glacial seismicity from a single-station on-ice record at Holtedahlfonna, Svalbard

ABSTRACTGlacial seismicity provides important insights into glacier dynamic processes. We study the temporal distribution of cryogenic seismic signals (icequakes) at Holtedahlfonna, Svalbard, between April and August 2016 using a single three-component sensor. We investigate sources of observed icequakes using polarization analysis and waveform modeling. Processes responsible for five icequake categories are suggested, incorporating observations of previous studies into our interpretation. We infer that the most dominant icequake type is generated by surface crevasse opening through hydrofracturing. Secondly, bursts of high-frequency signals are presumably caused by repeated near-surface crevassing due to high strain rates during glacier fast-flow episodes. Furthermore, signals related to resonance in water-filled cracks, fracturing or settling events in dry firn or snow before the melt season, and processes at the glacier bed are observed. Amplitude of seismic background noise is clearly related to glacier runoff. We process ambient seismic noise to invert horizontal-to-vertical spectral ratios for a sub-surface seismic velocity model used to model icequake signals. Our study shows that a single seismic sensor provides useful information about seasonal ice dynamics in case deployment of a network is not feasible.

High-Resolution Velocity Analysis Method Using the ℓ-1 Norm Regularized Least-Squares Method for Pavement Inspection

We propose a high-resolution velocity analysis method to estimate the electromagnetic wave propagation velocity in subsurface medium. The estimation is achieved by applying the l-1 norm regularized least-squares method to the conventional common-midpoint (CMP) velocity analysis algorithm. The proposed method can provide not only higher resolution than the conventional velocity analysis method, but can also be applied with a coarse sampling array system, such as our array ground penetrating radar YAKUMO, which returns eight CMP traces within a two meter width. The main purpose of this approach is for precise pavement inspection at shallow depths. We applied this method to both a simulated dataset and real data acquired by YAKUMO at a model airport taxiway to detect the slight velocity changes caused by millimeter-thin cracks filled with air or water within the 15 cm-thick asphalt pavement. In both cases slight velocity changes of about 0.005 m/ns can be detected, and the difference between air- and water-filled cracks can be distinguished. Also, this method is applied to a data acquired at airport taxi-way, the damaged parts are detected successfully and shows good agreement with the corning results. The results indicate that the proposed method is effective for pavement inspection, especially in the presence of thin cracks that cannot be seen directly with the reflected signal.

Effects of tension crack on stability of bentonite-water slurry trenches in clay

Bentonite-water slurry trenches are utilised not only in underground excavations, but also in geo-environmental protections. Slurry trench can be used to prevent leaks and spills of the contaminant from landfills. This paper presented a procedure for evaluation of the impact of possible tension crack on stability of slurry trench in clay. The procedure is based on variational limiting equilibrium approach to slope stability. A straight extension was made to assess the safety of slurry trenches with cracks. Closed-form solutions were derived for the factor of safety and the potential crack depth for undrained total stress and drained effective stress conditions. Using normalisation presented the derived results in the form of stability chart. These charts provide a convenient tool for selection of the preliminary parameters for the slurry trench and determination of the potential crack depth. The results demonstrate the overestimation on the stability of slurry trenches when neglecting the tension crack, especially for undrained conditions. As the groundwater level increases or slurry level decreases, the depth of tension crack increases. To avoid the water-filled crack occurred, the groundwater level is suggested to be decreased by 10% of the trench depth below the ground surface.

