Collagen gel contraction assays: From modelling wound healing to quantifying cellular interactions with three-dimensional extracellular matrices.

Cells respond to and actively remodel the extracellular matrix (ECM). The dynamic and bidirectional interaction between cells and ECM, especially their mechanical interactions, has been found to play an essential role in triggering a series of complex biochemical and biomechanical signal pathways and in regulating cellular functions and behaviours. The collagen gel contraction assay (CGCA) is a widely used method to investigate cell-ECM interactions in 3D environments and provides a mechanically associated readout reflecting 3D cellular contractility. In this review, we summarize various versions of CGCA, with an emphasis on recent high-throughput and low-consumption CGCA techniques. More importantly, we focus on the technique of force monitoring during the contraction of collagen gel, which provides a quantitative characterization of the overall forces generated by all the resident cells in the collagen hydrogel. Accordingly, we present recent biological applications of the CGCA, which have expanded from the initial wound healing model to other studies concerning cell-ECM interactions, including fibrosis, cancer, tissue repair and the preparation of biomimetic microtissues.

Hydrogel-based patient-friendly photodynamic therapy of oral potentially malignant disorders.

Oral potentially malignant disorders (OPMDs) are precursor lesions with an increased risk of malignant transformation. Topical photodynamic therapy (PDT) mediated by 5-aminolevulinic acid (ALA) (ALA-PDT) is a promising therapeutic method in the treatment of OPMDs. However, the clinical application of topical ALA-PDT is restricted by several limitations, including low delivery efficiency, poor comfort, and easy influence by saliva. Here, we designed a highly adhesion-strength dry polyacrylic acid (PAA)-chitosan (CHI)-ALA interpenetrating network hydrogel (PACA) patch after investigating the spatiotemporal dynamics of ALA drug delivery via diffusion-based finite-element models. The PACA patch could adhere to the moist oral mucosa fast and stably and deliver ALA. PACA hydrogel-mediated PDT (PACA-PDT) effectively improved OPMDs in vitro and in a hamster oral carcinogenesis model. In particular, we conducted a trial to recruit 60 OPMD volunteers to demonstrate the feasibility and comfort of the PACA hydrogel patch. This study provides evidence that PACA hydrogel-mediated PDT could be a patient-friendly treatment modality for OPMDs.

Electrically Programmable Interfacial Adhesion for Ultrastrong Hydrogel Bonding.

Adjustable interfacial adhesion is of great significance in smart-hydrogel-related engineering fields. This study presents an electroadhesion strategy for universal and ultrastrong hydrogel bonding with electrically programmable strength. An ionic hydrogel containing lithium ions is designed to achieve hydrated-ion-diffusion-mediated interfacial adhesion, where external electric fields are employed to precisely control spatiotemporal dynamics of the ion diffusion across ionic adhesion region (IAR). The hydrogel can realize a universal, ultrastrong, efficient, tough, reversible, and environmentally tolerant electroadhesion to diverse hydrogels, whose peak adhesion strength and interfacial adhesion toughness are as high as 1.2 MPa and 3750 J m-2 , respectively. With a mechanoelectric coupling model, the dominant role of the hydrated ions in IAR played in the interfacial electroadhesion is further quantitatively revealed. The proposed strategy opens a door for developing high-performance adhesion hydrogels with electrically programmable functions, which are indispensable for various emerging fields like flexible electronics and soft robotics.

Spontaneous formation and spatial self-organization of mechanically induced mesenchymal-like cells within geometrically confined cancer cell monolayers.

There is spatiotemporal heterogeneity in cell phenotypes and mechanical properties in tumor tissues, which is associated with cancer invasion and metastasis. It is well-known that exogenous growth factors like transforming growth factor (TGF)-ß, can induce epithelial-mesenchymal transition (EMT)-based phenotypic transformation and the formation of EMT patterning on geometrically confined monolayers with mechanics heterogeneity. In the absence of exogenous TGF-ß stimulation, however, whether geometric confinement-caused mechanics heterogeneity of cancer cell monolayers alone can trigger the EMT-based phenotypic heterogeneity still remains mysterious. Here, we develop a micropattern-based cell monolayer model to investigate the regulation of mechanics heterogeneity on the cell phenotypic switch. We reveal that mechanics heterogeneity itself is enough to spontaneously induce the emergence of mesenchymal-like phenotype and asymmetrical activation of TGF-ß-SMAD signaling. Spatiotemporal dynamics of patterned cell monolayers with mesenchymal-like phenotypes is essentially regulated by tissue-scale cell behaviors like proliferation, migration as well as heterogeneous cytoskeletal contraction. The inhibition of cell contraction abrogates the asymmetrical TGF-ß-SMAD signaling activation level and the emergence of mesenchymal-like phenotype. Our work not only sheds light on the key regulation of mechanics heterogeneity caused by spatially geometric confinement on regional mesenchymal-like phenotype of cancer cell monolayers, but highlights the key role of biophysical/mechanical cues in triggering phenotypic switch.

