Tuning the Mechanical Properties of Hydrogel Core-Shell Particles by Inwards Interweaving Self-Assembly.

Mechanical properties of hydrogel particles are of importance for their interactions with cells or tissue, apart from their relevance to other applications. While so far the majority of works aiming at tuning particle mechanics relied on chemical cross-linking, we report a novel approach using inwards interweaving self-assembly of poly(allylamine) (PA) and poly(styrenesulfonic acid) (PSSA) on agarose gel beads. Using this technique, shell thicknesses up to tens of micrometers can be achieved from single-polymer incubations and accurately controlled by varying the polymer concentration or incubation period. We quantified the changes in mechanical properties of hydrogel core-shell particles. The effective elastic modulus of core-shell particles was determined from force spectroscopy measurements using the colloidal probe-AFM (CP-AFM) technique. By varying the shell thickness between 10 and 24 µm, the elastic modulus of particles can be tuned in the range of 10-190 kPa and further increased by increasing the layer number. Through fluorescence quantitative measurements, the polymeric shell density was found to increase together with shell thickness and layer number, hence establishing a positive correlation between elastic modulus and shell density of core-shell particles. This is a valuable method for constructing multidensity or single-density shells of tunable thickness and is particularly important in mechanobiology as studies have reported enhanced cellular uptake of particles in the low-kilopascal range (<140 kPa). We anticipate that our results will provide the first steps toward the rational design of core-shell particles for the separation of biomolecules or systemic study of stiffness-dependent cellular uptake.

Artificial microniches for probing mesenchymal stem cell fate in 3D.

Droplet microfluidics is combined with bio-orthogonal thiol-ene click chemistry to fabricate micrometer-sized, monodisperse fibrinogen-containing hyaluronic acid hydrogel microbeads in a mild, radical-free procedure in the presence of human mesenchymal stem cells (hMSCs). The gel beads serve as microniches for the 3D culture of single hMSCs, containing hyaluronic acid and additional fibrinogen for cell surface binding, and they are porous and stable in tissue culture medium for up to 4 weeks with mechanical properties right in the range of soft solid tissues (0.9-9.2 kPa). The encapsulation procedure results in 70% viable hMSCs in the microbeads after 24 hours of culture and a very high degree of viability of the cells after long term culture of 2 weeks. hMSCs embedded in the microniches display an overall rounded morphology, consistent with those previously observed in 3D culture. Upon induction, the multipotency and differentiation potential of the hMSCs are characterized by staining of corresponding biomarkers, demonstrating a clear heterogeneity in the cell population. These hydrogel microbeads represent a versatile microstructured material platform with great potential for studying the differences of material cues and soluble factors in stem cell differentiation in a 3D tissue-like environment at the single cell level.

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.

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.

Micromechanical characterization of spider silk particles.

Spider silk fibers are well known for their mechanical properties, and they are therefore in the focus of materials scientists. Additionally, silks display biocompatibility making them interesting materials for applications in medicine or cosmetics. Due to the low abundance of natural spider silk proteins because of the spider's cannibalism, the recombinant spider silk protein eADF4 has been established for material science applications. Once processed into micron-sized particles by controlled salting-out, these particles can be used as drug delivery vehicles. For any application of the silk particles it is important to know their mechanical characteristics for processing and storage reasons. Here, we examine the swelling behavior and mechanics of these particles. Upon hydration, a drastic drop in elastic modulus occurs by orders of magnitude, from 0.8 GPa in the dry state to 2.99 MPa in the wet state. Importantly, the elastic modulus of recombinant silk particles can be tuned by varying the molecular weight of the used proteins, as well as chemical crosslinking thereof.

Monodisperse collagen-gelatin beads as potential platforms for 3D cell culturing.

A droplet-based microfluidics technique is used to produce monodisperse, 80 µm collagen-gelatin beads with tunable mechanical properties in the range of 1-10 kPa after photo-crosslinking. The gel beads are porous, mechanically robust and stable in buffer, but can be degraded enzymatically. Encapsulated fibroblast cells maintain 70% viability after one-week encapsulation and preliminary results show that the degree of spreading of cells in gels is correlated with the stiffness of the material.