Structure of oceanic crust and serpentinization at subduction trenches

The subducting oceanic lithosphere may carry a large amount of chemically bound water into the deep Earth interior, returning water to the mantle, facilitating melting, and hence keeping the mantle mobile and, in turn, nurturing plate tectonics. Bending-related faulting in the trench–outer rise region prior to subduction has been recognized to be an important process, promoting the return flux of water into the mantle. Extensional faults in the trench–outer rise are opening pathways into the lithosphere, supporting hydration of the lithosphere, including alteration of dry peridotite to water-rich serpentine. In this paper, we review and summarize recent work suggesting that bend faulting is indeed a key process in the global water cycle, albeit not yet well understood. Two features are found in a worldwide compilation of tomographic velocity models derived from wide-angle seismic data, indicating that oceanic lithosphere is strongly modified when approaching a deep-sea trench: (1) seismic velocities in both the lower crust and upper mantle are significantly reduced compared to the structure found in the vicinity of mid-ocean ridges and in mature crust away from subduction zones; and (2) profiles shot perpendicular to the trench show both crustal and upper mantle velocities decreasing systematically approaching the trench axis, highlighting an evolutionary process because velocity reduction is related to deformation, alteration, and hydration. P-wave velocity anomalies suggest that mantle serpentinization at trenches is a global feature of all subducting oceanic plates older than 10–15 Ma. Yet, the degree of serpentinization in the uppermost mantle is not firmly established, but may range from 1.9 in the trench, supporting the development of water-filled cracks where bend faulting occurs. In the mantle, the Vp/Vs ratio increases from ∼1.75 in the outer rise to values of >1.8 at the trench, indicating the increasing intensity of serpentinization.

Width estimation of a subsurface water-filled crack using internal multiple reflections from electromagnetic scattering model of a cylinder

A width estimation method for a subsurface water-filled crack using internal multiple reflections is proposed. The energy transport and path length of internal multiple reflections are analyzed from electromagnetic scattering model of a dielectric cylinder using the extended ray theory. The energy transport shows that some multiple rays transport along the surface of the cylinder to the receiver, which form the creeping waves, and some multiple rays refract toward the receiver, focusing on the radial direction. The conclusion that the path lengths are multiples of a diameter distance is utilized to estimate the width of the cylinder. In order to distinguish the adjacent reflections with small time interval, wavefronts and resonances of internal multiple reflections are also analyzed, based on the fact that the creeping waves and refracted waves at the radial direction show different resonances in the time-frequency domain. In order to verify the proposed method, experiments both in free space and subsurface scene are given to estimate the width of water-filled cracks.

HOW THE GEYSERS, TIDAL STRESSES, AND THERMAL EMISSION ACROSS THE SOUTH POLAR TERRAIN OF ENCELADUS ARE RELATED

We present the first comprehensive examination of the geysering, tidal stresses, and anomalous thermal emission across the south pole of Enceladus and discuss the implications for the moon's thermal history and interior structure. A 6.5 yr survey of the moon's south polar terrain (SPT) by the Cassini imaging experiment has located ~100 jets or geysers erupting from four prominent fractures crossing the region. Comparing these results with predictions of diurnally varying tidal stresses and with Cassini low resolution thermal maps shows that all three phenomena are spatially correlated. The coincidence of individual jets with very small (~10 m) hot spots detected in high resolution Cassini VIMS data strongly suggests that the heat accompanying the geysers is not produced by shearing in the upper brittle layer but rather is transported, in the form of latent heat, from a sub-ice-shell sea of liquid water, with vapor condensing on the near-surface walls of the fractures. Normal stresses modulate the geysering activity, as shown in the accompanying paper; we demonstrate here they are capable of opening water-filled cracks all the way down to the sea. If Enceladus' eccentricity and heat production are in steady state today, the currently erupting material and anomalous heat must have been produced in an earlier epoch. If regional tidal heating is occurring today, it may be responsible for some of the erupting water and heat. Future Cassini observations may settle the question.

Investigation by limit analysis on the stability of slopes with cracks

A full set of solutions for the stability of homogeneous c, φ slopes with cracks has been obtained by the kinematic method of limit analysis, providing rigorous upper bounds to the true collapse values for any value of engineering interest of φ, the inclination of the slope, and the depth and location of cracks. Previous stability analyses of slopes with cracks are based mainly on limit equilibrium methods, which are not rigorous, and are limited in their capacity for analysis, since they usually require the user to assume a crack depth and location in the slope. Conversely, numerical methods (e.g. finite-element method) struggle to deal with the presence of cracks in the slope, because of the discontinuities introduced in both the static and kinematic fields by the presence of cracks. In this paper, solutions are provided in a general form considering cases of both dry and water-filled cracks. Critical failure mechanisms are determined for cracks of known depth but unspecified location, cracks of known location but unknown depth, and cracks of unspecified location and depth. The upper bounds are achieved by assuming a rigid rotational mechanism (logarithmic spiral failure line). It is also shown that the values obtained provide a significant improvement on the currently available upper bounds based on planar failure mechanisms, providing a reduction in the stability factor of up to 85%. Charts of solutions are presented in dimensionless form for ease of use by practitioners.

