pH-Triggered softening of crosslinked hydrogen-bonded capsules

Softening implantable bioelectronics: Material designs, applications, and future directions

Implantable bioelectronics, integrated directly within the body, represent a potent biomedical solution for monitoring and treating a range of medical conditions, including chronic diseases, neural disorders, and cardiac conditions, through personalized medical interventions. Nevertheless, contemporary implantable bioelectronics rely heavily on rigid materials (e.g., inorganic materials and metals), leading to inflammatory responses and tissue damage due to a mechanical mismatch with biological tissues. Recently, soft electronics with mechanical properties comparable to those of biological tissues have been introduced to alleviate fatal immune responses and improve tissue conformity. Despite their myriad advantages, substantial challenges persist in surgical handling and precise positioning due to their high compliance. To surmount these obstacles, softening implantable bioelectronics has garnered significant attention as it embraces the benefits of both rigid and soft bioelectronics. These devices are rigid for easy standalone implantation, transitioning to a soft state in vivo in response to environmental stimuli, which effectively overcomes functional/biological problems inherent in the static mechanical properties of conventional implants. This article reviews recent research and development in softening materials and designs for implantable bioelectronics. Examples featuring tissue-penetrating and conformal softening devices highlight the promising potential of these approaches in biomedical applications. A concluding section delves into current challenges and outlines future directions for softening implantable device technologies, underscoring their pivotal role in propelling the evolution of next-generation bioelectronics.

Impact of chemistry on the preparation and post-modification of multilayered hollow microcapsules

In the last few years, various chemical bondings and interactions were rationally adopted to develop different multilayered microcapsules, where the empty interior accommodated various important cargoes, including bioactive molecules, nanoparticles, antibodies, enzymes, etc., and the thin membrane protected/controlled the release of the loaded cargo. Eventually, such materials are with immense potential for a wide range of prospective applications related to targeted drug delivery, sensing, bio-imaging, developing biomimetic microreactors, and so on. The emphasis on the use of various chemistries for the development of functional and useful microcapsules is rarely illustrated in the literature in the past. In this feature article, the rational uses of different chemistries for (a) preparing and (b) post-modifying various functional microcapsules are accounted. The appropriate selection of chemical bondings/interactions, including electrostatic interaction, host-guest interaction, hydrogen bonding, and covalent bonding, allowed the integration of essential constituents during the layer-by-layer deposition process for 'in situ' tailoring of the relevant and diverse properties of the hollow microcapsules. Recently, different chemically reactive hollow microcapsules were also introduced through the strategic association of 'click chemistry', ring-opening azlactone reaction, thiol-ene reaction, and 1,4-conjugate addition reaction for facile and desired post covalent modifications of the multilayer membrane. The strategic selection of chemistry remained as the key basis to synthesize smart and useful microcapsules.

Dynamic Solid-State Ultrasound Contrast Agent for Monitoring pH Fluctuations In Vivo

The key challenge for in vivo biosensing is to design biomarker-responsive contrast agents that can be readily detected and monitored by broadly available biomedical imaging modalities. While a range of biosensors have been designed for optical, photoacoustic, and magnetic resonance imaging (MRI) modalities, technical challenges have hindered the development of ultrasound biosensors, even though ultrasound is widely available, portable, safe, and capable of both surface and deep tissue imaging. Typically, contrast-enhanced ultrasound imaging is generated by gas-filled microbubbles. However, they suffer from short imaging times because of the diffusion of the gas into the surrounding media. This demands an alternate approach to generate nanosensors that reveal pH-specific changes in ultrasound contrast in biological environments. Silica cores were coated with pH-responsive poly(methacrylic acid) (PMA_SH) in a layer-by-layer (LbL) approach and subsequently covered in a porous organosilica shell. Transmission electron microscopy (TEM) and confocal laser scanning microscopy (CLSM) were employed to monitor the successful fabrication of multilayered particles and prove the pH-dependent shrinkage/swelling of the PMA_SH layer. This demonstrates that reduction in pH below healthy physiological levels resulted in significant increases in ultrasound contrast, in gel phantoms, mouse cadaver tissue, and live mice. The future of such materials could be developed into a platform of biomarker-responsive ultrasound contrast agents for clinical applications.

Shape Recovery of Spherical Hydrogen-Bonded Multilayer Capsules after Osmotically Induced Deformation

