Microgels as viscosity modifiers influence lubrication performance of continuum

Bulk and Interfacial Behavior of Potato Protein-Based Microgels

This study aims to understand the bulk and interfacial performance of potato protein microgels. Potato protein (PoP) was used to produce microgels of submicrometer diameter via a top-down approach of thermal cross-linking followed by high-shear homogenization of the bulk gel. Bulk "parent" gels were formed at protein concentrations [PoP] = 5-18 wt %, which subsequently varied in their bulk shear elastic modulus (G') by several orders of magnitude (1-100 kPa), G' increasing with increasing [PoP]. The PoP microgels (PoPM) formed from these parent gels had diameters varying between 100 and 300 nm (size increasing with increasing G' and [PoP]), as observed via dynamic light scattering and atomic force microscopy (AFM) of PoPM adsorbed onto silicon. Interfacial rheology (interfacial shear storage and loss moduli, Gi' and Gi″) and interfacial tension (γ) of adsorbed films of PoP (i.e., nonheated PoP) and PoPM (both at tetradecane-water interfaces) were also studied, as well as the bulk rheology of the PoPM dispersions. The results showed that PoPM dispersions (at 50 vol %) had significantly higher bulk viscosity and shear thinning properties compared to the nonmicrogelled PoP at the same overall [PoP], but the bulk rheological behavior was in sharp contrast to the interfacial rheological performance, where Gi' and Gi″ of PoP were higher than for any of the PoPM. This suggests that the deformability and size of the microgels were key in determining the interfacial rheology of the PoPM. These findings may be attributed to the limited capacity for "unfolding" and lateral interactions of the larger PoPM at the interface, which are presumed to be stiffer due to their production from the strongest PoP gels. Our study further confirmed that heating and cooling the adsorbed films of PoPM after their adsorption showed little change, highlighting that hydrogen bonding was limited between the microgel particles.

Precise Lubrication and Protection of Cartilage Damage by Targeting Hydrogel Microsphere

Osteoarthritis (OA) is a degenerative bone and joint disease characterized by decreased cartilage lubrication, leading to continuous wear and ultimately irreversible damage. This situation is particularly challenging for early-stage OA, as current bio-lubricants lack precise targeting for small inflammatory lesions. In this work, an antibody-mediated targeting hydrogel microspheres (HMS) is developed to precisely lubricate the local injury site of cartilage and prevent the progression of early OA. Anti-Collagen type I (Anti-Col1) is an antibody that targets cartilage injury sites in early OA stages. It is anchored on a HMS matrix made of Gelatin methacrylate (GelMA) and poly (sulfobetaine methacrylate) (PSBMA) to create targeted HMS (T-G/S HMS). The T-G/S HMS's high hydrophilicity, along with the dynamic interaction between its surficial Anti-Col1 and the Col1 on cartilage injury site, ensures its precise and effective lubrication of early OA lesions. Consequently, injecting T-G/S HMS into rats with early OA significantly slows disease progression and reduces symptoms. In conclusion, the developed injectable targeted lubricating HMS and the precisely targeted lubrication strategy represent a promising, convenient technique for treating OA, particularly for slowing the early-stage OA progression.

Demulsification of Pickering emulsions: advances in understanding mechanisms to applications

Pickering emulsions are ultra-stable dispersions of two immiscible fluids stabilized by solid or microgel particles rather than molecular surfactants. Although their ultra-stability is a signature performance indicator, often such high stability hinders their demulsification, i.e., prevents the droplet coalescence that is needed for phase separation on demand, or release of the active ingredients encapsulated within droplets and/or to recover the particles themselves, which may be catalysts, for example. This review aims to provide theoretical and experimental insights on demulsification of Pickering emulsions, in particular identifying the mechanisms of particle dislodgment from the interface in biological and non-biological applications. Even though the adhesion of particles to the interface can appear irreversible, it is possible to detach particles via (1) alteration of particle wettability, and/or (2) particle dissolution, affecting the particle radius by introducing a range of physical conditions: pH, temperature, heat, shear, or magnetic fields; or via treatment with chemical/biochemical additives, including surfactants, enzymes, salts, or bacteria. Many of these changes ultimately influence the interfacial rheology of the particle-laden interface, which is sometimes underestimated. There is increasing momentum to create responsive Pickering particles such that they offer switchable wettability (demulsification and re-emulsification) when these conditions are changed. Demulsification via wettability alteration seems like the modus operandi whilst particle dissolution remains only partially explored, largely dominated by food digestion-related studies where Pickering particles are digested using gastrointestinal enzymes. Overall, this review aims to stimulate new thinking about the control of demulsification of Pickering emulsions for release of active ingredients associated with these ultra-stable emulsions.

Effect of in vitro food oral coating and lubricity on satiety: A randomized controlled trial using milk protein beverages

We investigated the effects of complex textural attributes of food i.e. lubricity and oral coating, on appetite ratings, food intake, salivary and gut peptides for the first time. Milk protein-rich beverages (whey and casein) were instrumentally analyzed (tribology, viscosity and adsorption, latter representing oral coating) using in vitro measurements. Then these protein beverage preloads differing in their coating properties (low coating, medium coating and high coating) were assessed in two cross-over satiety trials (Study 1, n=37; Study 2, n=15; Total n= 52). Fullness ratings increased in the high coating beverage condition (p < .05) only after 20 min with limited effects on other time points, suggesting a sporadic effect of oral coating on appetite ratings (n=37). There was a correlation between concentration of protein in saliva and appetite ratings; the higher the concentration of protein in saliva the lower the desire to eat (r = - 0.963; p < 0.05) and prospective food consumption ratings (r =- 0.980; p < 0.05). Human saliva was more lubricating after ingesting preload with high coating properties, thus explaining the results on appetite ratings. There was no effect of oral coating on energy intake and gut peptides (n=15), suggesting that complex textural attributes having influence on oral processing might not have any effect on the later parts of the satiety cascade. Oral coating/ lubricity appears to have a subtle and sporadic effect on appetite suppression, which needs further investigation with changing macronutrients/energy load and degree of coating/ lubricity.