Release Behavior of Tanshinone IIA Sustained-Release Pellets Based on Crack Formation Theory

The objective of this study was to investigate the drug release mechanism and in vivo performance of Tanshinone IIA sustained-release pellets, coated with blends of polyvinyl acetate (PVAc) and poly(vinyl alcohol)-poly(ethylene glycol) (PVA-PEG) graft copolymer. A formulation screening study showed that pellets coated with PVAc-PVA-PEG at a ratio of 70:30 (w/w) succeeded in achieving a 24 h sustained release, irrespective of the coating weight (from 2% to 10%). Both the microscopic observation and mathematical model gave further insight into the underlying release mechanism, indicating that diffusion through water-filled cracks was dominant for the control of drug release. In vivo test showed that the maximum plasma concentration of sustained-release pellets was decreased from 82.13 ± 17.05 to 40.50 ± 11.72 ng mL as that of quick-release pellets. The time of maximum concentration, half time, and mean residence time were all prolonged from 3.80 ± 0.40 to 8.02 ± 0.81 h, 4.28 ± 1.21 to 8.18 ± 2.06 h, and 8.60 ± 1.59 to 17.50 ± 2.78 h, compared with uncoated preparations. A good in vitro-in vivo correlation was characterized by a high coefficient of determination (r = 0.9772). In conclusion, pellets coated with PVAc-PVA-PEG could achieve a satisfactory sustained-release behavior based on crack formation theory.

Physical modelling analysis of the New Orleans levee breaches

As part of the Interagency Performance Evaluation Task Force (IPET) investigation into levee breaches in New Orleans following Hurricane Katrina, centrifuge modelling was undertaken of representative levee cross-sections on the 17th Street, Orleans and London Avenue Canals. Two mechanisms were observed leading to breaching of the levee in the models, both of which stemmed from a water-filled crack that formed in front of the flood wall. Depending on the foundation conditions and geometry of the levee and flood wall, the crack led either to a rotation of the flood wall landwards, with uplift and sliding on the top of the sand towards the landward toe of the levee, or to a translational (sliding) failure in the clay layer commencing from the bottom or toe of the flood wall. In the Orleans models no breach ensued, although it was clear these sections were close to failure. The centrifuge model tests identified, at an early stage in the IPET investigation, the importance of the ‘gap' mechanism affecting the stability of the flood walls, and confirmed that levee geometry and flood wall depth of penetration, together with the underlying soil profile, were critical to the performance of the system under flood loading.

Water Content Measurement in Expansive Soils Using the Neutron Probe

Abstract Capacitance-type methods for measuring soil water content are known to be unreliable in expansive soils, as cracking disrupts the intimate contact between the soil and the measuring device. The neutron probe, which infers water content from the thermalisation of a cloud of neutrons, is potentially less affected by cracking. The effect of cracking on neutron probe measurements was investigated by a series of numerical simulations using an axisymmetric finite element model based on seven-group neutron-diffusion theory. The simulations employed a consistent soil cracking model based on Maryland clay, in which crack volumes are determined from the changes in void ratio in the shrinking bulk soil. The results show that the presence of cracks in a clay soil affects the inferred water content and that measurements affected by air-filled cracking under-predict not only the water content in the uncracked soil peds but also the average water content in the larger cracked soil mass. The reason for this under-prediction is understood by considering the spatial distribution of the thermalised neutrons in the cracked and uncracked soils. The fast neutrons emitted from the source are seen to diffuse preferentially along air-filled cracks, traveling a large distance from the detector before they become thermalised, thus reducing their likelihood of being back-scattered to the detector where they can be counted. The proximity of the first crack to the probe in the ground also affects the measurement. Water-filled cracks are seen to have the opposite (but lesser) effect to air-filled cracks. A comparison of a simple uniform width crack model to a more realistic model in which crack width varies with changing water content shows that the model is sensitive to crack distribution and that the linear calibration expressions that are typically employed for neutron probes are likely to be unreliable in cracked clay soils.