The mechanical properties of microparticles intended for in vivo applications as drug delivery vehicles are among important parameters that influence their circulation in the blood and govern particle biodistribution. We report on the synthesis of soft but mechanically robust spherical capsules via a hydrogen-bonded multilayer assembly of (poly(N-vinylpyrrolidone), M_w = 10 000 g mol^-1) with (poly(methacrylic acid) M_w = 100 000 g mol^-1)) (PVPON/PMAA)_n in methanol using 4 μm nonporous silica microparticles as sacrificial templates, where n = 5 and 10 and represents the bilayer number. The mechanical properties of (PVPON/PMAA)_n spherical capsules were assessed using the osmotic pressure difference method and resulted in an elasticity modulus of 97 ± 8 MPa, which is in the range of Young's modulus for elastomeric networks. We also found that hydrogen-bonded (PVPON/PMAA)₁₀ capsules demonstrated almost complete recovery from a concave buckled inward shape induced by the osmotic pressure difference from the addition of polystyrene sulfonate (PSS) to the capsule solution to their initial spherical shape within 12 h after the PSS solution was rinsed off. The permeability measurements through the capsule shell using fluorescently labeled dextran molecular probes revealed that the average mesh size of the hydrogen-bonded network assembled in methanol is in the range of 3 to 9 nm and is not permeable to FITC-dextran with a molecular weight of >40 000 g mol^-1. Our study shows that physically cross-linked polyelectrolyte multilayer capsules are capable of withstanding large deformations, which is essential to the development of adaptable particles for controlled delivery.

Encapsulation and Ultrasound-Triggered Release of G-Quadruplex DNA in Multilayer Hydrogel Microcapsules

Nucleic acid therapeutics have the potential to be the most effective disease treatment strategy due to their intrinsic precision and selectivity for coding highly specific biological processes. However, freely administered nucleic acids of any type are quickly destroyed or rendered inert by a host of defense mechanisms in the body. In this work, we address the challenge of using nucleic acids as drugs by preparing stimuli responsive poly(methacrylic acid)/poly(N-vinylpyrrolidone) (PMAA/PVPON)n multilayer hydrogel capsules loaded with ~7 kDa G-quadruplex DNA. The capsules are shown to release their DNA cargo on demand in response to both enzymatic and ultrasound (US)-triggered degradation. The unique structure adopted by the G-quadruplex is essential to its biological function and we show that the controlled release from the microcapsules preserves the basket conformation of the oligonucleotide used in our studies. We also show that the (PMAA/PVPON) multilayer hydrogel capsules can encapsulate and release ~450 kDa double stranded DNA. The encapsulation and release approaches for both oligonucleotides in multilayer hydrogel microcapsules developed here can be applied to create methodologies for new therapeutic strategies involving the controlled delivery of sensitive biomolecules. Our study provides a promising methodology for the design of effective carriers for DNA vaccines and medicines for a wide range of immunotherapies, cancer therapy and/or tissue regeneration therapies in the future.

Small Angle Scattering for Pharmaceutical Applications: From Drugs to Drug Delivery Systems

The sub-nanometer scale provided by small angle neutron and X-ray scattering is of special importance to pharmaceutical and biomedical investigators. As drug delivery devices become more functionalized and continue decreasing in size, the ability to elucidate details on size scales smaller than those available from optical techniques becomes extremely pertinent. Information gathered from small angle scattering therefore aids the endeavor of optimizing pharmaceutical efficacy at its most fundamental level. This chapter will provide some relevant examples of drug carrier technology and how small angle scattering (SAS) can be used to solve their mysteries. An emphasis on common first-step data treatments is provided which should help clarify the contents of scattering data to new researchers. Specific examples of pharmaceutically relevant research on novel systems and the role SAS plays in these studies will be discussed. This chapter provides an overview of the current applications of SAS in drug research and some practical considerations for selecting scattering techniques.

Temperature-responsive nanogel multilayers of poly(N-vinylcaprolactam) for topical drug delivery

We report nanothin temperature-responsive hydrogel films of poly(N-vinylcaprolactam) nanoparticles (νPVCL) with remarkably high loading capacity for topical drug delivery. Highly swollen (νPVCL)_n multilayer hydrogels, where n denotes the number of nanoparticle layers, are produced by layer-by-layer hydrogen-bonded assembly of core-shell PVCL-co-acrylic acid nanoparticles with linear PVPON followed by cross-linking of the acrylic acid shell with either ethylene diamine (EDA) or adipic acid dihydrazide (AAD). We demonstrate that a (νPVCL)₅ film undergoes dramatic and reversible swelling up to 9 times its dry thickness at pH = 7.5, indicating 89v/v % of water inside the network. These hydrogels exhibit highly reversible ∼3-fold thickness changes with temperature variations from 25 to 50°C at pH = 5, the average pH of human skin. We also show that a (νPVCL)₃₀ hydrogel loaded with ∼120µgcm^-2 sodium diclofenac, a non-steroidal anti-inflammatory drug used for osteoarthritis pain management, provides sustained permeation of this drug through an artificial skin membrane for up to 24h at 32°C (the average human skin surface temperature). The cumulative amount of diclofenac transported at 32°C from the (νPVCL)₃₀ hydrogel after 24h is 12 times higher than that from the (νPVCL)₃₀ hydrogel at 22°C. Finally, we demonstrate that the (νPVCL) hydrogels can be used for multiple drug delivery by inclusion of Nile red, fluorescein and DAPI dyes within the νPVCL nanoparticles prior to hydrogel assembly. Using confocal microscopy we observed the presence of separate dye-loaded νPVCL compartments within the hydrogel matrix with all three dyes confined to the nanogel particles without intermixing between the dyes. Our study provides opportunity for development of temperature-responsive multilayer hydrogel coatings made via the assembly of core-shell nanogel particles which can be used for skin-sensitive materials for topical drug delivery.

