Visualizing flow dynamics and restart of Carbopol gel solutions in tube and parallel-plates geometries with wall slip

The present study examines the impact of slip in Carbopol solutions during the restart flow in pipelines utilizing in situ visualization techniques. Rheological tests were conducted using smooth and hatched parallel plate geometries to obtain the rheological characteristics of the solutions. The behavior of the solutions in the creep tests is compared with those in the experimental unit. Three flow regimes were identified through rheological and experimental setup tests: non-flow, slip, and yielded. The slip regime allowed the establishment of a slip static yield stress value, indicating significant deformation states, and a restart pressure decrease of about 61% when compared to the static yield stress. The flow dynamics under the wall slip effect is captured by velocity profiles, velocity contour maps and velocity gradient. Transient correlations of the scaling law type were determined, with wall slip in proportion to the velocity gradient and wall shear stress. Additionally, the concentration of viscoplastic material in the solution increased the scaling law index. This research seeks to provide valuable findings by quantifying the effects of apparent wall slip through in situ measurements. Such insights are crucial for designing and managing pipeline transport systems that handle yield stress fluids, applicable across various industries including cosmetics, food processing, and the production of waxy oils.

An Insight Into Risk Assessment and Reformulation of Drug Products Manufactured Using Benzene Grade Carbomer: A Regulatory Perspective

Cancer has been an enormous pain point for patients and regulatory bodies across the globe. In Dec. 2023, the US FDA released guidance on benzene-grade carbomer formulations, which triggered pharmaceutical manufacturers to assess risk, test finished products, and reformulate drug products with benzene-grade carbomer. The immediate implementation of the stoppage of finished products with benzene-grade carbomers has threatened pharmaceutical excipients and finished product manufacturers. The gravity of this situation prompted the US Pharmacopeia to extend the deadline for discontinuation from August 1, 2025, to August 1, 2026, allowing manufacturers ample time for reformulation and regulatory compliance.There is an immediate need to understand the guidance and to learn how manufacturers should do the risk assessment and approach reformulation. This review provides an in-depth analysis of the risk assessment and reformulation processes involved in various dosage forms utilizing benzene-grade carbomer, supported by specific case studies.This review offers insights into navigating the USFDA guidelines to ensure formulation safety and compliance, thus enabling pharmaceutical practitioners to uphold the highest standards of patient care and tackle life cycle management challenges.The decision of the USFDA to restrict the usage of high benzene content of carbomer in the formulation is a welcome move. This article has shown a way for researchers to see opportunities in the path and provide best-in-class medicines to patients with a better formulation safety profile.

Low yield stress measurements with a microfluidic rheometer

Yield stress, τy, is a key rheological property of complex materials such as gels, dense suspensions, and dense emulsions. While there is a range of established techniques to measure τy in the order of tens to thousands of pascals, the measurement of low τy, specifically below 1 Pa, remains underexplored. In this article, we present the measurement of low apparent τy using a Hele-Shaw microfluidic extensional flow device (MEFD). Using the MEFD, we observe a gradient in shear stress, τ, such that τ is lower near the center or stagnation point, and higher away from the stagnation point. For a yield stress fluid, we observe that, below a certain flow rate, τ exceeds τy only in the outer region, leading to stagnation or unyielding of the fluid in the inner region. We use scaling analysis based on a Hele-Shaw linear extensional flow to deduce τy by measuring the size of the unyielded region, S. We validate this scaling relationship using Carbopol solutions with concentrations ranging between 0.015 to 0.3%, measuring τy as low as ∼10 mPa to ∼1 Pa, and comparing it with τy measured using a standard rheometer. While the experimental lower limit of our technique is 5 mPa, modifying the geometry or improving the image analysis can reduce this limit to the order of 10-4 Pa. The MEFD facilitates rapid measurement of τy, allowing for its real-time assessment. We further report τy of human blood samples between 30 to 80 mPa with their hematocrit ranging between 14 to 63%. Additionally, we determine τy for a mucus simulant (∼0.7 Pa), and lactic drink (∼7 mPa) to demonstrate the versatility of the MEFD technique.

