Proteins are the fundamental building blocks for high-performance materials in nature. Such materials fulfill structural roles, as in the case of silk and collagen, and can generate active structures including the cytoskeleton. Attention is increasingly turning to this versatile class of molecules for the synthesis of next-generation green functional materials for a range of applications. Protein nanofibrils are a fundamental supramolecular unit from which many macroscopic protein materials are formed. In this Review, we focus on the multiscale assembly of such protein nanofibrils formed from naturally occurring proteins into new supramolecular architectures and discuss how they can form the basis of material systems ranging from bulk gels, films, fibers, micro/nanogels, condensates, and active materials. We review current and emerging approaches to process and assemble these building blocks in a manner which is different to their natural evolutionarily selected role but allows the generation of tailored functionality, with a focus on microfluidic approaches. We finally discuss opportunities and challenges for this class of materials, including applications that can be involved in this material system which consists of fully natural, biocompatible, and biodegradable feedstocks yet has the potential to generate materials with performance and versatility rivalling that of the best synthetic polymers.

A vital challenge in complex organ manufacturing is to vascularize large combined tissues. The aim of this study is to vascularize in vitro an adipose-derived stem cell (ADSC)/fibrin/collagen incorporated three-dimensional (3D) poly(d,l-lactic-co-glycolic acid) (PLGA) scaffold (10 × 10 × 10 mm ) with interconnected channels. A low-temperature 3D printing technique was employed to build the PLGA scaffold. A step-by-step cocktail procedure was designed to engage or steer the ADSCs in the PLGA channels towards both endothelial and smooth muscle cell lineages. The combined system had sufficient mechanical properties to support the cell/fibrin/collagen hydrogel inside the predefined PLGA channels. The ADSCs encapsulated in the fibrin/collagen hydrogel differentiated to endothelial and smooth muscle cell lineage, respectively, corresponding to their respective locations in the construct and formed vascular-like structures. This technique allows in vitro vascularization of the predefined PLGA channels and provides a choice for complex organ manufacture. Copyright © 2014 John Wiley & Sons, Ltd.

Abstract3D scaffolds in the form of hydrogels and microgels have allowed for more native cell‐culture systems to be developed relative to flat substrates. Native biological tissues are, however, usually spatially inhomogeneous and anisotropic, but regulating the spatial density of hydrogels at the microscale to mimic this inhomogeneity has been challenging to achieve. Moreover, the development of biocompatible synthesis approaches for protein‐based microgels remains challenging, and typical gelation conditions include UV light, extreme pH, extreme temperature, or organic solvents, factors which can compromise the viability of cells. This study addresses these challenges by demonstrating an approach to fabricate protein microgels with controllable radial density through microfluidic mixing and physical and enzymatic crosslinking of gelatin precursor molecules. Microgels with a higher density in their cores and microgels with a higher density in their shells are demonstrated. The microgels have robust stability at 37 °C and different dissolution rates through enzymolysis, which can be further used for gradient scaffolds for 3D cell culture, enabling controlled degradability, and the release of biomolecules. The design principles of the microgels could also be exploited to generate other soft materials for applications ranging from novel protein‐only micro reactors to soft robots.

Three-dimensional (3D) cell manipulation is available with the integration of microfluidic technology and rapid prototyping techniques. High-Fidelity (Hi-Fi) constructs hold enormous therapeutic potential for organ manufacturing and regenerative medicine. In the present paper we introduced a quasi-three-dimensional (Q3D) model with parallel biocompatible alginate/gelatin/fibrin hurdles. The behaviors of fluids and cells along the microfluidic channels with various widths were studied. Cells inside the newly designed microfluidic channels attached and grew well. Morphological changes of adipose-derived stem cells (ADSCs) in both two-dimensional (2D) and 3D milieu were found on the printed constructs. Endothelialization occurred with the co-cultures of ADSCs and hepatocytes. This study provides insights into the interactions among fluids, cells and biomaterials, the behaviors of fluids and cells along the microfluidic channels, and the applications of Q3D techniques.