Size-Controllable and pH-Sensitive Whey Protein Microgels as High-Performance Aqueous Biolubricants

Developing efficient aqueous biolubricants has become a significant focus of research due to their prevalence in biotribological contacts and enormous potential in soft matter applications. In this study, size-controllable, pH-sensitive whey protein microgels were prepared using a water-in-water emulsion template method from protein-polysaccharide phase separation. The granular hydrogel from the protein microgels exhibited superior lubricity, obtaining 2.7-fold and 1.7-fold reductions in coefficient of friction (μ) compared to native protein and human saliva (μ = 0.30 compared to 0.81 and 0.52, respectively). The microgels also exhibited outstanding load-bearing capabilities, sustaining lubrication under normal forces up to 5 N. Microgels with a smaller size (1 μm) demonstrated better lubricating performance than 6 and 20 μm microgels. The exceptional lubricity was from a synergistic effect of the ball-bearing mechanism and the hydration state of the microgels. Particularly at pH 7.4, the hydration layer surrounding highly negative charges contributed to the electrostatic repulsion among the swollen microgels, leading to an improved buffer ability to separate contact surfaces and effective rolling behavior. Such pH-dependent repulsion was evidenced using a surface forces apparatus that the adhesion between the whey protein-coated surfaces and protein-mica surfaces decreased from 4.49 to 0.97 mN/m and from 7.89 to 0.36 mN/m, respectively, with pH increasing from the isoelectronic point to 7.4. Our findings fundamentally elucidated the tribo-rheological properties and lubrication mechanisms of the whey protein microgels with excellent biocompatibility and environmental responsiveness, offering novel insights for their food and biomedical applications requiring aqueous biolubrication.

Selective Segregation of Thermo-Responsive Microgels via Microfluidic Technology

Separation of equally sized particles distinguished solely by material properties remains still a very challenging task. Here a simple separation of differently charged, thermo-responsive polymeric particles (for example microgels) but equal in size, via the combination of pressure-driven microfluidic flow and precise temperature control is proposed. The separation principle relies on forcing thermo-responsive microgels to undergo the volume phase transition during heating and therefore changing its size and correspondingly the change in drift along a pressure driven shear flow. Different thermo-responsive particle types such as different grades of ionizable groups inside the polymer matrix have different temperature regions of volume phase transition temperature (VPTT). This enables selective control of collapsed versus swollen microgels, and accordingly, this physical principle provides a simple method for fractioning a binary mixture with at least one thermo-responsive particle, which is achieved by elution times in the sense of particle chromatography. The concepts are visualized in experimental studies, with an intend to improve the purification strategy of the broad distribution of charged microgels into fractioning to more narrow distribution microgels distinguished solely by slight differences in net charge.

Effects of dispersing media on the rheological and tribological properties of basil seed mucilage-based thickened liquids

The development of thickening powders for the management of dysphagia is imperative due to the rapid growth of aging population and prevalence of the dysphagia. One promising thickening agent that can be used to formulate dysphagia diets is basil seed mucilage (BSM). This work investigates the effects of dispersing media, including water, milk, skim milk, and apple juice, on the rheological and tribological properties of the BSM-thickened liquids. Shear rheology results revealed that the thickening ability of BSM in these media in ascending order is milk < skim milk ≈ apple juice < water. On the other hand, extensional rheology demonstrated that the longest filament breakup time was observed when BSM was dissolved in milk, followed by skim milk, water, and apple juice. Furthermore, tribological measurements showed varying lubrication behavior, depending on the BSM concentration and dispersing media. Dissolution of BSM in apple juice resulted in the most superior lubrication property compared with that in other dispersing media. Overall, this study provides insights on BSM's application as a novel gum-based thickening powder in a range of beverages and emphasizes how important it is for consumers to have clear guidance for the use of BSM in dysphagia management.

Double-crosslinked self-healing hydrogel alleviates osteoarthritis by protecting from wearing and targeting NF-kB signaling

Osteoarthritis (OA) is a chronic disease that causes pain, morbidity, and disability. The main strategy for OA treatment focuses on inflammation suppression, inhibition of osteoclastogenesis, and protection of articular cartilage. These functions cannot be performed effectively by monotherapy. Therefore, an effective drug delivery system is required, capable of containing and controlling the efflux of various drugs to alleviate osteoclastogenesis, protect cartilage and subchondral bone, and suppress inflammation. In this work, an encapsulation system is constructed using a self-healing chitosan hydrogel and allocated compound drugs. The self-healing gel is composed of branched-functionalized chitosan, created by simultaneously using polycaprolactone polyethylene glycol azide as a block polymer and the host-guest assembly of β-cyclodextrin and adamantane. Inhibitors of the NFkB pathway are loaded into the cavities of β-cyclodextrin and the spring-like structure of the block polymer, which can be rapidly released upon joint friction (due to the reassembly of β-cyclodextrin and adamantane by shear stress and the stretch of the block polymer). In vitro experiments using BMMs and the ATDC5 cell line confirm that the developed hydrogel can simultaneously suppress osteoclastogenesis and induce chondrogenesis. Additionally, a model of knee arthritis in C57 mice was used to confirm that this double-crosslinked encapsulation system can lubricate the knee joint surface and provide adequate protection on demand through shear-responsive drug release.