Silver Alginate Hydrogel Micro- and Nanocontainers for Theranostics: Synthesis, Encapsulation, Remote Release, and Detection

We have designed multifunctional silver alginate hydrogel microcontainers referred to as loaded microcapsules with different sizes by assembling them via a template assisted approach using natural, highly porous calcium carbonate cores. Sodium alginate was immobilized into the pores of calcium carbonate particles of different sizes followed by cross-linking via addition of silver ions, which had a dual purpose: on one hand, the were used as a cross-linking agent, albeit in the monovalent form, while on the other hand they have led to formation of silver nanoparticles. Monovalent silver ions, an unusual cross-linking agent, improve the sensitivity to ultrasound, lead to homogeneous distribution of silver nanoparticles. Silver nanoparticles appeared on the shell of the alginate microcapsules in the twin-structure as determined by transmission electron microscopy. Remote release of a payload from alginate containers by ultrasound was found to strongly depend on the particle size. The possibility to use such particles as a platform for label-free molecule detection based on the surface enhanced Raman scattering was demonstrated. Cytotoxicity and cell uptake studies conducted in this work have revealed that microcontainers exhibit nonessential level of toxicity with an efficient uptake of cells. The above-described functionalities constitute building blocks of a theranostic system, where detection and remote release can be achieved with the same carrier.

Theranostic Multilayer Capsules for Ultrasound Imaging and Guided Drug Delivery

Despite the accessibility of ultrasound, the clinical potential of ultrasound-active theranostic agents has not been fully realized because it requires combining sufficient imaging contrast, high encapsulation efficiency, and ultrasound-triggered release in one entity. We report on theranostic polymer microcapsules composed of hydrogen-bonded multilayers of tannic acid and poly(N-vinylpyrrolidone) that produce high imaging contrast and deliver the anticancer drug doxorubicin upon low-power diagnostic or high-power therapeutic ultrasound irradiation. These capsules exhibit excellent imaging contrast in both brightness and harmonic modes and show prolonged contrast over six months, unlike commercially available microbubbles. We also demonstrate low-dose gradual and high-dose fast release of doxorubicin from the capsules by diagnostic (∼100 mW/cm2) and therapeutic (>10 W/cm2) ultrasound irradiation, respectively. We show that the imaging contrast of the capsules can be controlled by varying the number of layers, polymer type (relatively rigid tannic acid versus more flexible poly(methacrylic acid)), and polymer molecular weight. In vitro studies demonstrate that 50% doxorubicin release from ultrasound-treated capsules induces 97% cytotoxicity to MCF-7 human cancer cells, while no cytotoxicity is found without the treatment. Considering the strong ultrasound imaging contrast, high encapsulation efficiency, biocompatibility, and tunable drug release, these microcapsules can be used as theranostic agents for ultrasound-guided chemotherapy.

Impact of particle elasticity on particle-based drug delivery systems

Modification of nano/micro-particle physical parameters (e.g. size, shape, surface charge) has proven to be an effective method to enhance their delivery abilities. Recently, advances in particle synthesis have facilitated investigations into the role that particle elasticity plays in modulating drug delivery processes. This review will highlight: (i) methods to tune particle elasticity, (ii) the role particle elasticity plays in cellular internalization, (iii) the role of particle elasticity in modulating circulation times, (iv) the effect of particle elasticity on altering biodistribution and tissue targeting, and (v) the application of computational methods to explain the differences in cellular internalization of particles of different elasticities. Overall, literature reports suggest a complex relationship between particle elasticity and drug delivery processes. Despite this complex relationship, it is clear from numerous in vitro and in vivo studies that particle elasticity is an important parameter that can be leveraged to improve blood circulation, tissue targeting, and specific interactions with cells.

Shaped stimuli-responsive hydrogel particles: syntheses, properties and biological responses

This review summarizes a pool of current experimental approaches and discusses perspectives in the development of the synergistic combination of shape and stimuli-response in particulate hydrogels.

Interfacial Rheology of Hydrogen-Bonded Polymer Multilayers Assembled at Liquid Interfaces: Influence of Anchoring Energy and Hydrophobic Interactions

