Max Julius Männel

Concentration gradients of soluble signaling molecules—morphogens—determine the cellular organization in tissue development. Morphogen‐releasing microgels have shown potential to recapitulate this principle in engineered tissue constructs, however, with limited control over the molecular cues in space and time. Inspired by the functionality of sulfated glycosaminoglycans (sGAGs) in morphogen signaling in vivo, a library of sGAG‐based microgels is developed and designated as µGel Units to Instruct Development (µGUIDEs). Adjustment of the microgel's sGAG sulfation patterns and concentration enabled the programming of electrostatic affinities that control the release of morphogens. Based on computational analyses of molecular transport processes, µGUIDEs provided unprecedented precision in the spatiotemporal modulation of vascular endothelial growth factor (VEGF) gradients in a microgel‐in‐gel vasculogenesis model and kidney organoid cultures. The versatile approach offers new options for creating morphogen signaling centers to advance the understanding of tissue and organ development.

Droplet microfluidics—the art and science of forming droplets—has been revolutionary for high-throughput screening, directed evolution, single-cell sequencing, and material design. However, traditional fabrication techniques for microfluidic devices suffer from several disadvantages, including multistep processing, expensive facilities, and limited three-dimensional (3D) design flexibility. High-resolution additive manufacturing—and in particular, projection micro-stereolithography (PµSL)—provides a promising path for overcoming these drawbacks. Similar to polydimethylsiloxane-based microfluidics 20 years ago, 3D printing methods, such as PµSL, have provided a path toward a new era of microfluidic device design. PµSL greatly simplifies the device fabrication process, especially the access to truly 3D geometries, is cost-effective, and it enables multimaterial processing. In this review, we discuss both the basics and recent innovations in PµSL; the material basis with emphasis on custom-made photopolymer formulations; multimaterial 3D printing; and, 3D-printed microfluidic devices for emulsion formation as our focus application. Our goal is to support researchers in setting up their own PµSL system to fabricate tailor-made microfluidics.

We introduce a novel concept for mechanosensitive hydrogel microparticles, which translate deformation into changes in fluorescence and can thus function as mechanical probes. The hydrogel particles with controlled polymer network are produced via droplet microfluidics from poly(ethylene glycol) (PEG) precursors. Förster resonance energy transfer donors and acceptors are coupled to the PEG hydrogel network for reporting local deformations as fluorescence shifts. We show that global network deformations, which occur upon drying/rehydration, can be detected via a characteristic fluorescence shift. Combined characterization with confocal laser scanning microscopy and atomic force microscopy (AFM) shows that also local deformation of the particles can be detected. Using AFM, the mechanical properties of the particles can be quantified, which allows linking strain with stress and thus force sensing in a three-dimensional environment. Microfluidic material design allows for precisely varying the size of our hydrogel microparticles as well as their mechanical properties and polymer network structure with regard to the choice of the macromolecular precursors and their functionalization with fluorophores. Thus, concomitant changes in mechanical properties and mechanosensitivity qualify these hydrogel microparticles as an adjustable material platform for force sensing in structural mechanics or cell culturing.

Here, we demonstrate the additive manufacturing of two key microvalve designs, namely Nordin’s and Quake’s microvalves, based on a formulation consisting of tri(propylene glycol) diacrylate (TPGDA) as a base material, diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO) as a photoinitiator and Sudan1 as the UV-absorber via micro-stereolithography (μSL). Mechanical measurements of test prints show an average Young’s modulus of 15.7 MPa, which is eight times lower compared to several previous studies on 3D-printed microvalves and micropumps based on poly(ethylene glycol diacrylate) 255 (PEGDA-252). We use a high-resolution Cerafab7500 printer (Lithoz GmbH, Vienna) with a minimal lateral resolution of 10.3 μm to print membrane valves with voxel dimensions down to 60μm. Particularly, we study the effect of different comonomers added to the photopolymer formulation – neopentyl glycol propoxylate (1 PO/OH) diacrylate (NPGPDA), 1,6- hexanediol diacrylate (HDDA) and 2-phenoxyethyl acrylate (POEA) – on the layer thickness, which is identified to be a crucial parameter. 3D-printed valves are tested regarding maximum operating pressure withstanding pressures of up to 5 bar. We show that TPGDA-based resins combine high flexibility, mechanical stability, and sufficient resolution for the future design of flow control units in microfluidics.