Oral tribology of dairy protein-rich emulsions and emulsion-filled gels affected by colloidal processing and composition

Designing nutritious food for the elderly population often requires significant quantities of leucine-rich whey proteins to combat malnutrition, yet high-protein formulations can cause mouth dryness and increased oral friction. This study investigated how various colloidal processing methods and compositions impact the in vitro oral tribological properties of protein-rich emulsions and emulsion-filled gels. Oil-in-water emulsions with oil fractions from 1 wt% to 20 wt% were prepared, alongside emulsion-filled gels containing whey protein isolate (WPI), hydrolysed whey protein (HWP), or a blend of both (10 wt% protein content). Two processing approaches were employed: creating emulsions with an initial 10 w% protein content (M1) and initially forming emulsions with 0.1 wt% protein content, then enriching to a final 10 wt% concentration (M2). The hypothesis was that formulations with HWP or method 2 (M2) would offer lubrication benefits by inducing droplet coalescence, aiding in the formation of a lubricating boundary tribofilm. Surprisingly, the tribological behavior of high-protein emulsions showed minimal dependence on oil droplet volume fraction. However, both HWP-based emulsions and those processed with M2 for WPI exhibited significant friction reduction, which may be attributed to the presence of coalesced oil droplets, supporting our hypothesis. Substituting 50 wt% of WPI with HWP in emulsion-filled gel boli resulted in very low friction coefficients in the boundary lubrication regime, suggesting oil droplet release from the gel matrix. These findings provide insights into designing high-protein foods with improved mouthfeel for the elderly population, necessitating further validation through sensory studies.

Influence of Architecture on the Interfacial Properties of Polymers: Linear Chains, Stars, and Microgels

Surface-active polymers have important applications as effective and responsive emulsifiers, foaming agents, and coatings. In this contribution, we explore the impact of the polymer architecture on the behavior at oil-water interfaces by comparing different poly(N-isopropylacrylamide) (pNIPAM)-based systems, namely, monolayers of linear and star-shaped macromolecules, ultralow cross-linked, regular cross-linked, and hollow microgels. Compression isotherms were determined experimentally as well as by computer simulations. The latter provides information about the conformational changes of the individual macromolecules as well as the interfacial properties of the monolayer, including the surface structure and the density distribution of an ensemble of interacting macromolecules near an interface. Surprisingly, the isotherms of the linear polymer, of the star polymer, and of the ultralow cross-linked microgel have an identical shape that differs from the isotherms of regular and hollow microgels. We introduced the mass fraction of adsorbed polymer, which gives a measure of the polymer segments contributing to the isotherm in relation to the most flexible architecture, i.e., the linear polymer, and allows a comparison of polymers with different architectures. The data demonstrate that increasing the number of cross-links leads to a significantly lower amount of polymer in the proximity of the interface as the increase in cross-linker reduces the deformability or softness of the polymers at the interface. The volume fraction profiles along the normal to the interface are essentially different in the microgel monolayers as compared to those in the linear and star polymer. The profiles through the microgel contact line and their growth upon initial compression are similar to those of the linear chains. Herewith, the profiles through the center of mass practically do not change upon compression. Therefore, the initial growth in the microgel surface pressure reveals the polymer-like behavior and is related to the deformation of the peripheral part of the microgel. Further compression of the microgel monolayer leads to 3D interactions of the microgels within the aqueous side of the interface and soft colloid-like behavior.

Synthetic and biopolymeric microgels: Review of similarities and difference in behaviour in bulk phases and at interfaces

This review discusses the current knowledge of interfacial and bulk interactions of biopolymeric microgels in relation to the well-established properties of synthetic microgels for applications as viscosity modifiers and Pickering stabilisers. We present a timeline showing the key milestones in designing microgels and their bulk/ interfacial performance. Poly(N-isopropylacrylamide) (pNIPAM) microgels have remained as the protagonist in the synthetic microgel domain whilst proteins or polysaccharides have been primarily used to fabricate biopolymeric microgels. Bulk properties of microgel dispersions are dominated by the volume fraction (ϕ) of the microgel particles, but ϕ is difficult to pinpoint, as addressed by many theoretical models. By evaluating recent experimental studies over the last five years, we find an increasing focus on the analysis of microgel elasticity as a key parameter in modulating their packing at the interfaces, within the provinces of both synthetic and biopolymeric systems. Production methods and physiochemical factors shown to influence microgel swelling in the aqueous phase can have a significant impact on their bulk as well as interfacial performance. Compared to synthetic microgels, biopolymer microgels show a greater tendency for polydispersity and aggregation and do not appear to have a core-corona structure. Comprehensive studies of biopolymeric microgels are still lacking, for example, to accurately determine their inter- and intra- particle interactions, whilst a wider variety of techniques need to be applied in order to allow comparisons to real systems of practical usage.