We study the 2D rheological properties of hydrogen-bonded polymer multilayers assembled directly at dodecane-water and air-water interfaces using pendant drop/bubble dilation and the double-wall ring method for interfacial shear. We use poly(vinylpyrrolidone) (PVP) as a proton acceptor and a series of polyacrylic acids as proton donors. The PAA series of chains with varying hydrophobicity was fashioned from poly(acrylic acid), (PAA), polymethacrylic acid (PMAA), and a homemade hydrophobically modified polymer. The latter consisted of a PAA backbone covalently grafted with C12 moieties at 1% mol (referred to as PAA-1C12). Replacing PAA with the more hydrophobic PMAA provides a route for combining hydrogen bonding and hydrophobic interactions to increase the strength and/or the number of links connecting the polyacid chains to PVP. This systematic replacement allows for control of the ability of the monomer units inside the absorbed polymer layer to reorganize as the interface is sheared or compressed. Consequently, the interplay of hydrogen bonding and hydrophobic interactions leads to control of the resistance of the polymer multilayers to both shear and dilation. Using PAA-1C12 as the first layer improves the anchoring energy of a few monomers of the chain without changing the strength of the monomer-monomer contact in the complex layer. In this way, the layer does not resist shear but resists compression. This strategy provides the means for using hydrophobicity to control the interfacial dynamics of the complexes adsorbed at the interface of the bubbles and droplets that either elongate or buckle upon compression. Moreover, we demonstrate the pH responsiveness of these interfacial multilayers by adding aliquots of NaOH to the acidic water subphase surrounding the bubbles and droplets. Subsequent pH changes can eventually break the polymer complex, providing opportunities for encapsulation/release applications.

pH-Induced Softening of Polyelectrolyte Microcapsules without Apparent Swelling

Polyelectrolyte microcapsules represent a versatile platform to encapsulate and release active ingredients. Understanding the effect of environmental conditions on the mechanical properties of microcapsules is critically important for enabling their applications under various settings. In this report, we investigate the effect of solution pH on the mechanical properties of polyelectrolyte microcapsules made of two weak polyelectrolytes (poly(acrylic acid) and branched poly(ethylenimine)), formed via recently introduced nanoscale interfacial complexation in emulsion (NICE). Interestingly, the stiffness of the NICE microcapsule shell is reduced significantly (by 2 orders of magnitude) when the solution pH is raised from 2 to 6 even though there is little change in the size of microcapsules. These seemingly counterintuitive results suggest that the molecular structure of NICE microcapsules may be different from those of conventional polyelectrolyte microcapsules. The possibility of tuning the shell stiffness without inducing significant changes in the microcapsule size could offer unique advantages in practical applications.

Cellular uptake of poly(allylamine hydrochloride) microcapsules with different deformability and its influence on cell functions

It is important to understand the safety issue and cell interaction pattern of polyelectrolyte microcapsules with different deformability before their use in biomedical applications. In this study, SiO2, poly(sodium-p-styrenesulfonate) (PSS) doped CaCO3 and porous CaCO3 spheres, all about 4μm in diameter, were used as templates to prepare microcapsules with different inner structure and subsequent deformability. As a result, three kinds of covalently assembled poly(allylaminehydrochloride)/glutaraldehyde (PAH/GA) microcapsules with similar size but different deformability under external osmotic pressure were prepared. The impact of different microcapsules on cell viability and functions are studied using smooth muscle cells (SMCs), endothelial cells (ECs) and HepG2 cells. The results demonstrated that viabilities of SMCs, ECs and HepG2 cells were not significantly influenced by either of the three kinds of microcapsules. However, the adhesion ability of SMCs and ECs as well as the mobility of SMCs, ECs and HepG2 cells were significantly impaired after treatment with microcapsules in a deformability dependent manner, especially the microcapsules with lower deformability caused higher impairment on cell functions. The cellular uptake kinetics, uptake pathways, intracellular distribution of microcapsules are further investigated in SMCs to reveal the potential mechanism. The SMCs showed faster uptake rate and exocytosis rate of microcapsules with lower deformability (Cap@CaCO3/PSS and Cap@CaCO3), leading to higher intracellular accumulation of microcapsules with lower deformability and possibly larger retardation of cell functions. The results pointed out that the deformability of microcapsules is an important factor governing the biological performance of microcapsules, which requires careful adjustment for further biomedical applications.

Cellulose Nanocrystal Microcapsules as Tunable Cages for Nano- and Microparticles

We demonstrate the fabrication of highly open spherical cages with large through pores using high aspect ratio cellulose nanocrystals with "haystack" shell morphology. In contrast to traditional ultrathin shell polymer microcapsules with random porous morphology and pore sizes below 10 nm with limited molecular permeability of individual macromolecules, the resilient cage-like microcapsules show a remarkable open network morphology that facilitates across-shell transport of large solid particles with a diameter from 30 to 100 nm. Moreover, the transport properties of solid nanoparticles through these shells can be pH-triggered without disassembly of these shells. Such behavior allows for the controlled loading and unloading of solid nanoparticles with much larger dimensions than molecular objects reported for conventional polymeric microcapsules.

Hierarchically functionalized magnetic core/multishell particles and their postsynthetic conversion to polymer capsules

The controlled synthesis of hierarchically functionalized core/multishell particles is highly desirable for applications in medicine, catalysis, and separation. Here, we describe the synthesis of hierarchically structured metal-organic framework multishells around magnetic core particles (magMOFs) via layer-by-layer (LbL) synthesis. The LbL deposition enables the design of multishell systems, where each MOF shell can be modified to install different functions. Here, we used this approach to create controlled release capsules, in which the inner shell serves as a reservoir and the outer shell serves as a membrane after postsynthetic conversion of the MOF structure to a polymer network. These capsules enable the controlled release of loaded dye molecules, depending on the surrounding media.