Moldable Tissue-Sealant Hydrogels Composed of In Situ Cross-Linkable Polyethylene Glycol via Thiol-Michael Addition and Carbomers

Moldable tissue-sealant hydrogels were developed herein by combining the yield stress fluidity of a Carbomer and in situ cross-linking of 3-arm PEG-thiol (PEG-SH) and 4-arm PEG-acrylate (PEG-AC). The Carbomer was mixed with each PEG oligomer to form two aqueous precursors: Carbomer/PEG-SH and Carbomer/PEG-AC. The two hydrogel precursors exhibited sufficient yield stress (>100 Pa) to prevent dripping from their placement on the tissue surface. Moreover, these hydrogel precursors exhibited rapid restructuring when the shear strain was repeatedly changed. These rheological properties contribute to the moldability of these hydrogel precursors. After mixing these two precursors, they were converted from yield-stress fluids to chemically cross-linked hydrogels, Carbomer/PEG hydrogel, via thiol-Michael addition. The gelation time was 5.0 and 11.2 min at 37 and 25 °C, respectively. In addition, the Carbomer/PEG hydrogels exhibited higher cellular viability than the pure Carbomer. They also showed stable adhesiveness and burst pressure resistance to various tissues, such as the skin, stomach, colon, and cecum of pigs. The hydrogels showed excellent tissue sealing in a cecum ligation and puncture model in mice and improved the survival rate due to their tissue adhesiveness and biocompatibility. The Carbomer/PEG hydrogel is a potential biocompatible tissue sealant that surgeons can mold. It was revealed that the combination of in situ cross-linkable PEG oligomers and yield stress fluid such as Carbomer is effective for developing the moldable tissue sealant without dripping of its hydrogel precursors.

Carbomer Hydrogels with Microencapsulated α-Tocopherol: Focus on the Biocompatibility of the Microcapsules, Topical Application Attributes, and In Vitro Release Study

The microencapsulation of α-tocopherol based on the complex coacervation of low-molecular-weight chitosan (LMWC) and sodium lauryl ether sulphate (SLES) without harmful crosslinkers can provide biocompatible carriers that protect it from photodegradation and air oxidation. In this study, the influence of the microcapsule wall composition on carrier performance, compatibility with a high-water-content vehicle for topical application, and release of α-tocopherol were investigated. Although the absence of aldehyde crosslinkers decreased the encapsulation efficiency of α-tocopherol (~70%), the variation in the LMWC/SLES mass ratio (2:1 or 1:1) had no significant effect on the moisture content and microcapsule size. The prepared microcapsule-loaded carbomer hydrogels were soft semisolids with pseudoplastic flow behavior. The integrity of microcapsules embedded in the hydrogel was confirmed by light microscopy. The microcapsules reduced the pH, apparent viscosity, and hysteresis area of the hydrogels, while increasing their spreading ability on a flat inert surface and dispersion rate in artificial sweat. The in vitro release of α-tocopherol from crosslinker-free microcapsule-loaded hydrogels was diffusion-controlled. The release profile was influenced by the LMWC/SLES mass ratio, apparent viscosity, type of synthetic membrane, and acceptor medium composition. Better data quality for the model-independent analysis was achieved when a cellulose nitrate membrane and ethyl alcohol 60% w/w as acceptor medium were used.

Rheology of Highly Filled Polymer Compositions-Limits of Filling, Structure, and Transport Phenomena

The current state of the rheology of various polymeric and other materials containing a high concentration of spherical solid filler is considered. The physics of the critical points on the concentration scale are discussed in detail. These points determine the features of the rheological behavior of the highly filled materials corresponding to transitions from a liquid to a yielding medium, elastic-plastic state, and finally to an elastic solid-like state of suspensions. Theoretical and experimental data are summarized, showing the limits of the most dense packing of solid particles, which is of key importance for applications and obtaining high-quality products. The results of model and fine structural studies of physical phenomena that occur when approaching the point of filling the volume, including the occurrence of instabilities, are considered. The occurrence of heterogeneity in the form of individual clusters is also described. These heterogeneous objects begin to move as a whole that leads to the appearance of discontinuities in the suspension volume or wall slip. Understanding these phenomena is a key for particle technology and multiphase processing.

Acanthus mollis Formulations for Transdermal Delivery: From Hydrogels to Emulsions

Topical formulations of Acanthus mollis L. leaf and the optimization of the release of their active compounds and their topical bioavailability were investigated for the first time. In vitro, the release of active compounds from three formulations-an oil-in-water cream and two hydrogels (Carbopol 940 and Pluronic F-127)-was determined using Franz diffusion cells. Detection and quantification of the compounds was performed via high-performance liquid chromatography with a photodiode array (HPLC-PDA). DIBOA, a bioactive compound of this medicinal plant, exhibited release kinetics of the Weibull model for the Carbopol and Pluronic F-127 formulation, identifying it as a potential active agent to optimize the topical distribution of the formulations. The implications extend to applications in inflammation treatment and tyrosinase inhibition, suggesting that it can make a significant contribution to addressing skin conditions, including melanoma and various inflammatory diseases.