Mathematical modelling of anisotropy of illite-rich shale

SUMMARY The estimation of illite-rich shale anisotropy to account for the alignment of clays and gas- or brine-filled cracks is presented via mathematical modelling. Such estimation requires analysis to interpret the dominance of one effect over another. This knowledge can help to evaluate the permeability in the unconventional reservoir, stress orientation, and the seal capacity for the conventional reservoir. Effective media modelling is used to predict the elastic properties of the illite-rich shale and to identify the dominant contributions to the shale anisotropy. We consider two principal reasons of the shale anisotropy: orientation of clay platelets and orientation of fluid-filled cracks. In reality, both of these two factors affect the shale anisotropy. The goal of this study is, first, to separately analyse the effect of these two factors to reveal the specific features in P- and S-wave velocity behaviour typical of each of the factors, and, then, consider a combined effect of the factors when the cracks are horizontally or vertically aligned. To do this, we construct four models of shale. The behaviour of P- and S-wave velocities is analysed when gas- and water-filled cracks embedded in a host matrix are randomly oriented, or horizontally or vertically aligned. The host matrix can be either isotropic or anisotropic (of VTI symmetry). In such a modelling, we use published data on mineralogy and clay platelet alignment along with other micromechanical measurements. In the model, where the host matrix is isotropic, the presence of a singularity point (when the difference changes its sign) in shear wave velocities is an indicator of brine-filled aligned cracks. In the model with the VTI host matrix and horizontally aligned cracks filled with gas, an increase in their volume concentration leads to that the azimuth at which the singularity is observed moves toward the symmetry axis. In this case, if the clay content is small (around 20 per cent), the singularity point may even vanish. The Thomsen parameters are helpful in fluid type indication in shale. An indicator of gas-filled aligned cracks is ɛ > γ. If aligned cracks in illite-rich shale are brine-filled, ɛ < γ. Negative value of δ indicates brine-filled cracks in illite-rich shale. A shale with brine-filled cracks exhibits higher Vp/Vs ratio in the vertical direction as compared to the gas-filled shale. A disorientation of clay platelets and brine-filled cracks may lead to that the singularity point is absent for brine-saturated shale as well. In this case one can also observe ɛ > γ and decreased values of Vp/Vs in the vertical direction as in the case of gas-filled cracks. In the presence of vertically aligned cracks, shales exhibit distinctly revealed features of orthorhombic symmetry. The results have important applications where seismic measurements are applied to predict the maturity state of the shale.

New Orleans Levee System Performance during Hurricane Katrina: 17th Street Canal and Orleans Canal North

Centrifuge modeling of the 17th Street Canal and Orleans Canal North levees was performed in this study. During hurricane Katrina the levees on the 17th Street Canal failed, leading to breaches in the outfall canal in the city. Two mechanisms were observed in the centrifuge modeling that could cause a breach. First, a water-filled crack formed in front of the floodwall as the water in the canal rose above the top of the levee. The levees on the 17th Street Canal, which were supported on clay foundations, failed when this cracking led to a translational (sliding) failure in the clay layer commencing at the toe of the floodwall. The levees at Orleans Canal North, where failure did not occur, were also modeled to demonstrate that the model tests could successfully simulate failure and nonfailure conditions. The centrifuge model tests identified the importance of the crack formation in relation to the stability of the floodwall. These tests also confirmed that levee geometry, floodwall depth of penetration, and the underlying soil profile were all critical to the performance of the system under flood loading.

