Book excerpt: Lower Limb Exoskeleton, a wearable robot that is designed to provide lower limb assistance to users, has been rapidly developed in the previous decade. The goal of these robots is to replace human labor with robots while still having humans involved. However, while these robot suits provide sufficient assistance to the users, the efficiency of the robot is often overseen. Thus, restrict the exoskeleton's operating time or required it to connect to an external power supply. However, there is plenty of energy wasted in human motions. In this study, we target "loaded bipedal walking" as the primary motion to assist. In chapter 2, we applied trajectory optimization on different mechanical designs for lower-limb exoskeletons. It is commonly known that humans tend to use more energy to walk compared to other limb-based locomotion animals. This higher energy usage is due to "heel strikes" and "negative work" during human gait. Passive walkers elevate this phenomenon by utilizing elastic joints that absorb/reuse some of the negative work. The objective of this study is to absorb energy at one phase of the gait cycle, store it, and then release it at a later phase through the use of a lower limb exoskeleton. Knee geometry is one important factor in energy efficiency during gait. Animals with reversed knees compared to humans (backward knee), such as ostriches, exhibit improved energy efficiency. As part of this study, new energy optimization strategies were developed utilizing collision-based ground reaction forces and a discrete lagrangian. The minimal cost of transport (CoT) gait patterns were calculated with both forward-knee and backward-knee human-exoskeleton models. Simulation results show that wearing a backward-knee exoskeleton can reduce the CoT by 15% of while carrying external loads ranging from 20 to 60 kg. In addition, when the exoskeleton utilized energy recycling, the CoT was shown to be further reduced to 35%. These simulation results suggested that the optimal design for an exoskeleton aimed at utilizing energy recycling principles should incorporate backward-knee configurations much like those found in energy-efficient biped/quadruped animals. In fact, since the potential energy sources (heel strikes, negative work) and the main energy consumer (ankle push-off) occurs in the opposite legs, the ideal actuators for the exoskeleton need to be able to recycle, store, and transfer energy between different legs. To satisfy the actuator's requirements from chapter 2, in chapter 3 we choose pneumatic actuators as the actuator for our exoskeleton. Pneumatic actuators are a popular choice for wearable robotics due to their high force-to-weight ratio and natural compliance, which allows them to absorb and reuse wasted energy during movement. However, traditional pneumatic control is energy inefficient and difficult to precisely control due to nonlinear dynamics, latency, and the challenge of quantifying mechanical properties. To address these issues, In chapter 3, we developed a wearable pneumatic actuator with energy recycling capabilities and applied the sparse identification of nonlinear dynamics (SINDy) algorithm to generate a nonlinear delayed differential model from simple pressure measurements. Using only basic knowledge of thermal dynamics, SINDy was able to train models of solenoid valve-based pneumatic systems with a training accuracy of 90.58% and a test accuracy of 86.44%. The generated model, when integrated with model predictive control (MPC), resulted in a 5% error in pressure control. By using MPC for human assistive impedance control, the actuator was able to output the desired force profile and recycle around 88% of the energy used in negative work. These results demonstrate an energy-efficient and easily calibrated actuation scheme for designing assistive devices such as exoskeletons and orthoses. In chapter 4, we presented Pneumatic Exoskeleton with Reversible Knee (PERK). It utilizes the pneumatic actuators we developed in chapter 3 and the control strategies we concluded in chapter 2. Three clinical trials were done on three different test subjects. The results showed despite different walking patterns across different test subjects, there is less potential energy change during the swing phase of walking, potentially reducing the energy loss during the heel strike. In addition, during the double support phase, there is less energy consumption in the pneumatic system while configuring it as backward-knee, indicating it is easier or more intuitive for the user to have the exoskeleton recycling the dissipated energy with the backward-knee mechanism.