Engineering Nano/Microscale Chiral Self-Assembly in 3D Printed Constructs

Helical hierarchy found in biomolecules like cellulose, chitin, and collagen underpins the remarkable mechanical strength and vibrant colors observed in living organisms. This study advances the integration of helical/chiral assembly and 3D printing technology, providing precise spatial control over chiral nano/microstructures of rod-shaped colloidal nanoparticles in intricate geometries. We designed reactive chiral inks based on cellulose nanocrystal (CNC) suspensions and acrylamide monomers, enabling the chiral assembly at nano/microscale, beyond the resolution seen in printed materials. We employed a range of complementary techniques including Orthogonal Superposition rheometry and in situ rheo-optic measurements under steady shear rate conditions. These techniques help us to understand the nature of the nonlinear flow behavior of the chiral inks, and directly probe the flow-induced microstructural dynamics and phase transitions at constant shear rates, as well as their post-flow relaxation. Furthermore, we analyzed the photo-curing process to identify key parameters affecting gelation kinetics and structural integrity of the printed object within the supporting bath. These insights into the interplay between the chiral inks self-assembly dynamics, 3D printing flow kinematics and photo-polymerization kinetics provide a roadmap to direct the out-of-equilibrium arrangement of CNC particles in the 3D printed filaments, ranging from uniform nematic to 3D concentric chiral structures with controlled pitch length, as well as random orientation of chiral domains. Our biomimetic approach can pave the way for the creation of materials with superior mechanical properties or programable photonic responses that arise from 3D nano/microstructure and can be translated into larger scale 3D printed designs.

Preparation and comparison of two medical dressings made from the collagens from fish and bovine

Collagen is used in medical dressings because of its high hydrophilicity, low immunogenicity, excellent biocompatibility, and degradability. These features can promote cell proliferation and platelet agglomeration. Herein, we studied the preparation of gel dressing by using silver carp skin collagen and bovine collagen as raw materials. Their properties and the application effects of collagen gel dressing were evaluated and compared. The centrifugal stability, rheology, and water-loss rate of silver carp skin collagen gel (SCG) and bovine tendon collagen gel (CTG) were determined. Results showed that the two gels were stable, and SCG had better rheology and ductility than CTG. However, the denaturation temperature and water-retention rate of SCG were slightly lower than those of CTG. Two collagen gels were used in the burn-repair experiment of KM mice. Results showed that the SCG and CTG were consistent with the wound-repair effect of commercially available products for shallow II-degree scald and deep II-degree scald. In the superficial shallow II scald experiment, SCG had a faster healing rate in the first 8 days and a shorter recovery time than CTG. In the deep II-degree scald experiment, the wound-healing rate of SCG on the 14th day reached 94.24%, which was 2 days faster than the recovery time of CTG. Moreover, the skin after wound healing was shallower than the scar produced after CTG treatment. Therefore, SCG had the potential to be used as the medical dressing.

High-Fidelity Extrusion Bioprinting of Low-Printability Polymers Using Carbopol as a Rheology Modifier

Extrusion three-dimensional (3D) bioprinting is a promising technology with many applications in the biomedical and tissue engineering fields. One of the key limitations for the widespread use of this technology is the narrow window of printability that results from the need to have bioinks with rheological properties that allow the extrusion of continuous filaments while maintaining high cell viability within the materials during and after printing. In this work, we use Carbopol (CBP) as rheology modifier for extrusion printing of biomaterials that are typically nonextrudable or present low printability. We show that low concentrations of CBP can introduce the desired rheological properties for a wide range of formulations, allowing the use of polymers with different cross-linking mechanisms and the introduction of additives and cells. To explore the opportunities and limitations of CBP as a rheology modifier, we used ink formulations based on poly(ethylene glycol)diacrylate with extrusion 3D printing to produce soft, yet stable, hydrogels with tunable mechanical properties. Cell-laden constructs made with such inks presented high viability for cells seeded on top of cross-linked materials and cells incorporated within the bioink during printing, showing that the materials are noncytotoxic and the printed structures do not degrade for up to 14 days. To our knowledge, this is the first report of the use of CBP-containing bioinks to 3D-print complex cell-laden structures that are stable for days and present high cell viability. The use of CBP to obtain highly printable inks can accelerate the evolution of extrusion 3D bioprinting by guaranteeing the required rheological properties and expanding the number of materials that can be successfully printed. This will allow researchers to develop and optimize new bioinks focusing on the biochemical, cellular, and mechanical requirements of the targeted applications rather than the rheology needed to achieve good printability.

