Paper [3] reviews the state-of-the-art of gas foil bearings in China, many of which have been developed for use in high-speed turbo machinery. Different types of foil bearings are described (bump, multi-leaf, wire, viscoelastic, spring and protuberant), as are their different applications (e.g., fuel cell air compressors, blowers, turbo-expanders, and oil-free turbochargers). The addressed challenges concern thermal management, rotor-dynamic stability, and wear-resistant coatings.  
High-tech industry products such as solar panels, chips, and displays are composed of very thin layers of brittle materials that need to be handled with care. An original solution to handle the thin substrates without mechanical contact is proposed in [4]. The system works by floating the substrate on a thin film of air and by creating a viscous traction force on it. The actuator is improved to reach a bandwidth of 300 Hz, while the position control loop of the substrate reaches a bandwidth of 10 Hz with a position error of less than 13 \(\upmu\)m. It is a good example of the so-called tribotronics discipline.  
It was shown in the literature that the radial error motion of an aerostatic journal bearing is mainly influenced by the shape errors of the shaft. The influence of the shaft roundness and the cylindricity errors on the aerostatic spindle's rotation accuracy is evaluated in [5]. The research proposes an index that is more effective in predicting the spindle's rotation accuracy than the roundness and cylindricity.  
Paper [6] presents an elegant analytical model of a porous gas journal bearing after a literature survey on the topic. The analytical model is able to predict the flow and the dynamic force coefficients. An FE model is compared to the analytical one, and a perfect match is found. A validation of the static results with experimental data from the literature is also carried out. The paper represents a useful design guide for the quick selection of physical parameters, both for static performance and for stability analysis.  
A partial arc annular-thrust porous journal bearing that can carry both radial and axial loads is investigated in [7]. The analysis is carried out using commercial multi-physical software in order to evaluate the pressure distribution for low eccentricity as well as for low tilting angles and tilting speeds. The dynamic coefficients of tilting motion are then determined using the finite difference technique.  
Finally, the modal parameters of an aerostatic spindle are identified experimentally based on the impulse response in paper [8]. Different methods are proposed to identify the damped natural frequencies and the damping ratios of the conical and cylindrical modes of the spindle. The same modal parameters are also evaluated with a non-linear numerical model, and a comparison with experimental results is provided to validate the model.  
Writing--original draft preparation, F.C.; Writing--review and editing F.C., L.L., T.R. and A.T. All authors have read and agreed to the published version of the manuscript. This research received no external funding. The editors would like to acknowledge all of the authors for their high-quality contributions to this Special Issue. The MDPI management and staff are also acknowledged for their continuous support. The authors declare no conflict of interest.