In the growing fields of wearable robotics, rehabilitation robotics, prosthetics, and walking k robots, variable stiffness actuators (VSAs) or adjustable compliant actuators are being designed and implemented because of their ability to minimize large forces due to shocks, to safely interact with the user, and their ability to store and release energy in passive elastic elements. This review article describes the state of the art in the design of actuators with adaptable passive compliance. This new type of actuator is not preferred for classical position-controlled applications such as pick and place operations but is preferred in novel robots where safe human- robot interaction is required or in applications where energy efficiency must be increased by adapting the actuator's resonance frequency. The working principles of the different existing designs are explained and compared. The designs are divided into four groups: equilibrium-controlled stiffness, antagonistic-controlled stiffness, structure-controlled stiffness (SCS), and mechanically controlled stiffness.

Compliant actuator designs

Microfluidics consist of microfabricated structures for liquid handling, with cross-sections in the 1–500 μm range, and small volume capacity (fL-nL) Capillary tubes connected with fittings,1 although utilizing small volumes, are not considered microfluidics for the purposes of this paper since they are not microfabricated Likewise, millifluidic systems, made by conventional machining tools, are excluded due to their larger feature sizes (>500 μm)

Though micromachined systems for gas chromatography were introduced in the 1970’s,2 the field of microfluidics did not gain much traction until the 1990’s3 Silicon and glass were the original materials used, but then the focus shifted to include polymer substrates, and in particular, polydimethylsiloxane (PDMS) Since then the field has grown to encompass a wide variety of materials and applications The successful demonstration of electrophoresis and electroosmotic pumping in a microfluidic device provided a nonmechanical method for both fluid control and separation4 Laser induced fluorescence (LIF) enabled sensitive detection of fluorophores or fluorescently labeled molecules The expanded availability of low-cost printing allowed for cheaper and quicker mask fabrication for use in soft lithography5 Commercial microfluidic systems are now available from Abbott, Agilent, Caliper, Dolomite, Micralyne, Microfluidic Chip Shop, Micrux Technologies and Waters, as a few prominent examples For a more thorough description of the history of microfluidics, we refer the reader to a number of comprehensive, specialized reviews,3, 6–11 as well as a more general 2006 review12

The field of microfluidics offers many advantages compared to carrying out processes through bulk solution chemistry, the first of which relates to a lesson taught to every first-year chemistry student Simply stated, diffusion is slow! Thus, the smaller the distance required for interaction, the faster it will be Smaller channel dimensions also lead to smaller sample volumes (fL-nL), which can reduce the amount of sample or reagents required for testing and analysis Reduced dimensions can also lead to portable devices to enable on-site testing (provided the associated hardware is similarly portable) Finally, integration of multiple processes (like labeling, purification, separation and detection) in a microfluidic device can be highly enabling for many applications

Microelectromechanical systems (MEMS) contain integrated electrical and mechanical parts that create a sensor or system Applications of MEMS are ubiquitous, including automobiles, phones, video games and medical and biological sensors13 Micro-total analysis systems, also known as labs-on-a-chip, are the chemical analogue of MEMS, as integrated microfluidic devices that are capable of automating multiple processes relevant to laboratory sciences For example, a typical lab-on-a-chip system might selectively purify a complex mixture (through filtering, antibody capture, etc), then separate target components and detect them

Microfluidic devices consist of a core of common components Areas defined by empty space, such as reservoirs (wells), chambers and microchannels, are central to microfluidic systems Positive features, created by areas of solid material, add increased functionality to a chip and can consist of membranes, monoliths, pneumatic controls, beams and pillars Given the ubiquitous nature of negative components, and microchannels in particular, we focus here on a few of their properties Microfluidic channels have small overall volumes, laminar flow and a large surface-to-volume ratio Dimensions of a typical separation channel in microchip electrophoresis (μCE) are: 50 μm width, 15 μm height and 5 cm length for a volume of 375 nL Flow in these devices is normally nonturbulent due to low Reynolds numbers For example, water flowing at 20°C in the above channel at 1 μL/min (222 cm/s) results in a Reynolds number of ~05, where <2000 is laminar flow Since flow is nonturbulent, mixing is normally diffusion-limited Small channel sizes also have a high surface-to-volume ratio, leading to different characteristics from what are commonly found in bulk volumes The material surface can be used to manipulate fluid movement (such as by electroosmotic flow, EOF) and surface interactions For a solution in contact with a charged surface, a double layer of charge is created as oppositely charged ions are attracted to the surface charges This electrical double layer consists of an inner rigid or Stern Layer and an outer diffuse layer An electrostatic potential known as the zeta potential is formed, with the magnitude of the potential decreasing as distance from the surface increases The electrical double layer is the basis for EOF, wherein an applied voltage causes the loosely bound diffuse layer to move towards an electrode, dragging the bulk solution along Charges on the exposed surface also exert a greater influence on the fluid in a channel as its size decreases Larger surface-to-volume ratios are more prone to nonspecific adsorption and surface fouling In particular, non-charged and hydrophobic microdevice surfaces can cause proteins in solution to denature and stick