EHO-85, Novel Amorphous Antioxidant Hydrogel, Containing Olea europaea Leaf Extract-Rheological Properties, and Superiority over a Standard Hydrogel in Accelerating Early Wound Healing: A Randomized Controlled Trial

Many advanced wound healing dressings exist, but there is little high-quality evidence to support them. To determine the performance of a novel amorphous hydrogel (EHO-85) in relation to its application, we compared its rheological properties with those of other standard hydrogels (SH), and we assessed the induction of acceleration of the early stages of wound healing as a secondary objective of a prospective, multicenter, randomized, observer-blinded, controlled trial. The patients were recruited if they had pressure, venous, or diabetic foot ulcers and were treated with EHO-85 (n = 103) or VariHesive^® (SH) (n = 92), and their response was assessed by intention-to-treat as wound area reduction (WAR (%)) and healing rate (HR mm²/day) in the second and fourth weeks of treatment. Results: EHO-85 had the highest shear thinning and G'/G″ ratio, the lowest viscous modulus, G″, and relatively low cohesive energy; EHO-85 had a significantly superior effect over SH in WAR and HR, accelerating wound healing in the second and fourth weeks of application (p: 0.002). This superiority is likely based on its optimal moisturizing capacity and excellent pH-lowering and antioxidant properties. In addition, the distinct shear thinning of EHO-85 facilitates spreading by gentle hand pressure, making it easier to apply to wounds. These rheological properties contribute to its improved performance.

Cavitation Rheology of Model Yield Stress Fluids Based on Carbopol

Measuring the surface tension of yield stress fluids has remained a critical challenge due to limitations of the traditional tensiometry techniques. Here, we overcome those limits and successfully measure the surface tension and mechanical properties of a model yield stress fluid based on Carbopol gels via a needle-induced cavitation (NIC) technique. Our results indicate that the surface tension is approximately 70 ± 3 mN/m, and is independent of the rheology of yield stress fluid over a wide range of yield stress values σ_y = 0.5-120 Pa. In addition, we demonstrate that a Young modulus smaller than E < 1 kPa can be successfully measured for Carbopol gels with NIC method. Finally, we present a time-resolved flow structure around the cavity in a host of yield stress fluids, and assess the impact of fluid rheology on the detailed form of flow around the cavity. Interestingly, prior to the critical point associated with cavitation, the yield stress fluid is weakly deformed suggesting that the measured surface tension data reflect the near equilibrium values. Beyond the critical point, the yield stress fluid experiences a strong flow that is controlled by both the critical pressure and the non-Newtonian rheology of the yield stress fluid.

3D Printing-Assisted Self-Assembly to Bio-Inspired Bouligand Nanostructures

Architected materials with nano/microscale orders can provide superior mechanical properties; however, reproducing such levels of ordering in complex structures has remained challenging. Inspired by Bouligand structures in nature, here, 3D printing of complex geometries with guided long-order radially twisted chiral hierarchy, using cellulose nanocrystals (CNC)-based inks is presented. Detailed rheological measurements, in situ flow analysis, polarized optical microscopy (POM), and director field analysis are employed to evaluate the chiral assembly over the printing process. It is demonstrated that shear flow forces inside the 3D printer's nozzle orient individual CNC particles forming a pseudo-nematic phase that relaxes to uniformly aligned concentric chiral nematic structures after the flow cessation. Acrylamide, a photo-curable monomer, is incorporated to arrest the concentric chiral arrangements within the printed filaments. The time series POM snapshots show that adding the photo-curable monomer at the optimized concentrations does not interfere with chiral self-assemblies and instead increases the chiral relaxation rate. Due to the liquid-like nature of the as-printed inks, optimized Carbopol microgels are used to support printed filaments before photo-polymerization. By paving the path towards developing bio-inspired materials with nanoscale hierarchies in larger-scale printed constructs, this biomimetic approach expands 3D printing materials beyond what has been realized so far.

2.5D printing of a yield-stress fluid

We report on direct ink writing of a model yield-stress fluid and focus on the printability of the first layer, the one in contact with the supporting substrate. We observe a diversity of deposition morphologies that depends on a limited set of operational parameters, mainly ink flow rate, substrate speed and writing density, and also on material properties (e.g., yield-stress). Among these morphologies, one of them does not depend on fluid properties (as long as the fluid displays some yield-stress) and consists of flat films whose thickness is controllable in a significant range, about $$0.1{-}1$$ 0.1 - 1  mm, and tunable in real time during printing. We thus demonstrate the ability to print films with thickness gradients and prove that the printing fidelity is mainly due to a competition between yield-stress and capillarity.