Microcapsules are a key class of microscale materials with applications in areas ranging from personal care to biomedicine, and with increasing potential to act as extracellular matrix (ECM) models of hollow organs or tissues. Such capsules are conventionally generated from non-ECM materials including synthetic polymers. Here, we fabricated robust microcapsules with controllable shell thickness from physically-and enzymatically-crosslinked gelatin and achieved a core-shell architecture by exploiting a liquid-liquid phase separated aqueous dispersed phase system in a one-step microfluidic process. Microfluidic mechanical testing revealed that the mechanical robustness of thicker-shell capsules could be controlled through modulation of the shell thickness. Furthermore, the microcapsules demonstrated environmentally-responsive deformation, including buckling by osmosis and external mechanical forces. Finally, a sequential release of cargo species was obtained through the degradation of the capsules. Stability measurements showed the capsules were stable at 37 • C for more than two weeks. These smart capsules are promising models of hollow biostructures, microscale drug carriers, and building blocks or compartments for active soft materials and robots.

The assembly of intracellular proteins into biomolecular condensates via liquid-liquid phase separation (LLPS) has emerged as a fundamental process underlying the organisation and regulation of cellular space and function. Physicochemical characterisation of the parameters that control and modulate phase separation is therefore essential for an improved understanding of protein phase behaviour, including for the therapeutic modulation of LLPS phenomena. A fundamental measure with which to describe protein phase behaviour in chemical space is the phase diagram. Characterisation of phase diagrams requires measuring the presence or absence of the condensed phase under a multitude of conditions and, as such, is associated with significant consumption of time and sample volume even when performed in microwell format.However, due to the rapidly increasing number of biologically and disease-relevant condensate systems, experimental techniques that enable high-throughput analysis of protein phase behaviour are required. To address this challenge, we present here a combinatorial droplet microfluidic platform, termed PhaseScan, for the rapid and high-resolution acquisition of protein phase diagrams. Using this platform, we demonstrate characterisation of the phase behaviour of a pathologically relevant mutant of the protein fused in sarcoma (FUS) in a highly parallelised manner, with significantly improved assay throughput and reduced sample consumption. We demonstrate the capability of the platform by finding the phase boundary at which FUS transitions from a one-phase to a two-phase state as modulated by 1,6-hexanediol, and estimate the free-energy landscape of this system using Flory-Huggins theory. These results thus provide a basis for the rapid acquisition of phase diagrams through the application of microdroplet techniques and pave the way for a wide range of applications, enabling rapid characterisation of the effect of environmental conditions and coacervate species on the thermodynamics of phase separation.

How to release growth factors (GFs) scientifically to promote stem cell proliferation and differentiation is one of the most significant research focuses in the field of regenerative medicine. In a controlled release system, growth factors, extracellular matrices or biomaterial carriers, and sometimes stem cells together form a geometric entirety. Biomaterial carriers provide GFs with a support structure to be adhered, immobilized, encapsulated or/and protected. As a unity, the release rate and rhythm of GFs on cells are normally very delicate and precise. Up to now, the best strategy for clinical applications is the combination systems that encapsulate GFs in microspheres, particularly the nano- or micro-encapsulation techniques integrated GFs with biomaterial carriers. In this mini review, we summarize the current progress in GF delivery systems for regenerative medicine and provide an outlook on two main aspects: one is the classes of stem cells and GFs that have been used frequently in regenerative medicine, including their respective application conditions and functions; the other is the controlled GF release systems, in which various GFs are released orderly and continuously without diffusing simply and rapidly, including their respective opportunities and challenges.

Liquid–liquid phase‐separated biomolecular systems are increasingly recognized as key components in the intracellular milieu where they provide spatial organization to the cytoplasm and the nucleoplasm. The widespread use of phase‐separated systems by nature has given rise to the inspiration of engineering such functional systems in the laboratory. In particular, reversible gelation of liquid–liquid phase‐separated systems could confer functional advantages to the generation of new soft materials. Such gelation processes of biomolecular condensates have been extensively studied due to their links with disease. However, the inverse process, the gel–sol transition, has been less explored. Here, a thermoresponsive gel–sol transition of an extracellular protein in microgel form is explored, resulting in an all‐aqueous liquid–liquid phase‐separated system with high homogeneity. During this gel–sol transition, elongated gelatin microgels are demonstrated to be converted to a spherical geometry due to interfacial tension becoming the dominant energetic contribution as elasticity diminishes. The phase‐separated system is further explored with respect to the diffusion of small particles for drug‐release scenarios. Together, this all‐aqueous system opens up a route toward size‐tunable and monodisperse synthetic biomolecular condensates and controlled liquid–liquid interfaces, offering possibilities for applications in bioengineering and biomedicine.