We focus our review on advances in microfluidic systems since 2008 In doing this, we occasionally must cover foundational work in microfluidics that is considerably less recent We do not focus on chemical synthesis applications of microfluidics although it is an expanding area, nor do we delve into lithography, device fabrication or production costs Our specific emphasis herein is on four areas within microfluidics: properties and applications of commonly used materials, basic functions, integration, and selected applications For each of these four topics we provide a concluding section on opportunities for future development, and at the end of this review, we offer general conclusions and prospective for future work in the field Due to the considerable scope of the field of microfluidics, we limit our discussion to selected examples from each area, but cite in-depth reviews for the reader to turn to for further information about specific topics We also refer the reader to recent comprehensive reviews on advances in lab-on-a-chip systems by Arora et al10 and Kovarik et al14

/pdf/advances-in-microfluidic-materials-functions-integration-and-27mmao3n1o.pdf

Advances in microfluidic materials, functions, integration, and applications.

Microfluidics is an evolving scientific field with immense commercial potential: analytical applications, such as biochemical assay development, biochemical analysis and biosensors as well as chemical synthesis applications essentially require microfluidics for sample handling, treatment or readout. A number of techniques are available to create microfluidic structures today. On industrial scale replication techniques such as injection molding are the gold standard whereas academic research mostly focuses on replication by casting of soft elastomers such as polydimethylsiloxane (PDMS). Both of these techniques require the creation of a replication master thus creating the microfluidic structure only in the second process step—they can therefore be termed two-(or multi-)step manufacturing techniques. However, very often the number of pieces to be created of one specific microfluidic design is low, sometimes even as low as one. This raises the question if two-step manufacturing is an appropriate choice, particularly if short concept-to-chip times are required. In this case one-step manufacturing techniques that allow the direct creation of microfluidic structures from digital three-dimensional models are preferable. For these processes the number of parts per design is low (sometimes as low as one), but quick adaptation is possible by simply changing digital data. Suitable techniques include, among others, maskless or mask based stereolithography, fused deposition molding and 3D printing. This work intends to discuss the potential and application examples of such processes with a detailed view on applicable materials. It will also point out the advantages and the disadvantages of the respective technique. Furthermore this paper also includes a discussion about non-conventional manufacturing equipment and community projects that can be used in the production of microfluidic devices.

/pdf/let-there-be-chip-towards-rapid-prototyping-of-microfluidic-3yd1tiia54.pdf

Let there be chip—towards rapid prototyping of microfluidic devices: one-step manufacturing processes

Robotic artificial muscles are a subset of artificial muscles that are capable of producing biologically inspired motions useful for robot systems, i.e., large power-to-weight ratios, inherent compliance, and large range of motions. These actuators, ranging from shape memory alloys to dielectric elastomers, are increasingly popular for biomimetic robots as they may operate without using complex linkage designs or other cumbersome mechanisms. Recent achievements in fabrication, modeling, and control methods have significantly contributed to their potential utilization in a wide range of applications. However, no survey paper has gone into depth regarding considerations pertaining to their selection, design, and usage in generating biomimetic motions. In this paper, we discuss important characteristics and considerations in the selection, design, and implementation of various prominent and unique robotic artificial muscles for biomimetic robots, and provide perspectives on next-generation muscle-powered robots.

https://ieeexplore.ieee.org/ielaam/8860/8731790/8725920-aam.pdf

Robotic Artificial Muscles: Current Progress and Future Perspectives

In this paper, biomimetic underwater robots built using smart actuators, e.g., a shape memory alloy (SMA), an ionic polymer metal composite (IPMC), lead zirconate titanate (PZT), or a hybrid SMA and IPMC actuator, are reviewed. The effects of underwater environment were also considered because smart actuators are often affected by their external environment. The characteristics of smart actuators are described based on their actuating conditions and motion types. Underwater robots are classified based on different swimming modes. We expanded our classification to non-fish creatures based on their swimming modes. The five swimming modes are body/caudal actuation oscillatory (BCA-O), body/caudal actuation undulatory (BCA-U), median/paired actuation oscillatory (MPA-O), median/paired actuation undulatory (MPA-U), and jet propulsion (JET). The trends of biomimetic underwater robots were analyzed based on robot speed (body length per second, BL/s). For speed per body length, robots using an SMA as an actuator are faster than robots using an IPMC when considering a similar length or weight. Robots using a DC motor are longer while their speeds per body length are similar, which means that robots using smart actuators have an advantage of compactness. Finally, robots (using smart actuators or a motor) were compared with underwater animals according to their speed and different swimming modes. This review will help in setting guidelines for the development of future biomimetic underwater robots, especially those that use smart actuators.

