In the medical field and in soft robotics, flexible devices are required for safe human interaction, while rigid structures are required to withstand the force application and accuracy in motion. This paper aims at presenting controllable stiffness mechanisms described in the literature for applications with or without shape-locking performances. A classification of the solutions based on their working principle is proposed. The intrinsic properties of these adaptive structures can be modified to change their mechanical characteristics from a geometrical point of view or equivalent elastic properties (with internal mechanisms or with a change in material properties). These solutions are compared quantitatively, based on selected criteria linked to the medical field as the stiffness range, the activation time and the working conditions. Depending on the application and its requirements, the most suitable solution can be selected following the quantitative comparisons. Several applications of these tunable stiffness structures are proposed and illustrated by examples of the literature.

During the last years, great progress was made in material science in terms of concept, design and fabrication of new composite materials with conferred properties and desired functionalities. The scientific community paid particular interest to active soft materials, such as soft actuators, for their potential as transducers responding to various stimuli aiming to produce mechanical work. Inspired by this, materials engineers today are developing multidisciplinary approaches to produce new active matters, focusing on the kinematics allowed by the material itself more than on the possibilities offered by its design. Traditionally, more complex motions beyond pure elongation and bending are addressed by the robotics community. The present review targets encompassing and rationalizing a framework which will help a wider scientific audience to understand, sort and design future soft actuators and methods enabling complex motions. Special attention is devoted to recent progress in developing innovative stimulus-responsive materials and approaches for complex motion programming for soft robotics. In this context, a challenging overview of the new materials as well as their classification and comparison (performances and characteristics) are proposed. In addition, the great potential of soft transducers are outlined in terms of kinematic capabilities, illustrated by the related application. Guidelines are provided to design actuators and to integrate asymmetry enabling motions along any of the six basic degrees of freedom (translations and rotations), and strategies towards the programming of more complex motions are discussed. As a final note, a series of manufacturing methods are described and compared, from molding to 3D and 4D printing. The review ends with a Perspectives section, from material science and microrobotic points of view, on the soft materials’ future and close future challenges to be overcome.

The emerging field of plasma medicine opens new therapeutic opportunities with the use of cold atmospheric plasma as a versatile tool for the treatment of tissues in various medical indications. Yet, the complexity of this very reactive medium combined with a high dependence on its environment and generation parameters make it difficult to predict and optimize such treatment. To this end, a simple yet robust and accurate tissue phantom allowing to study the penetration and distribution of plasma action in simulated in vivo conditions has been developed by several groups. It combines a hydrogel-based matrix closely resembling tissues and chemical reporters incorporated in the gel to measure the delivery of reactive oxygen and nitrogen species (RONS) by the plasma. This paper reports the use of these models in the literature to give an overview of the state of the art, their capabilities and the further research required to improve it. First the hydrogels composition (i.e. gelatin and agarose) is discussed, as well as the parameters allowing to fine tune the model. In particular, we show that modifying mass fraction has been reported to mimic several types of tissues and that different model configurations allow to test different treatments conditions, including the barrier effect of skin or the direct treatment of a tissue bulk. The role of other critical parameters is highlighted, including manufacturing, diffusion, electrical characteristics but also liquid composition, thickness, aging and temperature influence. Secondly, RONS reporters used in the plasma medicine literature (colorimetric/fluorometric dyes) are summarized. The analysis techniques are discussed and the dyes characteristics (i.e. wavelength, specificity, and concentrations) are reported. Finally, the influence of medium and time on these measurements are covered. Main achievements, as well as limitations of these models are presented and linked to potential improvements and further research.

Cold atmospheric plasmas are a known source of reactive species enabling various treatments, from the healing of chronic wounds to the treatment of surface cancers. Therapeutic endoscopic procedures require developing specific flexible tools that can be used through or alongside endoscopes. Plasma devices for endoscopy have aroused significant research interest over the past few decades, but their electrical behaviour is not yet fully understood and predictable. There is thus a clear need for a robust model that provides a way to understand and optimize future devices. In this work, for the first time, an electrical equivalent model of a long plasma source (comprising plasma generation, transport and target interaction) was designed, implemented, and validated. System parameters were estimated based on the system geometry and independent measurements. The model reliably reproduces the double ignition (in the quartz chamber and at the treatment site) observed experimentally. Simulations globally agree with measurements taken for various gas gap distances and input voltages. Internal parameters that are difficult to measure, such as the electrical charge at the gas gaps, were inferred. The model can predict leakage current in the body and current at the target site. This work provides a new understanding of endoscopic plasma systems that could be used in the future to ensure patient and operator safety.

This experimental study characterized the effect of carrier gas flow on an endoscopic plasma jet. This system generates and transports helium plasma in a tube over several meters, along with a separate channel transporting oxygen to enhance plasma reactivity on site. The resulting plasma plume exiting the tube allows treatment of tissue surfaces in an endoscopic setting with a view to perform therapeutic operations in the gastrointestinal tract. In a closed cavity simulating the tract, the carrier gas flow was studied by a combined approach investigating plasma plume chemistry, fluid dynamics, and plasma effects on the surface of a hydrogel tissue model reporting oxidation. These three aspects are shown to be closely inter-related. Plasma plume length, intensity, and shape strongly depend on helium content, velocity, turbulence, and environment. Optical emission spectroscopy was used to show that the helium gas flow rate increases the amount of helium and reactive oxygen species (ROS) in the plume. Schlieren imaging was used to visualize the transition of the fluid from buoyant to laminar and finally turbulent depending on flow rate, with a backflow in a closed cavity. Finally, the frontal and radial treatment of the cavity was assessed by measuring ROS delivery to a KI-starch agarose gel model. Helium flow rate had a noticeable effect on the treatment distribution profile and treatment intensity, with different maxima for frontal and radial treatments. This combined approach, in an accurate simulation of the target configuration (i.e. a closed, cylindrical cavity), is necessary to optimize treatment, as its outcome depends on a balance between ROS production, transport, and distribution.

In soft robotics, the ability to generate advanced kinematics is a necessary step toward any more sophisticated tasks such as microobject manipulation, locomotion, or configuration changes. To this end, herein, a modular voxel‐based methodology adaptable to any scale and with any soft transducer is presented. The methodology is implemented at the micrometer scale with a one‐step fabrication process. An innovative gray‐tone lithography method using the two‐photon polymerization of photosensitive poly(N‐isopropylacrylamide) hydrogel is developed to print the voxels. Bending, compression, and twisting voxels are designed, printed, and characterized. A voxel consists of an isotropically shrinking active material reinforced adequately with a passive pattern. Each elementary voxel deforms along one degree of freedom and is a building block for superstructures able of advanced kinematics. With a side length of 40 μm, the bending voxel achieves a bending angle of 25º or curvature of . The compression voxel reaches an actuation strain of 40%, and the twisting voxel bends up to 18º. Advanced kinematics are demonstrated by printing complex structures composed of multiple elementary voxels. Herein, a foundation toward soft microrobots capable of performing complex tasks is constituted.