The fabrication of microfluidic devices with nonplanar microchannel design by micro‐stereolithography (µSL) for oil‐in‐water (O/W) and water‐in‐oil (W/O) single emulsion as well as oil‐in‐water‐in‐oil (O/W/O) and water‐in‐oil‐in‐water (W/O/W) double emulsion formation is presented. By investigating separation distance, printing direction and voxel compensation in X,Y‐plane as key printing parameters, microflow cells with channel cross‐sections varying from 500 to 75 µm are printed, which are thus truly similar to those yielded by conventional stamped microfluidics. Without the need of controlling microchannel wettability, these flow cells are employed for forming O/W and W/O microdroplets, ≈130–80 µm in diameter and narrow size distribution of 2%, and at frequencies of up to 2.9 kHz. The as‐formed emulsion droplets are then utilized as templates for preparing hydrophobic and hydrophilic polymer microparticles, respectively. Likewise, O/W/O and W/O/W double emulsions are prepared in a single microflow cell to extend the applicability of nonplanar, 3D‐printed microfluidic devices. For each set of experiments, the same single flow cell is used to prove the reusability and robustness of the 3D‐printed flow cells compared to microfluidic flow cells fabricated by conventional, combined photo‐ and soft‐lithography.

Enzyme-catalyzed controlled radical polymerization represents a powerful approach for the polymerization of a wide variety of water-soluble monomers. However, in such an enzyme-based polymerization system, the macromolecular catalyst (i.e., enzyme) has to be separated from the polymer product. Here, we present a compelling approach for the separation of the two macromolecular species, by taking the catalyst out of the molecular domain and locating it in the colloidal domain, ensuring quasi-homogeneous catalysis as well as easy separation of precious biocatalysts. We report on gold nanoparticles coated with horseradish peroxidase that can catalyze the polymerization of various monomers (e.g., N-isopropylacrylamide), yielding thermoresponsive polymers. Strikingly, these biocatalyst-coated nanoparticles can be recovered completely and reused in more than three independent polymerization cycles, without significant loss of their catalytic activity.

We investigate the effect of surfactant-mediated, asymmetric silver overgrowth of gold nanorods on their intrinsic optical properties. From concentration-dependent experiments, we established a close correlation of the extinction in the UV/vis/NIR frequency range and the morphological transition from gold nanorods to [email protected] cuboids. Based on this correlation, a generic methodology for in situ monitoring of the evolution of the cuboid morphology was developed and applied in time-dependent experiments. We find that growth rates are sensitive to the substitution of the surfactant headgroup by comparison of benzylhexadecyldimethylammonium chloride (BDAC) with hexadecyltrimethylammonium chloride (CTAC). The time-dependent overgrowth in BDAC proceeds about 1 order of magnitude slower than in CTAC, which allows for higher control during silver overgrowth. Furthermore, silver overgrowth results in a qualitatively novel optical feature: Upon excitation inside the overlap region of the interband transition of gold and intraband of silver, the gold core acts as a retarding element. The much higher damping of the gold core compared to the silver shell in [email protected] cuboids induces mirror charges at the core/shell interface as shown by electromagnetic simulations. Full control over the kinetic growth process consequently allows for precise tailoring of the resonance wavelengths of both modes. Tailored and asymmetric silver-overgrown gold nanorods are of particular interest for large-scale fabrication of nanoparticles with intrinsic metamaterial properties. These building blocks could furthermore find application in optical sensor technology, light harvesting, and information technology.

In this work, we investigate the ligand exchange of cetyltrimethylammonium bromide (CTAB) with bovine serum albumin for gold nanorods. We demonstrate by surface-enhanced Raman scattering measurements that CTAB, which is used as a shape-directing agent in the particle synthesis, is completely removed from solution and particle surface. Thus, the protein-coated nanorods are suitable for bioapplications, where cationic surfactants must be avoided. At the same time, the colloidal stability of the system is significantly increased, as evidenced by spectroscopic investigation of the particle longitudinal surface plasmon resonance, which is sensitive to aggregation. Particles are stable at very high concentrations (c_Au 20 mg/mL) in biological media such as phosphate buffer saline or Dulbecco’s Modified Eagle’s Medium and over a large pH range (2–12). Particles can even be freeze-dried (lyophilized) and redispersed. The protocol was applied to gold nanoparticles with a large range of aspect ratios and sizes with main absorption frequencies covering the visible and the near-IR spectral range from 600 to 1100 nm. Thus, these colloidally stable and surfactant-free protein-coated nanoparticles are of great interest for various plasmonic and biomedical applications.