Review of biomimetic underwater robots using smart actuators

This brief describes a new model for shape memory alloy (SMA) actuators based on the physics of the process and develops control strategies using the model. The model consists of three equations - the temperature dynamics described by Joules heating-convectional cooling, the mole fraction distribution with temperature given by statistics to describe a two state system, and a constitutive equation relating the changes in temperature and mole fraction to the stress and strain induced in the SMA. This model is used to develop two control schemes for controlling the strain in an SMA actuator. The first control scheme describes a gain-scheduled proportional-integral (PI) controller, the gains of which are obtained by means of linear quadratic regulator (LQR) optimization. The second control scheme is an Hinfin loop-shaping controller using normalized coprime stabilization which ensures robust stability by minimizing the effect of unmodeled dynamics at high frequencies. Simulation and experimental results show fast and accurate control of the strain in the SMA actuator for both control schemes.

Modeling and Control of Shape Memory Alloy Actuators

This paper describes a gain-scheduled controller for a shape memory alloy (SMA) actuator For accurate control of an SMA actuator, it is important to develop a precise model for the SMA A model has been proposed based on concepts from physics The equations include Joules heating - convectional cooling to explain the dynamics of temperature, Fermi-Dirac statistics to explain the variation of mole fraction with temperature, and a stress-strain constitutive equation to relate changes in mole fraction and temperature to changes in stress and strain of the SMA This model is then used to develop a gain-scheduled controller to control the strain in the SMA Simulation and experimental results show fast and accurate control of the strain in the SMA actuator

Modelling and gain scheduled control of shape memory alloy actuators

In this paper, friction compensation in robotic manipulators is studied. An observer based model of the friction force is utilized for the friction compensation algorithm. In order to evaluate the efficiency of this method two different manipulators with different friction characteristics are examined. A 2-DOF manipulator used for high-speed, micro-meter precision manipulation and a 4-DOF macro manipulator used for long reach positioning task are examined. These manipulators are characterized, according to compensation task classifications (Armstrong, B. et al., 1994), as high-reversal-velocity and low-reversal-velocity tracking tasks, respectively. In each case a steady-state model of friction is experimentally obtained. This model is further utilized in dynamic equations of the friction force during manipulation. It is shown that despite the different nature of the two manipulators the same method can effectively improve the speed and performance of the manipulation in both cases.

Friction compensation in low and high-reversal-velocity manipulators

A method of creating metallic micromolds with features that have high-aspect ratios is described in this paper The proposed manufacturing process utilizes laser micromachining to cut the negative two-dimensional profiles of the desired microfeatures and fluidic network patterns on a 100 μm thick brass sheet The positive relief of the cut pattern is then created by using electro-discharge micromachining (micro-EDM) die-sinking the metallic mask onto a brass substrate The final substrate with the desired relief pattern becomes the molding tool used for either elastomer casting or thermoplastic hot embossing To validate the proposed fabrication methodology and evaluate the quality of surface finishes, a brass mold master of a T-channel micromixer (50 μm width, 25 μm height) is developed and multiple replicated devices are cast on this mold using poly-di-methyl-siloxane (PDMS) The surface finish of both the original micromold master and final molded channels on PDMS are measured using an optical profiler and found to have a roughness of approximately 400 nm Ra The ability of the proposed fabrication technique to create high-aspect ratio features is illustrated by manufacturing a Y-channel micromixer with an aspect ratio of 4 Experimental results are discussed and suggestions for improvement are presented

/pdf/fabrication-of-metallic-micromolds-by-laser-and-electro-475urt0dnq.pdf

Fabrication of metallic micromolds by laser and electro-discharge micromachining

Lab-on-a-chip (LOC) and other microfluidic devices for medical applications need to be mass produced at a low fabrication cost because the disposable device is destroyed after a single use to avoid sample contamination. In this paper, a new method for rapidly fabricating metallic micromold masters for manufacturing large volumes of polymeric microfluidic devices is presented. Polymers are preferred over silicon as the device material due to their better compatibility with biological and chemical substances. The manufacturing method involves laser micromachining of the desired imprint features from thin metallic sheets and then microwelding them onto a substrate to form the final mold master. The polydimethylsiloxane (PDMS) elastomer is then poured over the mold and cured to produce the microfluidic device. The proposed method involves fewer processing steps than the soft lithography, electroplating and molding (LIGA) process. To verify the method, a metallic mold for a passive Y-channel microfluidic mixer was fabricated. The mold master was made from low-cost steel and the mold manufacturing process can be completed within an hour. PDMS elastomer is then poured over the mold and cured to produce the mixer. The channels of the mixer were 75 micrometers wide and 50 micrometers high. The mixer created from the mold was tested by mixing two streams of colored water in it. The maximum flow rate achieved by the prototype was 6.4 microlitres per minute. The experimental results confirm that a viable metallic mold master for microfluidic devices can be created by combining laser micromachining and microwelding processes. Finally, the limitations of the proposed rapid fabrication method are discussed.

/pdf/rapid-fabrication-of-micromolds-for-polymeric-microfluidic-3hss14939t.pdf

Rapid Fabrication of Micromolds for Polymeric Microfluidic Devices

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