Bioinspired Control of Calcium Phosphate Liesegang Patterns Using Anionic Polyelectrolytes

Biomimetic Calcium Phosphate Nanoparticles: Biomineralization Models and Precursors for Composite Materials

The formation of calcium phosphate under the control of water-soluble polymers is important for understanding bone growth in living organisms. These experiments also have spin-offs in the creation of composite materials, including for regenerative medicine applications. The formation of calcium phosphate (hydroxyapatite) from calcium chloride and diammonium phosphate was studied in the presence of polymers containing carboxyl, amine, and imidazole groups. Depending on the polymer composition, solid products and stable dispersions of positively or negatively charged nanoparticles were obtained. Oppositely charged nanoparticles can interact with each other to form a macroporous composite material, which holds promise as a filler for bone defects. The formation of a calcium phosphate layer around a living cell (dinoflagellate Gymnodinium corollarium A. M. Sundström, Kremp et Daugbjerg) using positive composite nanoparticles is a one-step approach to cell mineralization.

Anisotropic Liesegang pattern for the nonlinear elastic biomineral-hydrogel complex

The Liesegang pattern is a beautiful natural anisotropic patterning phenomenon observed in rocks and sandstones. This study reveals that the Liesegang pattern can induce nonlinear elasticity. Here, a Liesegang-patterned complex with biomineral-hydrogel repetitive layers is prepared. This Liesegang-patterned complex is obtained only when the biomineralization is performed under the supersaturated conditions. The Liesegang-patterned complex features a nonlinear elastic response, whereas a complex with a single biomineral shell shows a linear behavior, thus demonstrating that the Liesegang pattern is essential in achieving nonlinear elasticity. The stiff biomineral layers have buffered the concentrated energy on behalf of soft hydrogels, thereby exposing the hydrogel components to reduced stress and, in turn, enabling them to perform the elasticity continuously. Moreover, the nonlinear elastic Liesegang-patterned complex exhibits excellent stress relaxation to the external loading, which is the biomechanical characteristic of cartilage. This stress relaxation allows the bundle of fiber-type Liesegang-patterned complex to endure greater deformation.

A microfluidic double emulsion platform for spatiotemporal control of pH and particle synthesis

The temporal control of pH in microreactors such as emulsion droplets plays a vital role in applications including biomineralisation and microparticle synthesis. Typically, pH changes are achieved either by passive diffusion of species into a droplet or by acid/base producing reactions. Here, we exploit an enzyme reaction combined with the properties of a water-oil-water (W/O/W) double emulsion to control the pH-time profile in the droplets. A microfluidic platform was used for production of ∼100-200 μm urease-encapsulated double emulsions with a tuneable mineral oil shell thickness of 10-40 μm. The reaction was initiated on-demand by addition of urea and a pulse in base (ammonia) up to pH 8 was observed in the droplets after a time lag of the order of minutes. The pH-time profile can be manipulated by the diffusion timescale of urea and ammonia through the oil layer, resulting in a steady state pH not observed in bulk reactive solutions. This approach may be used to regulate the formation of pH sensitive materials under mild conditions and, as a proof of concept, the reaction was coupled to calcium phosphate precipitation in the droplets. The oil shell thickness was varied to select for either brushite microplatelets or hydroxyapatite particles, compared to the mixture of different precipitates obtained in bulk.

Non-equilibrium-Growing Aesthetic Ionic Skin for Fingertip-Like Strain-Undisturbed Tactile Sensation and Texture Recognition

Humans use periodically ridged fingertips to precisely perceive the characteristics of objects via ion-based fast- and slow-adaptive mechanotransduction. However, designing artificial ionic skins with fingertip-like tactile capabilities remains challenging because of the contradiction between structural compliance and pressure sensing accuracy (e.g., anti-interference from stretch and texture recognition). Inspired by the formation and modulus-contrast hierarchical structure of fingertips, an aesthetic ionic skin grown from a non-equilibrium Liesegang patterning process is introduced. This ionic skin with periodic stiff ridges embedded in a soft hydrogel matrix enables strain-undisturbed triboelectric dynamic pressure sensing as well as vibrotactile texture recognition. By coupling with another piezoresistive ionogel, an artificial tactile sensory system is further fabricated as a soft robotic skin to mimic the simultaneous fast- and slow-adaptive multimodal sensations of fingers in grasping actions. This approach may inspire the future design of high-performance ionic tactile sensors for intelligent applications in soft robotics and prosthetics.