Transforming sustainable plant proteins into high performance lubricating microgels

With the resource-intensive meat industry accounting for over 50% of food-linked emissions, plant protein consumption is an inevitable need of the hour. Despite its significance, the key barrier to adoption of plant proteins is their astringent off-sensation, typically associated with high friction and consequently poor lubrication performance. Herein, we demonstrate that by transforming plant proteins into physically cross-linked microgels, it is possible to improve their lubricity remarkably, dependent on their volume fractions, as evidenced by combining tribology using biomimetic tongue-like surface with atomic force microscopy, dynamic light scattering, rheology and adsorption measurements. Experimental findings which are fully supported by numerical modelling reveal that these non-lipidic microgels not only decrease boundary friction by an order of magnitude as compared to native protein but also replicate the lubrication performance of a 20:80 oil/water emulsion. These plant protein microgels offer a much-needed platform to design the next-generation of healthy, palatable and sustainable foods.

Progress of Microfluidic Hydrogel-Based Scaffolds and Organ-on-Chips for the Cartilage Tissue Engineering

Cartilage degeneration is among the fundamental reasons behind disability and pain across the globe. Numerous approaches have been employed to treat cartilage diseases. Nevertheless, none have shown acceptable outcomes in the long run. In this regard, the convergence of tissue engineering and microfabrication principles can allow developing more advanced microfluidic technologies, thus offering attractive alternatives to current treatments and traditional constructs used in tissue engineering applications. Herein, the current developments involving microfluidic hydrogel-based scaffolds, promising structures for cartilage regeneration, ranging from hydrogels with microfluidic channels to hydrogels prepared by the microfluidic devices, that enable therapeutic delivery of cells, drugs, and growth factors, as well as cartilage-related organ-on-chips are reviewed. Thereafter, cartilage anatomy and types of damages, and present treatment options are briefly overviewed. Various hydrogels are introduced, and the advantages of microfluidic hydrogel-based scaffolds over traditional hydrogels are thoroughly discussed. Furthermore, available technologies for fabricating microfluidic hydrogel-based scaffolds and microfluidic chips are presented. The preclinical and clinical applications of microfluidic hydrogel-based scaffolds in cartilage regeneration and the development of cartilage-related microfluidic chips over time are further explained. The current developments, recent key challenges, and attractive prospects that should be considered so as to develop microfluidic systems in cartilage repair are highlighted.

Current Advances of Polysaccharide-Based Nanogels and Microgels in Food and Biomedical Sciences

Polysaccharides are natural polymers with hydrophilic, biocompatible and biodegradable characteristics and have many opportunities in the food and pharmaceutical sectors. This review focuses on the field of nano and microstructures whose internal structure is based on networked polysaccharide chains in 3D i.e., polysaccharide nanogels (NGs) and microgels (MGs). As it is observed the number of articles on NGs and MGs in peer reviewed scientific journals has been increasing over the last two decades. At the same time, the relative contribution of polysaccharides in this field is gaining place. This review focuses on the different applied methods for the fabrication of a variety of polysaccharide-based NGs and MGs and aims to highlight the recent advances on the subject and present their potentials and properties with regards to their integration in aspects of medicinal and food sciences. The presentation of the recent advances in the application of polysaccharide NGs and MGs is divided in materials with potential as emulsion stabilizers and materials with potential as carriers of bioactives. For applications in the medical sector the division is based on the fabrication processes and includes self-assembled, electrostatically complexed/ionically crosslinked and chemically crosslinked NGs and MGs. It is concluded that many advances are expected in the application of these polysaccharide-based materials and in particular as nutrient-loaded emulsion stabilizers, viscosity modifiers and co-assembled structures in combination with proteins.

Injectable hydrogel microspheres with self-renewable hydration layers alleviate osteoarthritis

Introducing hydration layers to hydrogel microspheres (HMs) by coating the surface with liposomes can effectively reduce friction. However, the lubrication can be inactivated when the surface coatings are damaged. To endow HMs with the ability to form self-renewable hydration layers and maintain cellular homeostasis, rapamycin-liposome-incorporating hyaluronic acid-based HMs (RAPA@Lipo@HMs) were created using microfluidic technology and photopolymerization processes. The RAPA@Lipo@HMs improve joint lubrication by using a smooth rolling mechanism and continuously exposing liposomes on the outer surface to form self-renewable hydration layers via frictional wear. In addition, the released autophagy activator (rapamycin)-loaded cationic liposomes can target negatively charged cartilage through electrostatic interactions and maintain cellular homeostasis by increasing autophagy. Furthermore, the in vivo data showed that the RAPA@Lipo@HMs can alleviate joint wear and delay the progression of osteoarthritis. The RAPA@Lipo@HMs can provide efficient lubrication and potentially alleviate friction-related diseases such as osteoarthritis.

Emerging microfluidics-enabled platforms for osteoarthritis management: from benchtop to bedside

Osteoarthritis (OA) is a prevalent debilitating age-related joint degenerative disease. It is a leading cause of pain and functional disability in older adults. Unfortunately, there is no cure for OA once the damage is established. Therefore, it promotes an urgent need for early detection and intervention of OA. Theranostics, combining therapy and diagnosis, emerges as a promising approach for OA management. However, OA theranostics is still in its infancy. Three fundamental needs have to be firstly fulfilled: i) a reliable OA model for disease pathogenesis investigation and drug screening, ii) an effective and precise diagnostic platform, and iii) an advanced fabrication approach for drug delivery and therapy. Meanwhile, microfluidics emerges as a versatile technology to address each of the needs and eventually boost the development of OA theranostics. Therefore, this review focuses on the applications of microfluidics, from benchtop to bedside, for OA modelling and drug screening, early diagnosis, and clinical therapy. We first introduce the basic pathophysiology of OA and point out the major unfilled research gaps in current OA management including lack of disease modelling and drug screening platforms, early diagnostic modalities and disease-modifying drugs and delivery approaches. Accordingly, we then summarize the state-of-the-art microfluidics technology for OA management from in vitro modelling and diagnosis to therapy. Given the existing promising results, we further discuss the future development of microfluidic platforms towards clinical translation at the crossroad of engineering and biomedicine.

