Interfacial rheology of linearly growing polyelectrolyte multilayers at the water-air interface: from liquid to solid viscoelasticity.

Despite the long history of investigations of polyelectrolyte multilayer formation on solid or liquid surfaces, important questions remain open concerning the construction of the first set of layers. These are generally deposited on a first anchoring layer of different chemistry, influencing their construction and properties. We propose here an in-depth investigation of the formation of NaPSS/PAH multilayers at the air/water interface in the absence of a chemically different anchoring layer, profiting from the surface activity of NaPSS. To analyse the mechanical properties of the different layers, we combine recently established analysis techniques of an inflating/deflating bubble exploiting simultaneous shape and pressure measurement: bubble shape elastometry, general stress decomposition and capillary meniscus dynanometry. We complement these measurements by interfacial shear rheology. The obtained results allow us to confirm, first of all, the strength of the aforementioned techniques to characterize complex interfaces with non-linear viscoelastic properties. Furthermore, their sensitivity allows us to show that the multilayer properties are highly sensitive to the temporal and mechanical conditions under which they are constructed and manipulated. We nevertheless identify a robust trend showing a clear transition from a liquid-like viscoelastic membrane to a solid-like viscoelastic membrane after the deposition of 5 layers. We interpret this as the number of layers required to create a fully connected multilayer, which is consistent with previous results obtained on solid or liquid interfaces.

PEG-in-PDMS drops stabilised by soft silicone skins as a model system for elastocapillary emulsions with explicit morphology control.

HYPOTHESIS: The morphology of ordinary macro-emulsions is controlled by their high interfacial energies, i.e., by capillarity, leading to well-known structural features which can be tuned only over a narrow range. We claim here that a more explicit control over a much wider range of morphologies can be obtained by producing "elastocapillary emulsions" in which interfacial elasticity acts simultaneously with interfacial tension. EXPERIMENTS: We develop a model-system composed of PEG-in-PDMS emulsions, in which a catalyst diffuses from the PEG drops into the silicone matrix containing two reactive silicone polymers, which are cross-linked in a non-reactive silicone matrix to form a silicone gel of controlled thickness and mechanical properties on the drop surface. We characterise the cross-linking process of the gel in bulk and at the interface, and we analyse the skin growth kinetics. We then use the obtained understanding to produce emulsions with controlled elastocapillary interfaces using in-flow-chemistry in a purpose-designed millifluidic circuit. FINDINGS: We show that this approach allows to create interfaces over the full range of elastocapillary properties, and that very different emulsion morphologies can be generated depending on whether capillarity or elasticity dominates. These findings advance our fundamental understanding of the morphology of emulsions with complex interfaces, and they are of importance for the design of polymerised High Internal Phase Emulsions (polyHIPEs) with original structure/property relations. They will also be useful for the design of silicone capsules with fine-tuned mechanical properties.

One-Step Generation of Alginate-Based Hydrogel Foams Using CO2 for Simultaneous Foaming and Gelation.

The reliable generation of hydrogel foams remains a challenge in a wide range of sectors, including food, cosmetic, agricultural, and medical applications. Using the example of calcium alginate foams, we introduce a novel foam generation method that uses CO2 for the simultaneous foaming and pH reduction of the alginate solution to trigger gelation. We show that gelled foams of different gas fractions can be generated in a simple one-step process. We macroscopically follow the acidification using a pH-responsive indicator and investigate the role of CO2 in foam ageing via foam stability measurements. Finally, we demonstrate the utility of interfacial rheology to provide evidence for the gelation process initiated by the dissolution of the CO2 from the dispersed phase. Both approaches, gas-initiated gelation and interfacial rheology for its characterization, can be readily transferred to other types of gases and formulations.

Investigating Pore-Opening of Hydrogel Foams at the Scale of Freestanding Thin Films.

Controlling the pore connectivity of polymer foams is key for most of their applications, ranging from liquid uptake, mechanics, and acoustic/thermal insulation to tissue engineering. Despite their importance, the scientific phenomena governing the pore-opening processes remain poorly understood, requiring tedious trial-and-error procedures for property optimization. This lack of understanding is partly explained by the high complexity of the different interrelated, multiscale processes which take place as the foam transforms from an initially fluid foam into a solid foam. To progress in this field, this work takes inspiration from long-standing research on liquid foams and thin films to develop model experiments in a microfluidic "Thin Film Pressure Balance." These experiments allow the investigation of isolated thin films under well-controlled environmental conditions reproducing those arising within a foam undergoing cross-linking and drying. Using the example of alginate hydrogel films, the evolution of isolated thin films undergoing gelation and drying is correlated with the evolution of the rheological properties of the same alginate solution in bulk. The overall approach is introduced and a first set of results is presented to propose a starting point for the phenomenological description of the different types of pore-opening processes and the classification of the resulting pore-opening types.

