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Nanomechanical biosensors refer to a subfamily of micro-electromechanical systems (MEMS) consisting of movable suspended microstructures able to convert biological processes into measurable mechanical motion. Owing to this, nanomechanical biosensors have become a promising technology in the way to detect and manage bacterial pathogens with improved effectiveness. The precise treatment of an infection relies on its early diagnosis; however, the current standard culture-based methods for bacteria detection and antibiotic susceptibility testing involve long protocols and are labor intensive. Thanks to its high sensitivity, fast response, and high throughput capability, nanomechanical technology holds great potential for overcoming some of the limitations of conventional methods. This review aims to provide a perspective on the diverse transducer structures, working principles, and detection strategies of nanomechanical sensors for bacteria detection and antibiotic susceptibility testing. Their performance in terms of sensitivity and operation time is compared with standard methods currently used in clinical microbiology laboratories. In addition, commercial systems already developed and challenges in the way of reaching real sensing application beyond the research environment are discussed.

Materials science offers new perspectives in the clinical analysis of antimicrobial sensitivity. However, a biomaterial with the capacity to respond to living bacteria has not been developed to date. We present an electrochromic iron(III)-complexed alginate hydrogel sensitive to bacterial metabolism, here applied to fast antibiotic-susceptibility determination. Bacteria under evaluation are entrapped -and pre-concentrated- in the hydrogel matrix by oxidation of iron (II) ions to iron (III) and in situ formation of the alginate hydrogel in less than 2min and in soft experimental conditions (i.e. room temperature, pH 7, aqueous solution). After incubation with the antibiotic (10min), ferricyanide is added to the biomaterial. Bacteria resistant to the antibiotic dose remain alive and reduce ferricyanide to ferrocyanide, which reacts with the iron (III) ions in the hydrogel to produce Prussian Blue molecules. For a bacterial concentration above 10 colony forming units per mL colour development is detectable with the bare eye in less than 20min. The simplicity, sensitivity, low-cost and short response time of the biomaterial and the assay envisages a high impact of these approaches on sensitive sectors such as public health system, food and beverage industries or environmental monitoring.

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In biosensors development, alginate hydrogels are a first choice for enabling stable biomolecules entrapment in biocompatible membranes obtained under soft physiological conditions. Although widely exploited, most alginate membranes are isolating and poorly repetitive, which limit their application in biosensing. Significant steps forward on improving repeatability and conductivity have been performed, but to date there is no single protocol for controlled deposition of live cells in replicable conductive alginate layers. Here, cell electrotrapping in conductive alginate hydrogels is examined in order to overcome these limitations. Conductive alginate-coated electrodes are obtained after potentiostatic electrodeposition of graphite-doped alginate samples (up to 4% graphite). The presence of graphite reduces electrode passivation and improves the electrochemical response of the sensor, although still significantly lower than that recorded with the naked electrode. Bacterial electrotrapping in the conductive matrix is highly efficient (4.4 × 10 cells per gel) and repetitive (CV < 0.5%), and does not compromise bacterial integrity or activity (cell viability = 56%). Biosensing based on ferricyanide respirometry yielded a four times increase in biosensor response with respect to non-conductive alginate membrane, providing toxicity values completely comparable to those reported. Cell electrotrapping in conductive hydrogels represents a step forward towards in high-sensitive cell-based biosensors development with important influence in environmental analysis, food and beverage industry as well as clinical diagnosis.

respond to one or more external stimuli (e.g., light, temperature, strain, humidity, pH or electromagnetic fields) by changing their own properties (e.g., shape, size, viscosity, stiffness, color, among others). In this framework, soft optomechanical materials have the capacity to merge the unique optical properties of plasmonic and/or photonic structures, carbon-based nanomaterials (CNMs) or macromolecules with the high tunability and elasticity of soft polymers. Given the high transparency of many soft polymers in the visible and near-infrared (NIR) spectral ranges, together with the possibility to incorporate such optical structures with high compliancy in polymers that can withstand large mechanical deformations, makes this combination of optical and mechanical features ideal for the development of functional optomechanical devices. [1] Soft optomechanical systems can be designed to change their optical properties under mechanical stimuli induced, for example, by strain, temperature, pH and so on, to develop cost-effective and highly sensitive sensors and optical modulators handling large strains, keeping their properties after many deformation cycles and adapting to complex and irregular surfaces. Note that, for example, sensors detecting mechanical deformations (e.g., strain or displacements) are required in structures such as buildings, vehicles, packages or wearable devices, where it is necessary to determine the experienced mechanical stress. Conversely, mechanical sensors can be used as transducers to detect other kind of external stimuli (e.g., chemical, biochemical, optical, to mention a few) that are able to produce mechanical deformations in the material. This category includes, e.g., some biosensors and micro-electromechanical systems (MEMS) that have been widely developed during the last decade. Soft mechanical sensors require a compliant transducer, being the electrical and optical transduction methods the most widely developed. However, optical transduction offers several advantages, particularly, its wireless detection nature, high sensitivity, easy readout, and the capacity to provide 2D strain mapping, even with sub-micron scale spatial resolution. [2][3][4] The high optical tunability of soft optomechanical systems by external mechanical stimuli can also be exploited to build mechanical modulators to modify the color or transparency of surfaces, having great potential in the development of smart windows, [5] rewritable optical displays, encryption, and anticounterfeiting applications, [6] among others. [7]

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