Characteristics and source modeling of broadband seismic signals associated with the hydrothermal system at Satsuma–Iwojima volcano, Japan

The broadband seismic data observed at Satsuma–Iwojima volcanic island in southwestern Japan show a wide variety of interesting features. One of them is the periodic and frequency-dependent amplitude modulation of volcanic tremors. Another interesting feature is the occurrence of very long period (VLP) seismic pulses synchronized with the tremor amplitude modulation. Both the tremor amplitude modulation and the VLP pulses have fairly regular intervals of 46–50 min, suggesting a spatially fixed, non-destructive repetitive source. We performed waveform inversions to reveal the source mechanism of the VLP pulses, assuming a point source embedded in a medium that realistically represented the topography of the island. Numerical tests showed that the Green's functions used in the inversion were significantly affected by the topography. The observed VLP pulses were well explained by the rapid expansion of an inclined crack 100 m beneath the floor of the summit crater, which had a seismic moment of 3 × 10 10 Nm. Both the characteristic amplitude modulation and the VLP pulses are explained systematically by a model consisting of a water-filled crack and a network of gas paths. The water-filled crack was heated above the boiling temperature of the water in it. A sudden break of the unstable superheated state caused sudden vaporization and a pressure increase, which was observed as the VLP pulse. The network of gas paths consisted of swollen portions and constricted portions, which worked as tremor generators when vapor from the water-filled crack passed through.

Bugbuster—survivability of living bacteria upon shock compression

Shock recovery experiments were conducted on suspensions of 10 6/ml E. coli bacteria contained in a water-based medium that is emplaced within stainless steel containers. The water is shocked and recovered. These experiments simulate the environment of bacteria residing either in surface bodies of water or in subsurface water-filled cracks in rocks. Early Earth life is likely to have existed in such environments. However, the E. coli are not believed to be representative of early life and are merely used here for initial experiments. Some 10 − 2 to 10 − 4 of the bacteria population survived initial (800 ns duration) shock pressures in water of 220 and 260 MPa. TEM images of shock recovered bacteria indicate cell wall rupture and delamination. This appears to be the mortality mechanism. The TEM images indicate cell wall indentations may be occurring as would be consistent with Rayleigh–Taylor or Richtmyer–Meshkov fluid instabilities. In the present case, we consider the experiments as representing three layers of fluids: (1) The water-based medium, a stronger and possibly denser cell wall medium, and the interior of the cell cytoplasm. Variations of only 10–15% are expected in density. (2) A second mechanism that may cause cell wall failure is the multiple shock (nearly isentropic) compression freezing of liquid water medium into ice VI or ice VII high pressure phase that are 20% to 25% denser than the liquid. The decrease in volume associated with the transformation is expected to induce overpressures in the still liquid cell cytoplasm. Cell dynamic tensile wall strength thus appears to be a critical parameter from either of the above failure modes. Because the strain rate dependence of cell wall tensile strength is unstudied, we utilize the Grady and Lipkin [D.E. Grady, L. Lipkin, Criteria for impulsive rock fracture, Geophys. Res. Lett. 7 (1980) 255–258] model of tensile failure versus time scale (strain rate). Our single datum is fit to this law and we assume that at low strain rates, overpressures exceeding the cell Turgor pressure require on the order of ∼10 3 s. This model which has been applied to brittle media and metals for describing failure may permit application of short duration laboratory experiments as in the present ones to infer responses of organisms to much lower shock pressures, but for longer time scales (10 0 to 10 3 s) of planetary impacts. Using the present data for E. coli and applying the Grady and Lipkin model, we find that a 1.5 km diameter impactor will cause mortality of bacteria within a radius of 10 2 km but upon stress related attenuation the subsurface bacteria outside of this radius should survive.