Gel-to-gel non-variant transition of an organogel caused by polymorphism from nanotubes to crystallites.

An amide based gelator forms gels in trans-decalin. Below concentrations of 1 wt% the gels melt at temperatures varying with concentration. Above a concentration of 1 wt%, upon heating, the gel transforms into an opaque gel at an invariant temperature, and melts at higher temperature. The gel-to-gel transition is evidenced by several techniques: DSC, rheology, NMR, OM and turbidimetry. The phase diagram with the domain of the existence of both morphs was mapped by these techniques. Optical and electronic microscopy studies show that the first gel corresponds to the self-assembled nanotubes while the second gel is formed by crystalline fibers. The fibers are crystalline, as shown by the presence of Bragg peaks in the scattering curves. Both morphs correspond to a different H-bonding pattern as shown by FTIR. The first gel forms at a higher cooling rate, is metastable and transforms slowly into the second one. The second gel is stable. It forms at a low cooling rate, or by thermal annealing or aging of the first gel.

Validation of Milner's visco-elastic theory of sintering for the generation of porous polymers with finely tuned morphology.

Sacrificial sphere templating has become a method of choice to generate macro-porous materials with well-defined, interconnected pores. For this purpose, the interstices of a sphere packing are filled with a solidifying matrix, from which the spheres are subsequently removed to obtain interconnected voids. In order to control the size of the interconnections, viscous sintering of the initial sphere template has proven a reliable approach. To predict how the interconnections evolve with different sintering parameters, such as time or temperature, Frenkel's model has been used with reasonable success over the last 70 years. However, numerous investigations have shown that the often complex flow behaviour of the spheres needs to be taken into account. To this end, S. Milner [arXiv:1907.05862] developed recently a theoretical model which improves on some key assumptions made in Frenkel's model, leading to a slightly different scaling. He also extended this new model to take into account the visco-elastic response of the spheres. Using an in-depth investigation of templates of paraffin spheres, we provide here the first systematic comparison with Milner's theory. Firstly, we show that his new scaling describes the experimental data slightly better than Frenkel's scaling. We then show that the visco-elastic version of his model provides a significantly improved description of the data over a wide parameter range. We finally use the obtained sphere templates to produce macro-porous polyurethanes with finely controlled pore and interconnection sizes. The general applicability of Milner's theory makes it transferable to a wide range of formulations, provided the flow properties of the sphere material can be quantified. It therefore provides a powerful tool to guide the creation of sphere packings and porous materials with finely controlled morphologies.

How Promoting and Breaking Intersurfactant H-Bonds Impact Foam Stability.

On the basis of previous results revealing that intersurfactant H-bonds improve foam stability, we now focus on how foams stabilized by two different N-acyl amino acid surfactants are affected by different salts (NaF, NaCl, NaSCN), which can promote or break intersurfactant H-bonds. The chosen surfactants, namely, sodium N-lauroyl sarcosinate (C12SarcNa) and sodium N-lauroyl glycinate (C12GlyNa), differ only by one methyl group at the nitrogen of the amide bond that blocks intersurfactant H-bonds in the case of C12SarcNa. The salts were chosen because they are kosmotropic (NaF), chaotropic (NaSCN), and in between (NaCl) and thus influence the formation of an H-bond network in different ways. Surface tension measurements showed that the addition of salts decreased the cmcs of both surfactants and increased the packing density, as expected. Moreover, in presence of the salts, the head groups of the H-bond forming surfactant C12GlyNa were more tightly packed at the surface than the C12SarcNa head groups. The effect of the salts on foam stability was studied by analysis of the foam height, the foam liquid fraction, and by image analysis of the foam structure. As expected, the salts had no significant effect on foams stabilized by C12SarcNa, which is unable to form intersurfactant H-bonds. In contrast, the stability of C12GlyNa-containing foams followed the trend NaF > NaCl > NaSCN, which is in agreement with NaF promoting and NaSCN breaking intersurfactant H-bonds. Surface rheology measurements allowed us to correlate foam stability with surface elasticity. This study provides new insights into the importance of H-bond promoters and breakers, which should be used in the future design of tailor-made surfactants.

Motorizing fibres with geometric zero-energy modes.

