3D Bio-Printability of Hybrid Pre-Crosslinked Hydrogels

Maintaining shape fidelity of 3D bio-printed scaffolds with soft biomaterials is an ongoing challenge. Here, a rheological investigation focusing on identifying useful physical and mechanical properties directly related to the geometric fidelity of 3D bio-printed scaffolds is presented. To ensure during- and post-printing shape fidelity of the scaffolds, various percentages of Carboxymethyl Cellulose (CMC) (viscosity enhancer) and different calcium salts (CaCl2 and CaSO4, physical cross-linkers) were mixed into alginate before extrusion to realize shape fidelity. The overall solid content of Alginate-Carboxymethyl Cellulose (CMC) was limited to 6%. A set of rheological tests, e.g., flow curves, amplitude tests, and three interval thixotropic tests, were performed to identify and compare the shear-thinning capacity, gelation points, and recovery rate of various compositions. The geometrical fidelity of the fabricated scaffolds was defined by printability and collapse tests. The effect of using multiple cross-linkers simultaneously was assessed. Various large-scale scaffolds were fabricated (up to 5.0 cm) using a pre-crosslinked hybrid. Scaffolds were assessed for the ability to support the growth of Escherichia coli using the Most Probable Number technique to quantify bacteria immediately after inoculation and 24 h later. This pre-crosslinking-based rheological property controlling technique can open a new avenue for 3D bio-fabrication of scaffolds, ensuring proper geometry.

Optical interferometry based micropipette aspiration provides real-time sub-nanometer spatial resolution

Micropipette aspiration (MPA) is an essential tool in mechanobiology; however, its potential is far from fully exploited. The traditional MPA technique has limited temporal and spatial resolution and requires extensive post processing to obtain the mechanical fingerprints of samples. Here, we develop a MPA system that measures pressure and displacement in real time with sub-nanometer resolution thanks to an interferometric readout. This highly sensitive MPA system enables studying the nanoscale behavior of soft biomaterials under tension and their frequency-dependent viscoelastic response.

Author Correction: Assessment of nanoindentation in stiffness measurement of soft biomaterials: kidney, liver, spleen and uterus

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Programmable Viscoelasticity in Protein-RNA Condensates with Disordered Sticker-Spacer Polypeptides

The rheological properties of biological matters play a fundamental role in many cell processes. At the organelle level, liquid-liquid phase separation of multivalent proteins and RNAs drives the formation of biomolecular condensates that facilitate dynamic compartmentalization of cellular biochemistry1. With recent advances, it is becoming increasingly clear that the structure and rheological properties of these condensates are critical to their cellular functions2,3. Meanwhile, aberrant liquid-to-solid transitions in some cellular condensates are implicated in neurodegenerative disorders4. Within the limits of two extreme material states, viz., viscous liquid and amorphous or fibrillar solid, there lies a spectrum of materials known as viscoelastic fluids. Viscoelastic fluids behave as an elastic solid at time-scales shorter than their network reconfiguration time but as a viscous fluid at longer time-scales. Viscoelasticity of biomolecular condensates may constitute an adaptive mechanism for sensing mechanical stress and regulating biochemical processes5. From an engineering standpoint, viscoelastic fluids hold great potential for designing soft biomaterials with programmable mechanosensitivity6,7. Here, employing microrheology with optical tweezers, we demonstrate how multivalent disordered sticker-spacer8,9 polypeptides undergoing associative phase separation with RNA can be designed to control the frequency-dependent viscoelastic behavior of their condensates. Utilizing linear repeat polypeptides inspired by natural RNA-binding sequences, we show that polypeptide-RNA condensates behave as Maxwell fluids, with viscoelastic behavior that can be fine-tuned by the identity of the sticker and spacer residues. The sequence heuristics that we uncovered allowed us to create biomolecular condensates spanning two orders of magnitude in their viscous and elastic responses to the applied mechanical stress. This sequence-encoded regulation of viscoelasticity in disordered polypeptide-RNA condensates establishes a link between the molecular architecture of the polypeptide chains and the rheological properties of the resulting condensates at the mesoscale, enabling a route to engineer soft biomaterials with programmable mechanics.

Electrospun hydrogels for dynamic culture systems: advantages, progress, and opportunities

Hydrogel nanofibers build on established soft biomaterials to enable design and control of unique, dynamic cell culture systems.