On the Determination of Mechanical Properties of Aqueous Microgels-Towards High-Throughput Characterization

Aqueous microgels are distinct entities of soft matter with mechanical signatures that can be different from their macroscopic counterparts due to confinement effects in the preparation, inherently made to consist of more than one domain (Janus particles) or further processing by coating and change in the extent of crosslinking of the core. Motivated by the importance of the mechanical properties of such microgels from a fundamental point, but also related to numerous applications, we provide a perspective on the experimental strategies currently available and emerging tools being explored. Albeit all techniques in principle exploit enforcing stress and observing strain, the realization differs from directly, as, e.g., by atomic force microscope, to less evident in a fluid field combined with imaging by a high-speed camera in high-throughput strategies. Moreover, the accompanying analysis strategies also reflect such differences, and the level of detail that would be preferred for a comprehensive understanding of the microgel mechanical properties are not always implemented. Overall, the perspective is that current technologies have the capacity to provide detailed, nanoscopic mechanical characterization of microgels over an extended size range, to the high-throughput approaches providing distributions over the mechanical signatures, a feature not readily accessible by atomic force microscopy and micropipette aspiration.

Phytoglycogen Nanoparticles: Nature-Derived Superlubricants

Phytoglycogen nanoparticles (PhG NPs), a single-molecule highly branched polysaccharide, exhibit excellent water retention, due to the abundance of close-packed hydroxyl groups forming hydrogen bonds with water. Here we report lubrication properties of close-packed adsorbed monolayers of PhG NPs acting as boundary lubricants. Using direct surface force measurements, we show that the hydrated nature of the NP layer results in its striking lubrication performance, with two distinct confinement-controlled friction coefficients. In the weak- to moderate-confinement regime, when the NP layer is compressed down to 8% of its original thickness under a normal pressure of up to 2.4 MPa, the NPs lubricate the surface with a friction coefficient of 10^-3. In the strong-confinement regime, with 6.5% of the original layer thickness under a normal pressure of up to 8.1 MPa, the friction coefficient was 10^-2. Analysis of the water content and energy dissipation in the confined NP film reveals that the lubrication is governed by synergistic contributions of unbound and bound water molecules, with the former contributing to lubrication properties in the weak- to moderate-confinement regime and the latter being responsible for the lubrication in the strong-confinement regime. These results unravel mechanistic insights that are essential for the design of lubricating systems based on strongly hydrated NPs.

Probing gels and emulsions using large-amplitude oscillatory shear and frictional studies with soft substrate skin surrogates

Water swellable crosslinked polymers are widely used in oil-in-water emulsions for the healthcare and cosmetic industries due to their thickening properties. In this study, we investigate the rheological and lubrication behavior of a microgel-forming polymer, a lightly-crosslinked hydrophobically modified polyacrylic acid (HMPAA), in an aqueous medium and in an emulsion. Hydrogenated phosphatidylcholine, a class of phospholipids, is used as a surfactant in the emulsions composed of different oil content. Rheological behavior is probed both in the linear and non-linear regimes using small strain amplitude and large amplitude oscillatory shear (LAOS) experiments, respectively. We observe all systems to exhibit gel-like behavior with the elastic modulus (G') dominating and being frequency independent. Lissajous-Bowditch plots and nonlinear parameters obtained under large deformation show that the emulsions can resist greater deformations with smaller increase in the viscous dissipation when compared to a HMPAA gel. For tribology experiments, friction curves in a range of entrainment speeds are examined using substrates to mimic the skin surface (PDMS and Bioskin®). The role of polymer hydrophobicity on the different substrates are also explored by comparing the behavior of HMPAA to that of its hydrophilic analog, a polyacrylic acid highly crosslinked. We find the friction coefficient to be dependent on the hydrophobicity of the substrate and the polymer as well as the substrate roughness. These results taken together provide insights in the formulation of skincare products with efficient lubrication properties for different skin characteristics.

Linking polymer hydrophobicity and molecular interactions to rheology and tribology in phospholipid-containing complex gels

HYPOTHESIS

The rheological behavior and frictional properties (macroscopic level) of systems containing a hydrophobically modified polymer and phospholipids depend on the hydrophobic association that occur between the hydrophobic moiety of the polymer and the phospholipid tails (molecular level). The hydrophobicity of the polymer can thus be used to control its interactions with phospholipids, and manipulate complex gel macroscopic behavior.

EXPERIMENTS

By using systems composed of a crosslinked hydrophobically modified polyacrylic acid (HMPAA) or a crosslinked polyacrylic acid polymer (PAA) and phospholipids, we examine the underlying mechanisms through which the components interact using isothermal titration calorimetry (ITC) and their effect on rheological and tribological characteristics of complex gels.