Probiotic Properties of Chicken-Derived Highly Adherent Lactic Acid Bacteria and Inhibition of Enteropathogenic Bacteria in Caco-2 Cells.

Lactic acid bacteria (LAB) as probiotic candidates have various beneficial functions, such as regulating gut microbiota, inhibiting intestinal pathogens, and improving gut immunity. The colonization of the intestine is a prerequisite for probiotic function. Therefore, it is necessary to screen the highly adherent LAB. In this study, the cell surface properties, such as hydrophobicity, auto-aggregation, co-aggregation, and adhesion abilities of the six chicken-derived LAB to Caco-2 cells were investigated. All six strains showed different hydrophobicity (21.18-95.27%), auto-aggregation (13.61-30.17%), co-aggregation with Escherichia coli ATCC 25922 (10.23-36.23%), and Salmonella enterica subsp. enterica serovar Typhimurium ATCC 13311 (11.71-39.35%), and adhesion to Caco-2 cells (8.57-26.37%). Pediococcus pentosaceus 2-5 and Lactobacillus reuteri L-3 were identified as the strains with strong adhesion abilities (26.37% and 21.57%, respectively). Moreover, these strains could survive in a gastric acid environment at pH 2, 3, and 4 for 3 h and in a bile salt environment at 0.1%, 0.2%, and 0.3% (w/v) concentration for 6 h. Furthermore, the cell-free supernatant of P. pentosaceus 2-5 and L. reuteri L-3 inhibited the growth of enteropathogenic bacteria and the strains inhibited the adhesion of these pathogens to Caco-2 cells. In this study, these results suggested that P. pentosaceus 2-5 and L. reuteri L-3, isolated from chicken intestines might be good probiotic candidates to be used as feed additives or delivery vehicles of biologically active substances.

Extracellular matrix stiffness modulates host-bacteria interactions and antibiotic therapy of bacterial internalization.

Pathogenic bacteria evolve multiple strategies to hijack host cells for intracellular survival and persistent infections. Previous studies have revealed the intricate interactions between bacteria and host cells at genetic, biochemical and even single molecular levels. Mechanical interactions and mechanotransduction exert a crucial impact on the behaviors and functions of pathogenic bacteria and host cells, owing to the ubiquitous mechanical microenvironments like extracellular matrix (ECM) stiffness. Nevertheless, it remains unclear whether and how ECM stiffness modulates bacterial infections and the sequential outcome of antibacterial therapy. Here we show that bacteria tend to adhere to and invade epithelial cells located on the regions with relatively high traction forces. ECM stiffness regulates spatial distributions of bacteria during the invasion through arrangements of F-actin cytoskeletons in host cells. Depolymerization of cytoskeletons in the host cells induced by bacterial infection decreases intracellular accumulation of antibiotics, thus preventing the eradication of invaded bacterial pathogens. These findings not only reveal the key regulatory role of ECM stiffness, but suggest that the coordination of cytoskeletons may provide alternative approaches to improve antibiotic therapy against multidrug resistant bacteria in clinic.

Strong underwater adhesion of injectable hydrogels triggered by diffusion of small molecules.

It is challenging for injectable hydrogels to achieve high underwater adhesiveness. Based on this concern, we report a fully physically crosslinked injectable hydrogel composed of gelatin, tea polyphenols and urea, capable of realising smart adhesion to various materials, like glass and porcine skin, in diverse aqueous environments. The urea molecules are designed as crosslinking disruptors for interfering with the formation of hydrogen bonds in the hydrogel, therefore modulating its crosslinking density and mechanical properties such as tensile strength, toughness and adhesive strength. Triggered by physical diffusion of the urea molecules towards the surrounding liquid environment, the hydrogel can achieve efficient (∼10 s), self-strengthening and long-lasting (>2 weeks) underwater adhesion. Remarkably, for fresh porcine skin, the instantaneous underwater adhesive strength is 10.4 kPa whereas the peak strength is as high as 152.9 kPa with the aid of the self-strengthening effect. More interestingly, it can simultaneously form controllable underwater non-adhesive surfaces, regulated by changes in the diffusion-triggered local concentration of urea. Further, it is also biocompatible, antibacterial, biodegradable and 3D printable in water, which offers great convenience for various applications concerning smart interfacial adhesion, like biomedicine and flexible electronics. Likewise, the physical diffusion-mediated mechanism represents an innovative strategy for developing next-generation smart hydrogels.