Interplay of Hydrogen Bonding and Hydrophobic Interactions to Control the Mechanical Properties of Polymer Multilayers at the Oil-Water Interface

We probe the mechanical shear and compression properties of hydrogen-bonded polymer multilayers directly assembled at the oil-water interface using interfacial rheology techniques. We show that the polymer multilayers behave mechanically like a transient network, with elastic moduli that can be varied over 2 orders of magnitude by controlling the type and strength of physical interactions involved in the multilayers, which are controlled by the pH and the hydrophobicity of the polymer. Indeed, the interplay of hydrogen and hydrophobic interactions enables one to obtain a tighter and stronger network at the interface. Moreover, we show how a simple LBL process applied directly on emulsion droplets leads to encapsulation of a model oil, dodecane, as well as perfume molecules.

Internalization of red blood cell-mimicking hydrogel capsules with pH-triggered shape responses

We report on naturally inspired hydrogel capsules with pH-induced transitions from discoids to oblate ellipsoids and their interactions with cells. We integrate characteristics of erythrocytes such as discoidal shape, hollow structure, and elasticity with reversible pH-responsiveness of poly(methacrylic acid) (PMAA) to design a new type of drug delivery carrier to be potentially triggered by chemical stimuli in the tumor lesion. The capsules are fabricated from cross-linked PMAA multilayers using sacrificial discoid silicon templates. The degree of capsule shape transition is controlled by the pH-tuned volume change, which in turn is regulated by the capsule wall composition. The (PMAA)15 capsules undergo a dramatic 24-fold volume change, while a moderate 2.3-fold volume variation is observed for more rigid PMAA-(poly(N-vinylpyrrolidone) (PMAA-PVPON)5 capsules when solution pH is varied between 7.4 and 4. Despite that both types of capsules exhibit discoid-to-oblate ellipsoid transitions, a 3-fold greater swelling in radial dimensions is found for one-component systems due to a greater degree of the circular face bulging. We also show that (PMAA-PVPON)5 discoidal capsules interact differently with J774A.1 macrophages, HMVEC endothelial cells, and 4T1 breast cancer cells. The discoidal capsules show 60% lower internalization as compared to spherical capsules. Finally, hydrogel capsules demonstrate a 2-fold decrease in size upon internalization. These capsules represent a unique example of elastic hydrogel discoids capable of pH-induced drastic and reversible variations in aspect ratios. Considering the RBC-mimicking shape, their dimensions, and their capability to undergo pH-triggered intracellular responses, the hydrogel capsules demonstrate considerable potential as novel carriers in shape-regulated transport and cellular uptake.

Microcapsule mechanics: from stability to function

Microcapsules are reviewed with special emphasis on the relevance of controlled mechanical properties for functional aspects. At first, assembly strategies are presented that allow control over the decisive geometrical parameters, diameter and wall thickness, which both influence the capsule's mechanical performance. As one of the most powerful approaches the layer-by-layer technique is identified. Subsequently, ensemble and, in particular, single-capsule deformation techniques are discussed. The latter generally provide more in-depth information and cover the complete range of applicable forces from smaller than pN to N. In a theory chapter, we illustrate the physics of capsule deformation. The main focus is on thin shell theory, which provides a useful approximation for many deformation scenarios. Finally, we give an overview of applications and future perspectives where the specific design of mechanical properties turns microcapsules into (multi-)functional devices, enriching especially life sciences and material sciences.

pH-responsive hydrogel cubes for release of doxorubicin in cancer cells

Doxorubicin (DOX)-loaded poly(methacrylic acid) hydrogel cubes release the drug at pH <5. These hydrogels are developed for shape-directed cellular uptake for drug delivery.

Immersive polymer assembly on immobilized particles for automated capsule preparation

We report a versatile approach for polymer capsule preparation using immobilized particles, which are immersed into polymer solutions either manually or by using an automated robotic dipping machine. This technique produces polyelectrolyte capsules with improved retention over conventionally prepared capsules. Additionally, responsive hydrogel capsules of different diameter can be prepared simultaneously.