Blends of aqueous polymer dispersions used for pellet coating: Importance of the particle size

Blends of aqueous dispersions of a water-insoluble and an enteric polymer, namely ethyl cellulose:hydroxypropyl methylcellulose acetate succinate (EC:HPMCAS) and ethyl cellulose:methacrylic acid ethyl acrylate copolymer (EC:Eudragit® L), were used as coating materials to control theophylline release from matrix pellets. Varying the polymer blend ratio, broad ranges of drug release patterns were obtained at low as well as at high pH. Interestingly, the resulting release profiles were rather similar for both types of blends in 0.1 M HCl, whereas significant differences were observed in phosphate buffer pH 7.4. Surprisingly, drug release at high pH was much slower for EC:HPMCAS blends compared to EC:Eudragit® L blends, although HPMCAS leached out more rapidly (and to a higher extent) from the film coatings than Eudragit® L. To explain these phenomena and to better understand the underlying drug release mechanisms, thin polymeric films of identical composition as the pellet coatings were prepared and physicochemically characterized before and upon exposure to the release media. Importantly, the polymer particle size was identified to be a very crucial formulation parameter, determining the resulting film coating structure and properties. The Eudragit® L particles are much smaller than the HPMCAS particles (nano- vs. micrometer size range) and, thus, more effectively hinder the formation of a continuous and mechanically stable EC network. Consequently, the EC structures remaining after enteric polymer leaching at high pH are mechanically much weaker in the case of Eudragit® L. Upon exposure to phosphate buffer, water-filled cracks are formed, through which the drug rapidly diffuses out. In contrast, the EC structures remaining upon HPMCAS leaching are mechanically stronger and drug release is controlled by diffusion through the polymeric remnants.

Conditions for the reversal of ice/air surface slope on ice streams and shelves: a model study

AbstractReversals in the ice/air surface slope are important in geomorphic and glaciological contexts, thus motivating consideration of the conditions under which they form. Surface slope reversals are seen in numerous places, such as ice rumples on ice shelves, as surficial lakes, and at the down-glacier end of Vostok lake, Antarctica. Such slope reversals can reduce or reverse the subglacial hydrological gradient, thereby rerouting subglacial water transport and possibly leading to the creation of subglacial lakes. Supraglacial lakes produced by slope reversals in ablation zones may aid in driving water-filled cracks that allow surface water access to the bed. Surface slope reversals, in the absence of a concomitant reversal in ice flow, indicate a local violation of the so-called ‘shallow-ice’ approximation, and in this circumstance the longitudinal deviatoric stress becomes critical in the stress equilibrium. Using a simple numerical model, we have explored the conditions under which surface slope reversals form for certain simple scenarios. The results indicate that ice which initially possesses a normal slope will tend toward a reversed slope if the ice is thinned, the bed is strengthened or the downstream buttressing is increased.

Polymer blends used for the aqueous coating of solid dosage forms: importance of the type of plasticizer

The aim of this study was to investigate the importance of the type of plasticizer in polymer blends used for the coating of solid dosage forms, comparing a lipophilic and a hydrophilic plasticizer (dibutyl sebacate (DBS) and triethyl citrate (TEC)). In vitro drug release from propranolol hydrochloride (propranolol HCl)-loaded pellets coated with blends of ethyl cellulose (EC) and Eudragit® L (100:0, 75:25, 50:50, 25:75 and 0:100 w/w) was investigated at low as well as at high pH. To better understand the underlying mass transport mechanisms, the physicochemical properties of the film coatings (e.g. mechanical resistance, water uptake and dry weight loss behavior) were determined. Interestingly, drug release strongly depended on the type of plasticizer. Importantly, not only the slope but also the shape of the release curves was affected, indicating that the chemical nature of the plasticizer plays a major role for the underlying drug release mechanisms. Diffusion through the intact polymer coatings and/or through water-filled cracks was found to be dominating for the control of drug release. The relative importance of these pathways strongly depended on the polymer blend ratio and type of plasticizer. In contrast to DBS, TEC rapidly leached out of the coatings, resulting in decreasing mechanical resistances of the films and, thus, facilitated crack formation. In addition, the hydrophilicity of the plasticizer significantly affected the water uptake behavior of the film coatings and, hence, changes in the coatings' toughness and drug permeability. Also the relative affinity of the plasticizer to the different polymers was found to be of significance. In contrast to TEC, DBS has a higher affinity to EC than to Eudragit® L, resulting in potential redistributions of this plasticizer within the polymeric systems and changes in the release profiles during storage. Importantly, these effects could be avoided with appropriate curing conditions and preparation techniques for the coating dispersions.