FINDINGS

We find the systems containing HMPAA and phospholipid exhibit gel-like behavior with the elastic modulus increasing substantially upon phospholipid addition due to hydrophobic interactions that result in a more interconnected network formation, as evidenced by ITC measurements. Similar experiments with a crosslinked polyacrylic acid polymer (PAA) show no interactions, lending credence to our hypothesis. In addition, soft tribological behavior shows lower friction coefficients at low entrainment speeds with HMPAA concentration and the addition of phospholipid, while no change in friction coefficient was observed in the case of increasing PAA concentration, indicating HMPAA and phospholipids to be interacting with the soft PDMS contacts.

Surface adsorption and lubrication properties of plant and dairy proteins: A comparative study

The aim of this work was to compare the surface adsorption and lubrication properties of plant and dairy proteins. Whey protein isolate (WPI) and pea protein isolate (PPI) were chosen as model animal and plant proteins, respectively, and various protein concentrations (0.1-100 mg/mL) were studied with/without heat treatment (90 °C/60 min). Quartz crystal microbalance with dissipation monitoring (QCM-D) experiments were performed on hydrophilic (gold) and hydrophobic polydimethylsiloxane (PDMS) sensors, with or without a mucin coating, latter was used to mimic the oral surface. Soft tribology using PDMS tribopairs in addition to wettability measurements, physicochemical characterization (size, charge, solubility) and gel electrophoresis were performed. Soluble fractions of PPI adsorbed to significantly larger extent on PDMS surfaces, forming more viscous films as compared to WPI regardless of heat treatment. Introducing a mucin coating on a PDMS surface led to a decrease in binding of the subsequent dietary protein layers, with PPI still adsorbing to a larger extent than WPI. Such large hydrated mass of PPI resulted in superior lubrication performance at lower protein concentration (≤10 mg/mL) as compared to WPI. However, at 100 mg/mL, WPI was a better lubricant than PPI, with the former showing the onset of elastohydrodynamic lubrication. Enhanced lubricity upon heat treatment was attributed to the increase in apparent viscosity. Fundamental insights from this study reveal that pea protein at higher concentrations demonstrates inferior lubricity than whey protein and could result in unpleasant mouthfeel, and thus may inform future replacement strategies when designing sustainable food products.

Rheology and tribology of starch + κ-carrageenan mixtures

In this study, we investigated the rheological and tribological properties of biopolymer mixtures of gelatinized corn starches (0.5 - 10.0 wt%) and κ-carrageenan (κC) (0.05 - 1.0 wt%). Two different starch samples were used. The first starch (CS1), despite extensive heating and shearing contained "ghost" granules, while the second starch (CS2) had no visible ghost granules after the same gelatinization process as CS1. Apparent viscosity measurements demonstrated that κC + CS1 mixtures were shear thinning liquids, with viscosity values being lower than the corresponding weight average of the values of the individual equilibrium phases at shear rates < 50 s-1 . Tribological results revealed that κC ≥ 0.5 wt% was required to observe any decrease in friction coefficients in the mixed lubrication regime. Starch (CS1) showed an unusual behavior at ≥ 5 wt%, where the friction coefficient decreased not only in the mixed regime but also in the boundary regime, probably due to the presence of the "ghost" granules, as the latter became entrained in the contact region. The CS1 + κC mixtures showed significantly lower friction coefficients than that of pure CS1 and κC in the mixed regime. However, the CS2 + κC mixture (i.e., containing no ghost granules) showed similar behavior to pure κC in the mixed regime, while lower friction coefficients than that of the pure CS2 and κC in the boundary regime. These findings illustrate new opportunities for designing biopolymer mixtures with tunable lubrication performance, via optimizing the concentrations of the individual biopolymers and the gelatinization state of the starch.

Synergistic Interactions of Plant Protein Microgels and Cellulose Nanocrystals at the Interface and Their Inhibition of the Gastric Digestion of Pickering Emulsions

It is possible that Pickering emulsions can optimize the transport of nutraceuticals, pharmaceuticals, and other bioactive compounds in human physiology. So-called ultrastable Pickering emulsions are often destabilized in the gastric digestion regime if the particles are proteinaceous in nature. The present study seeks to test how the interfacial structure can be engineered via synergistic particle-particle interactions to impact the gastric coalescence of Pickering emulsions. In this study, we designed plant-based protein-particle-stabilized oil-in-water emulsions (PPM-E, with 20 wt % sunflower oil) via pea protein microgels (PPM at 1 wt %). The PPM hydrodynamic diameter is ∼250 nm. In vitro gastric digestion of PPM-E confirmed droplet coalescence within 30 min of pepsin addition. Supposedly surface-active cellulose nanocrystals (CNCs, 1-3 wt %) were added to PPM-E at pH 3.0 to determine if they could act as a barrier to interfacial pepsinolysis due to the CNC and PPM being oppositely charged at this gastric pH value. A combination of confocal microscopy, zeta potential, and Langmuir trough measurements suggested that CNCs and PPMs might form a combined layer at the O/W interface, owing to the electrostatic attraction between them. CNCs at >2 wt % inhibited the pepsinolyis of the adsorbed PPM film and thus droplet coalescence. However, increasing concentrations of CNC also increased the bulk viscosity of the PPM-E and eventually caused gelation of the emulsions, which would also delay their gastric breakdown. In conclusion, tuning the bulk and interfacial structure of Pickering emulsions via synergistic interactions between two types of particles could be an effective strategy to modify the enzymatic breakdown of such emulsions, which would have important applications in pharmaceuticals, foods, and other soft-matter applications.