Mechanics of pH-responsive hydrogel capsules

While soft hydrogel nano- and microstructures hold great potential for therapeutic treatments and in vivo applications, their nanomechanical characterization remains a challenge. In this paper, soft, single-component, supported hydrogel films were fabricated using pendant-thiol-modified poly(methacrylic acid) (PMASH). The influence of hydrogel architecture on deformation properties was studied by fabricating films on particle supports and producing free-standing capsules. The influence of the degree of thiol-based cross-linking on the mechanical properties of the soft hydrogel systems (core-shell and capsules) was studied using a colloidal-probe (CP) AFM technique. It was found that film mechanical properties, stability, and capsule swelling could be finely tuned by controlling the extent of poly(methacrylic acid) thiol modification. Furthermore, switching the pH from 7.4 to 4.0 led to film densification due to increased hydrogen bonding. Hydrogel capsule systems were found to have stiffness values ranging from 0.9 to 16.9 mN m(-1) over a thiol modification range of 5 to 20 mol %. These values are significantly greater than those for previously reported PMASH planar films of 0.7-5.7 mN m(-1) over the same thiol modification range (Best et al., Soft Matter 2013, 9, 4580-4584). Films on particle substrates had comparable mechanical properties to planar films, demonstrating that while substrate geometry has a negligible effect, membrane and tension effects may play an important role in capsule force resistance. Further, when transitioning from solid-supported films to free-standing capsules, simple predictions of shell stiffness based on modulus changes found for supported films are not valid. Rather, additional effects like diameter increases (geometrical changes) as well as tension buildup need to be taken into account. These results are important for research related to the characterization of soft hydrogel materials and control over their mechanical properties.

Tuning the mechanical properties of nanoporous hydrogel particles via polymer cross-linking

Soft hydrogel particles with tunable mechanical properties are promising for next-generation therapeutic applications. This is due to the increasingly proven role that physicochemical properties play in particulate-based delivery vectors, both in vitro and in vivo. The ability to understand and quantify the mechanical properties of such systems is therefore essential to optimize function and performance. We report control over the mechanical properties of poly(methacrylic acid) (PMA) hydrogel particles based on a mesoporous silica templating method. The mechanical properties of the obtained particles can be finely tuned through variation of the cross-linker concentration, which is hereby quantified using a cross-linking polymer with a fluorescent tag. We demonstrate that the mechanical properties of the particles can be elucidated using an atomic force microscopy (AFM) force spectroscopy method, which additionally allows for the study of hydrogel material properties at the nanoscale through high-resolution force mapping. Young's modulus and stiffness of the particles were tuned between 0.04 and 2.53 MPa and between 1.6 and 28.4 mN m(-1), respectively, through control over the cross-linker concentration. The relationship between the concentration of the cross-linker added and the amount of adsorbed polymer was observed to follow a Langmuir isotherm, and this relationship was found to correlate linearly with the particle mechanical properties.

Permeability and micromechanical properties of silk ionomer microcapsules

We studied the pH-responsive behavior of layer-by-layer (LbL) microcapsules fabricated from silk fibroin chemically modified with different poly amino acid side chains: cationic (silk-poly L-lysine, SF-PL) or anionic (silk-poly-L-glutamic acid, SF-PG). We observed that stable ultrathin shell microcapsules can be assembled with a dramatic increase in swelling, thickness, and microroughness at extremely acidic (pH < 2.5) and basic (pH > 11.0) conditions without noticeable disintegration. These changes are accompanied by dramatic changes in shell permeability with a 2 orders of magnitude increase in the diffusion coefficient. Moreover, the silk ionomer shells undergo remarkable softening with a drop in Young's modulus by more than 1 order of magnitude due to the swelling, stretching, and increase in material porosity. The ability to control permeability and mechanical properties over a wide range for the silk-based microcapsules, with distinguishing stability under harsh environmental conditions, provides an important system for controlled loading and release and applications in bioengineering.

UV-cross-linkable multilayer microcapsules made of weak polyelectrolytes

Microcapsules composed of weak polyelectrolytes modified with UV-responsive benzophenone (BP) groups were fabricated by the layer-by-layer (LbL) technique. Being exposed to UV lights, capsules shrunk in the time course of minutes at irradiation intensity of 5 mW/cm(2). The shrinkage adjusted the capsule permeability, providing a novel way to encapsulate fluorescence-labeled dextran molecules without heating. Cross-linking within the capsule shells based on hydrogen abstraction via excited benzophenone units by UV showed a reliable and swift approach to tighten and stabilize the capsule shell without losing the pH-responsive properties of the weak polyelectrolyte multilayers.

pH-responsive layer-by-layer nanoshells for direct regulation of cell activity

Saccharomyces cerevisiae yeast cells encapsulated with pH-responsive synthetic nanoshells from lightly cross-linked polymethacrylic acid showed a high viability rate of around 90%, an indication of high biocompatibility of synthetic pH-responsive shells. We demonstrated that increasing pH above the isoelectric point of the polymer shell leads to a delay in growth rate; however, it does not affect the expression of enhanced green fluorescent protein. We suggest that progressive ionization and charge accumulation within the synthetic shells evoke a structural change in the outer shells which affect the membrane transport. This change facilitates the ability to manipulate growth kinetics and functionality of the cells with the surrounding environment. We observed that hollow layer-by layer nanoshells showed a remarkable degree of reversible swelling/deswelling over a narrow pH range (pH 5.0-6.0), but their assembly directly on the cell surface resulted in the suppression of large dimensional changes. We suggest that the variation in surface charges caused by deprotonation/protonation of carboxylic groups in the nanoshells controlled cell growth and cell function, which can be utilized for external chemical control of cell-based biosensors.