Responsive materials1-3 have been used to generate structures with built-in complex geometries4-6, linear actuators7-9 and microswimmers10-12. These results suggest that complex, fully functional machines composed solely from shape-changing materials might be possible 13 . Nonetheless, to accomplish rotary motion in these materials still relies on the classical wheel and axle motifs. Here we explore geometric zero-energy modes to elicit rotary motion in elastic materials in the absence of a rigid wheel travelling around an axle. We show that prestrained polymer fibres closed into rings exhibit self-actuation and continuous motion when placed between two heat baths due to elastic deformations that arise from rotational-symmetry breaking around the rod's axis. Our findings illustrate a simple but robust model to create active motion in mechanically prestrained objects.

Novel Alkali Activation of Titanium Substrates To Grow Thick and Covalently Bound PMMA Layers.

Titanium (Ti) is the most widely used metal in biomedical applications because of its biocompatibility; however, the significant difference in the mechanical properties between Ti and the surrounding tissues results in stress shielding which is detrimental for load-bearing tissues. In the current study, to attenuate the stress shielding effect, a new processing route was developed. It aimed at growing thick poly(methyl methacrylate) (PMMA) layers grafted on Ti substrates to incorporate a polymer component on Ti implants. However, the currently available methods do not allow the development of thick polymeric layers, reducing significantly their potential uses. The proposed route consists of an alkali activation of Ti substrates followed by a surface-initiated atom transfer radical polymerization using a phosphonic acid derivative as a coupling agent and a polymerization initiator and malononitrile as a polymerization activator. The average thickness of the grown PMMA layers is approximately 1.9 µm. The Ti activation-performed in a NaOH solution-leads to a porous sodium titanate interlayer with a hierarchical structure and an open microporosity. It promotes the covalent grafting reaction because of high hydroxyl groups' content and enables establishing a further mechanical interlocking between the growing PMMA layer and the Ti substrate. As a result, the produced graduated structure possesses high Ti/PMMA adhesion strength (∼260 MPa). Moreover, the PMMA layer is (i) thicker compared to those obtained with the previously reported techniques (∼1.9 µm), (ii) stable in a simulated body fluid solution, and (iii) biocompatible. This strategy opens new opportunities toward hybrid prosthesis with adjustable mechanical properties with respect to host bone properties for personalized medicines.

Photoinduced Cross-Linking of Dynamic Poly(disulfide) Films via Thiol Oxidative Coupling.

Initially developed as an elastomer with an excellent record of barrier and chemical resistance properties, poly(disulfide) has experienced a revival linked to the dynamic nature of the S-S covalent bond. A novel photobase-catalyzed oxidative polymerization of multifunctional thiols to poly(disulfide) network is reported. Based solely on air oxidation, the single-step process is triggered by the photodecarboxylation of a xanthone acetic acid liberating a strong bicyclic guanidine base. Starting with a 1 µm thick film based on trithiol poly(ethylene oxide) oligomer, the UV-mediated oxidation of thiols to disulfides occurs in a matter of minutes both selectively, i.e., without overoxidation, and quantitatively as assessed by a range of spectroscopic techniques. Thiolate formation and film thickness determine the reaction rates and yield. Spatial control of the photopolymerization serves to generate robust micropatterns, while the reductive cleavage of S-S bridges allows the recycling of 40% of the initial thiol groups.

A living thick nanofibrous implant bifunctionalized with active growth factor and stem cells for bone regeneration.

New-generation implants focus on robust, durable, and rapid tissue regeneration to shorten recovery times and decrease risks of postoperative complications for patients. Herein, we describe a new-generation thick nanofibrous implant functionalized with active containers of growth factors and stem cells for regenerative nanomedicine. A thick electrospun poly(ε-caprolactone) nanofibrous implant (from 700 µm to 1 cm thick) was functionalized with chitosan and bone morphogenetic protein BMP-7 as growth factor using layer-by-layer technology, producing fish scale-like chitosan/BMP-7 nanoreservoirs. This extracellular matrix-mimicking scaffold enabled in vitro colonization and bone regeneration by human primary osteoblasts, as shown by expression of osteocalcin, osteopontin, and bone sialoprotein (BSPII), 21 days after seeding. In vivo implantation in mouse calvaria defects showed significantly more newly mineralized extracellular matrix in the functionalized implant compared to a bare scaffold after 30 days' implantation, as shown by histological scanning electron microscopy/energy dispersive X-ray microscopy study and calcein injection. We have as well bifunctionalized our BMP-7 therapeutic implant by adding human mesenchymal stem cells (hMSCs). The activity of this BMP-7-functionalized implant was again further enhanced by the addition of hMSCs to the implant (living materials), in vivo, as demonstrated by the analysis of new bone formation and calcification after 30 days' implantation in mice with calvaria defects. Therefore, implants functionalized with BMP-7 nanocontainers associated with hMSCs can act as an accelerator of in vivo bone mineralization and regeneration.