Review on fat replacement using protein-based microparticulated powders or microgels: A textural perspective

Background

Due to the growing rise in obesity and food-linked diseases, the replacement of calorie-dense fat has been a key focus of food industries in the last few decades with proteins being identified as promising fat replacers (FRs).

Scope and approach

This review aims to provide an overview of animal and plant protein-based FR studies that have been performed in the last 5 years. Protein isolates/concentrates, their microparticulated forms and protein microgels in model and real foods have been examined. Special emphasis has been given on the characterisation techniques that have been used to compare the full fat (FF) and low fat (LF) versions of the foods using FRs.

Key findings and conclusions

Microparticulated whey protein (MWP) has been the preferred choice FR with some success in replacing fat in model foods and dairy applications. Plant proteins on the other hand have attracted limited research attention as FRs, but show success similar to that of animal proteins. Key characterisation techniques used to compare full fat with low fat products containing FRs have been apparent viscosity, texture profile analysis, microscopy, particle size and sensory properties with oral tribology being a relatively recent undertaking. Coupling tribology with adsorption techniques (muco-adhesion) can be effective to bridge the instrumental-sensory property gap and might accelerate the development cycle of designing low/no fat products. From a formulation viewpoint, sub-micron sized microgels that show shear-thinning behaviour and have boundary lubrication properties offer promises with respect to exploiting their fat replacement potential in the future.

Interactions between a responsive microgel monolayer and a rigid colloid: from soft to hard interfaces

Responsive poly-N-isopropylacrylamide-based microgels are commonly used as model colloids with soft repulsive interactions. It has been shown that the microgel-microgel interaction in solution can be easily adjusted by varying the environmental parameters, e.g., temperature, pH, or salt concentration. Furthermore, microgels readily adsorb to liquid-gas and liquid-liquid interfaces forming responsive foams and emulsions that can be broken on-demand. In this work, we explore the interactions between microgel monolayers at the air-water interface and a hard colloid in the water. Force-distance curves between the monolayer and a silica particle were measured with the Monolayer Particle Interaction Apparatus. The measurements were conducted at different temperatures and lateral compressions, i.e., different surface pressures. The force-distance approach curves display long-range repulsive forces below the volume phase transition temperature of the microgels. Temperature and lateral compression reduce the stiffness of the monolayer. The adhesion increases with temperature and decreases with a lateral compression of the monolayer. When compressed laterally, the interactions between the microgels are hardly affected by temperature, as the directly adsorbed microgel fractions are nearly insensitive to temperature. In contrast, our findings show that the temperature-dependent swelling of the microgel fractions in the aqueous phase strongly influences the interaction with the probe. This is explained by a change in the microgel monolayer from a soft to a hard repulsive interface.

Synergistic Microgel-Reinforced Hydrogels as High-Performance Lubricants

The ability to create a superlubricious aqueous lubricant is important for various biological and technological applications. Here, a nonlipid biolubricant with strikingly low friction coefficients is fabricated (patented) by reinforcing a fluid-like hydrogel composed of biopolymeric nanofibrils with proteinaceous microgels, which synergistically provide superlubricity on elastomeric surfaces in comparison to any of the sole components. This two-component lubricant composed of positively charged lactoferrin microgels and negatively charged κ-carrageenan hydrogels is capable of exceeding the high lubricating performance of real human saliva in tribo tests using both smooth and textured surfaces, latter mimicking the human tongue's wettability, topography, and compliance. The favorable electrostatic attraction between mutually oppositely charged microgels and the hydrogel reinforces the mechanical properties of the hydrogel, allowing friction reduction by combining the benefits of both viscous and hydration lubrication. The superlubricity of these microgel-reinforced hydrogels offers a unique prospect for the fabrication of biocompatible aqueous lubricants for dry-mouth therapy and/or designing of nonobesogenic nutritional technologies.

3D Biomimetic Tongue-Emulating Surfaces for Tribological Applications

Oral friction on the tongue surface plays a pivotal role in mechanics of food transport, speech, sensing, and hedonic responses. The highly specialized biophysical features of the human tongue such as micropapillae-dense topology, optimum wettability, and deformability present architectural challenges in designing artificial tongue surfaces, and the absence of such a biomimetic surface impedes the fundamental understanding of tongue-food/fluid interaction. Herein, we fabricate for the first time, a 3D soft biomimetic surface that replicates the topography and wettability of a real human tongue. The 3D-printed fabrication contains a Poisson point process-based (random) papillae distribution and is employed to micromold soft silicone surfaces with wettability modifications. We demonstrate the unprecedented capability of these surfaces to replicate the theoretically defined and simulated collision probability of papillae and to closely resemble the tribological performances of human tongue masks. These de novo biomimetic surfaces pave the way for accurate quantification of mechanical interactions in the soft oral mucosa.