Shape deformation and recovery of multilayer microcapsules after being squeezed through a microchannel

The deformation and recovery behaviors of multilayer microcapsules were investigated after being forced to flow through a microchannel. The microchannel device with a constriction (5.7 μm in depth) in the middle was designed, and the multilayer microcapsules with different size and layer thickness (and thereby different mechanical strength) were used. Deformation in the microchannel was observed for all the capsules with a size larger than the constriction height, and its extent was mainly governed by the difference between capsule size and constriction height. The squeezed microcapsules could recover their original spherical shape when the deformation extent was smaller than 16%, whereas permanent physical deformation took place when the deformation extent was larger than 34%. The capsules filled with polyelectrolytes could greatly enhance their shape recovery ability due to the higher osmotic pressure in the capsule interior and could well maintain the preloaded low-molecular-weight dyes regardless of the squeezing.

The role of particle geometry and mechanics in the biological domain

Nanostructured particulate materials are expected to revolutionize diagnostics and the delivery of therapeutics for healthcare. To date, chemistry-derived solutions have been the major focus in the design of materials to control interactions with biological systems. Only recently has control over a new set of physical parameters, including size, shape, and rigidity, been explored to optimize the biological response and the in vivo performance of nanoengineered delivery vectors. This Review highlights the methods used to manipulate the physical properties of particles and the relevance of these physical properties to cellular and circulatory interactions. Finally, the importance of future work to synergistically tailor both physical and chemical properties of particulate materials is discussed, with the aim of improving control over particle interactions in the biological domain.

Robust and responsive silk ionomer microcapsules

We demonstrate the assembly of extremely robust and pH-responsive thin shell LbL microcapsules from silk fibroin counterparts modified with poly(lysine) and poly(glutamic) acid, which are based on biocompatible silk ionomer materials in contrast with usually exploited synthetic polyelectrolytes. The microcapsules are extremely stable in an unusually wide pH range from 1.5 to 12.0 and show a remarkable degree of reversible swelling/deswelling response in dimensions, as exposed to extreme acidic and basic conditions. These changes are accompanied by reversible variations in shell permeability that can be utilized for pH-controlled loading and unloading of large macromolecules. Finally, we confirmed that these shells can be utilized to encapsulate yeast cells with a viability rate much higher than that for traditional synthetic polyelectrolytes.

Direct probing of micromechanical properties of hydrogen-bonded layer-by-layer microcapsule shells with different chemical compositions

The mechanical properties of hydrogen-bonded layer-by-layer (LbL) microcapsule shells constructed from tannic acid (TA) and poly(vinylpyrrolidone) (PVPON) components have been studied in both the dry and swollen states. In the dry state, the value of the elastic modulus was measured to be within 0.6-0.7 GPa, which is lower than the typical elastic modulus for electrostatically assembled LbL shells. Threefold swelling of the LbL shells in water results in a significant reduction of the elastic modulus to values well below 1 MPa, which is typical value seen for highly compliant gel materials. The increase of the molecular weight of the PVPON component from 55 to 1300 kDa promotes chain entanglements and causes a stiffening of the LbL shells with a more than 2-fold increase in elastic modulus value. Moreover, adding a polyethylenimine prime layer to the LbL shell affects the growth of hydrogen-bonded multilayers which consequently results in dramatically stiffer, thicker, and rougher LbL shells with the elastic modulus increasing by more than an order of magnitude, up to 4.3 MPa. An alternation of the elastic properties of very compliant hydrogen-bonded shells by variation of molecular weight is a characteristic feature of weakly bonded LbL shells. Such an ability to alter the elastic modulus in a wide range is critically important for the design of highly compliant microcapsules with tunable mechanical stability, loading ability, and permeability.

Shape switching of hollow layer-by-layer hydrogel microcontainers

We present a novel type of ultrathin hydrogel microcapsules with pH-triggered shape switch. The capsules are produced as hollow hydrogel replicas of cubical inorganic templates and capable of keeping cubical geometries at neutral pH but transform into bulged structures at basic pH.

Dynamic aspects of films prepared by a sequential deposition of species: perspectives for smart and responsive materials

The deposition of surface coatings using a step-by-step approach from mutually interacting species allows the fabrication of so called "multilayered films". These coatings are very versatile and easy to produce in environmentally friendly conditions, mostly from aqueous solution. They find more and more applications in many hot topic areas, such as in biomaterials and nanoelectronics but also in stimuli-responsive films. We aim to review the most recent developments in such stimuli-responsive coatings based on layer-by-layer (LBL) depositions in relationship to the properties of these coatings. The most investigated stimuli are based on changes in ionic strength, temperature, exposure to light, and mechanical forces. The possibility to induce a transition from linear to exponential growth in thickness and to change the charge compensation from "intrinsic" to "extrinsic" by controlling parameters such as temperature, pH, and ionic strength are the ways to confer their responsiveness to the films. Chemical post-modifications also allow to significantly modify the film properties.