Effect of Hydrogel Substrate Components on the Stability of Tetracycline Hydrochloride and Swelling Activity against Model Skin Sebum

Due to its high instability and rapid degradation under adverse conditions, tetracycline hydrochloride (TC) can cause difficulties in the development of an effective but stable formulation for the topical treatment of acne. The aim of the following work was to propose a hydrogel formulation that would ensure the stability of the antibiotic contained in it. Additionally, an important property of the prepared formulations was the activity of the alcoholamines contained in them against the components of the model sebum. This feature may help effectively cleanse the hair follicles in the accumulated sebum layer. A series of formulations with varying proportions of anionic polymer and alcoholamine and containing different polymers have been developed. The stability of tetracycline hydrochloride contained in the hydrogels was evaluated for 28 days by HPLC analysis. Formulations containing a large excess of TRIS alcoholamine led to the rapid degradation of TC from an initial concentration of about 10 µg/mL to about 1 µg/mL after 28 days. At the same time, these formulations showed the highest activity against artificial sebum components. Thanks to appropriately selected proportions of the components, it was possible to develop a formulation that assured the stability of tetracycline for ca. one month, while maintaining formulation activity against the components of model sebum.

Design and Optimization of a Natural Medicine from Copaifera reticulata Ducke for Skin Wound Care

In this study, we developed a bioadhesive emulsion-filled gel containing a high amount of Copaifera reticulata Ducke oil-resin as a veterinary or human clinical proposal. The phytotherapeutic system had easy preparation, low cost, satisfactory healing ability, and fly repellency, making it a cost-effective clinical strategy for wound care and myiasis prevention. Mechanical, rheological, morphological, and physical stability assessments were performed. The results highlight the crosslinked nature of the gelling agent, with three-dimensional channel networks stabilizing the Copaifera reticulata Ducke oil-resin (CrD-Ore). The emulgel presented antimicrobial activity, satisfactory adhesion, hardness, cohesiveness, and viscosity profiles, ensuring the easy spreading of the formulation. Considering dermatological application, the oscillatory responses showed a viscoelastic performance that ensures emulgel retention at the action site, reducing the dosage frequencies. In Vivo evaluations were performed using a case report to treat ulcerative skin wounds aggravated by myiasis in calves and heifers, which demonstrated healing, anti-inflammatory, and repellent performance for the emulsion-filled gel. The emulgel preparation, which is low in cost, shows promise as a drug for wound therapy.

Producing shape-engineered alginate particles using viscoplastic fluids

Non-spherical hydrogel particles are of fundamental interest and can find use in a variety of applications ranging from pharmaceuticals to biomedical to food. Here, we report a new method that leverages the yield stress property of viscoplastic fluids to synthesize shape-engineered alginate particles. By dripping an aqueous viscoplastic solution composed of sodium alginate and a yield-stress material into an ionic gelation bath, droplets are controllably deformed and crosslinked, producing a wide assortment of shapes. We find that by tuning the yield stress of the solution and the nozzle tip orientation, a range of shapes from symmetric and near-spherical, to asymmetric and anisotropic (e.g., egg-, rice grain-, arc-, ring-, snail shell-, tear-, and tadpole-like) can be produced. We explain our observations using scaling analysis of the forces exerted on the droplet at different stages of particle production. We show that the main factors that determine the degree of droplet deformation during bath entry and the final appearance of the alginate particles are the initial shape of the droplets, the timescales of the viscoplastic fluid relaxation versus the crosslinking reaction, and the physico-chemical properties of the yield-stress material.

Evidence for Chaotic Behavior during the Yielding of a Soft Particle Glass

Materials comprised of deformable particles such as microgels and concentrated emulsions and foams display complex rheological behavior that includes a yielding transition from an elastic solid to viscous fluid. Most studies of this class of soft matter involve shear flows, and only a handful report both shear and normal stresses. We present measurements of the shear stress and two normal stress differences for a microgel subjected to constant shear rate flows. The shear stress evolves through the yield point in a manner indicative of simple yield stress fluid behavior. Prior to yielding, the normal stress differences are immeasurable; beyond the yield point, they evolve in a reproducibly chaotic manner.

Accessing new and scalable high molecular weight branched copolymer structures using transfer-dominated branching radical telomerisation (TBRT)

Three new synthesis strategies for branched statistical copolymers containing analogues of step-growth backbones are shown using free radical chemistries and transfer-dominated branching radical polymerisation (TBRT) conditions.