Ball-Bearing-Inspired Polyampholyte-Modified Microspheres as Bio-Lubricants Attenuate Osteoarthritis

Osteoarthritis, a lubrication dysfunction related disorder in joint, is characterized by articular cartilage degradation and joint capsule inflammation. Enhancing joint lubrication, combined with anti-inflammatory therapy, is considered as an effective strategy for osteoarthritis treatment. Herein, based on the ball-bearing-inspired superlubricity and the mussel-inspired adhesion, a superlubricated microsphere, i.e., poly (dopamine methacrylamide-to-sulfobetaine methacrylate)-grafted microfluidic gelatin methacrylate sphere (MGS@DMA-SBMA), is developed by fabricating a monodisperse, size-uniform microsphere using the microfluidic technology, and then a spontaneously modified microsphere with DMA-SBMA copolymer by a one-step biomimetic grafting approach. The microspheres are endowed with enhanced lubrication due to the tenacious hydration layer formed around the charged headgroups (-N⁺ (CH₃ )₂ - and -SO₃^- ) of the grafted poly sulfobetaine methacrylate (pSBMA), and simultaneously are capable of efficient drug loading and release capability due to their porous structure. Importantly, the grafting of pSBMA enables the microspheres with preferable properties (i.e., enhanced lubrication, reduced degradation, and sustained drug release) that are highly desirable for intraarticular treatment of osteoarthritis. In addition, when loaded with diclofenac sodium, the superlubricated microspheres with excellent biocompatibility can inhibit the tumor necrosis factor α (TNF-α)-induced chondrocyte degradation in vitro, and further exert a therapeutic effect toward osteoarthritis in vivo.

Pathways and challenges towards a complete characterization of microgels

Due to their controlled size, sensitivity to external stimuli, and ease-of-use, microgel colloids are unique building blocks for soft materials made by crosslinking polymers on the micrometer scale. Despite the plethora of work published, many questions about their internal structure, interactions, and phase behavior are still open. The reasons for this lack of understanding are the challenges arising from the small size of the microgel particles, complex pairwise interactions, and their solvent permeability. Here we describe pathways toward a complete understanding of microgel colloids based on recent experimental advances in nanoscale characterization, such as super-resolution microscopy, scattering methods, and modeling.

Protein Microgel-Stabilized Pickering Liquid Crystal Emulsions Undergo Analyte-Triggered Configurational Transition

Herein, we report a novel approach that involves Pickering stabilization of micometer-sized liquid crystal (LC) droplets with biocompatible soft materials such as a whey protein microgel (WPM) to facilitate the analysis of analyte-induced configurational transition of the LC droplets. The WPM particles were able to irreversibly adsorb at the LC-water interface, and the resulting WPM-stabilized LC droplets possessed a remarkable stability against coalescence over time. Although the LC droplets were successfully protected by a continuous network of the WPM layer, the LC-water interface was still accessible for small molecules such as sodium dodecyl sulfate (SDS) that could diffuse through the meshes of the adsorbed WPM network or through the interfacial pores and induce an LC response. This approach was exploited to investigate the dynamic range of the WPM-stabilized LC droplet response to SDS. Nevertheless, the presence of the unadsorbed WPM in the aqueous medium reduced the access of SDS molecules to the LC droplets, thus suppressing the configuration transition. An improved LC response to SDS with a lower detection limit was achieved after washing off the unadsorbed WPM. Interestingly, the LC exhibited a detection limit as low as ∼0.85 mM for SDS within the initial WPM concentration ranging from 0.005 to 0.1 wt %. Furthermore, we demonstrate that the dose-response behavior was strongly influenced by the number of droplets exposed to the aqueous analytes and the type of surfactants such as anionic SDS, cationic dodecyltrimethylammonium bromide (DTAB), and nonionic tetra(ethylene glycol)monododecyl ether (C₁₂E₄). Thus, our results address key issues associated with the quantification of aqueous analytes and provide a promising colloidal platform toward the development of new classes of biocompatible LC droplet-based optical sensors.

Slip and momentum transfer mechanisms mediated by Janus rods at polymer interfaces

As an incipient but preeminent technology for multiphase nanomaterials/fluids, exact compatibilizing mechanisms of Janus particles in polymer blends and the consequent morphology remain unknown. The contributions of Janus nanorods to slip suppression and momentum transfer across the interface have been explored through dissipative particle dynamics simulations under shear flow at unentangled polymer-polymer interfaces. Rods have been then grafted with flexible polymer chains to unveil interfacial structure-property relationships at a molecular level when compared with flexible diblock copolymer surfactants. When Janus rods are sparsely grafted with necessarily longer grafts, they favor a greater degree of graft interpenetration with polymer phases. This yields less effective momentum transfer that impacts droplet coalescence processes; dynamic heterogeneities at complex interfaces; and helps map their efficiency as compatibilizers.

Uncovering friction dynamics using hydrogel particles as soft ball bearings

Rolling ball bearings are widely known and applied to decrease friction between two surfaces. More recently, hydrogel-hydrogel tribopairs have also revealed good but rather complex lubrication properties. Here, we use hydrogels as ball bearings to elucidate that soft spherical particles have nontrivial rate-dependent lubrication behavior. Unlike Newtonian lubrication or dry solid friction, hydrogel particles in suspension transition through four frictional regimes as a function of sliding velocity. We relate the different regimes to the deformation of the particles at different gap sizes, which changes the effective contact area between the sliding surfaces. By systematically varying the particle characteristics and the surface properties of the sliding surfaces, we assign potential mechanisms for each of the different lubricating regimes as a function of velocity: (I) relatively high friction due to particle flattening and direct contact between interacting bodies (II) decrease of friction owing to the presence of rolling particles (III) large inflow of particles in a confined space leading to compressed particles and (IV) the formation of a thick lubricating layer. Using these suspensions with soft, deformable particles as a ball bearing system, we provide new insights into soft material friction with applications in emulsions, powders, pastes or other granular materials.