Engineered hydrogen-bonded polymer multilayers: from assembly to biomedical applications

In this tutorial review, developments in hydrogen-bonded LbL materials are discussed, with an emphasis on loading and release of cargo for biomedical applications.

Stimuli-responsive porous hydrogels at interfaces for molecular filtration, separation, controlled release, and gating in capsules and membranes

A continuously growing area of controlled and tunable transport and separation of biomolecules and drugs has recently attracted attention to the structures which can be referred to as stimuli-responsive porous hydrogel thin films. Because of spatial constraints, swelling/shrinking of the hydrogel films results in closing/opening (or vice versa) of the film's pores. Such responsive systems can be used in the configuration of plane films or capsules. The combination of a low thickness (translating into a low hydrodynamic flow resistance and rapid response) with well-defined size and shape of pores (translating into better control of transport and separation), which can be closed, opened, or tuned by an external signal (allowing a large amplitude of changes in diffusivity of solutes in the thin film and a precise control of the pore size), makes these materials very attractive for a range of applications, such as molecular filtration, separation, drug delivery, sensors, and actuators.

Probing the in situ competitive displacement of protein by nonionic surfactant using atomic force microscopy

Force-distance data obtained from an atomic force microscope have been used to follow the in situ displacement of beta-lactoglobulin from tetradecane droplets by Tween 20 (polyoxyethylenesorbitan monolaurate). Interpretation of the force-distance curves has shown that the slope of the region, traditionally termed the constant compliance region, is a useful indicator of droplet deformation within a given experiment. The magnitude of this slope can be used to monitor how the deformability of the droplet changes upon addition of surfactant. It has been found that, immediately after initial addition of surfactant, there is an increase in magnitude of this slope, indicating a stiffening of the droplet, attributed to a stiffening of the protein network formed at the surface of the droplet. Subsequent additions of Tween 20 reduce the magnitude of the slope until an equilibrium value is reached, where the interface becomes surfactant-dominated. These observations suggest that it is possible to monitor in situ the displacement of protein from individual oil droplets. The data have been interpreted in terms of the "orogenic" model of displacement, which is based on studies made on model interfaces. These data have been compared to those obtained using the more traditional techniques of dilatational rheology, surface loading, and surface potential measurements for analogous beta-lactoglobulin-stabilized droplets or emulsions.

Using nanoparticle-filled microcapsules for site-specific healing of damaged Substrates: creating a "repair-and-go" system

Using a hybrid computational approach, we simulate the behavior of nanoparticle-filled microcapsules that are propelled by an imposed shear to move over a substrate, which encompasses a microscopic crack. When the microcapsules become localized in the crack, the nanoparticles can penetrate the capsule's shell to bind to and fill the damaged region. Initially focusing on a simple shear flow, we isolate conditions where the microcapsules become arrested in the cracks and those where the capsules enter the cracks for a finite time but are driven to leave this region by the imposed flow. We also characterize the particle deposition process for these two scenarios, showing that the deposition is greater for the arrested capsules. We then determine the effect of utilizing a pulsatile shear flow and show that this flow field can lead to an effective "repair-and-go" system where the microcarriers not only deliver a high volume fraction of particles into the crack but also leave the fissure and, thus, can potentially repair additional damage within the system.

Polymer multilayer films obtained by electrochemically catalyzed click chemistry

We report the covalent layer-by-layer construction of polyelectrolyte multilayer (PEM) films by using an efficient electrochemically triggered Sharpless click reaction. The click reaction is catalyzed by Cu(I) which is generated in situ from Cu(II) (originating from the dissolution of CuSO(4)) at the electrode constituting the substrate of the film. The film buildup can be controlled by the application of a mild potential inducing the reduction of Cu(II) to Cu(I) in the absence of any reducing agent or any ligand. The experiments were carried out in an electrochemical quartz crystal microbalance cell which allows both to apply a controlled potential on a gold electrode and to follow the mass deposited on the electrode through the quartz crystal microbalance. Poly(acrylic acid) (PAA) modified with either alkyne (PAA(Alk)) or azide (PAA(Az)) functions grafted onto the PAA backbone through ethylene glycol arms were used to build the PEM films. Construction takes place on gold electrodes whose potentials are more negative than a critical value, which lies between -70 and -150 mV vs Ag/AgCl (KCl sat.) reference electrode. The film thickness increment per bilayer appears independent of the applied voltage as long as it is more negative than the critical potential, but it depends upon Cu(II) and polyelectrolyte concentrations in solution and upon the reduction time of Cu(II) during each deposition step. An increase of any of these latter parameters leads to an increase of the mass deposited per layer. For given buildup conditions, the construction levels off after a given number of deposition steps which increases with the Cu(II) concentration and/or the Cu(II) reduction time. A model based on the diffusion of Cu(II) and Cu(I) ions through the film and the dynamics of the polyelectrolyte anchoring on the film, during the reduction period of Cu(II), is proposed to explain the major buildup features.