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<p dir="ltr">To ensure safety, traditional industrial robots must operate within cages to separate them from human workers. This requirement has led to the rapid development of collaborative robots (cobots) designed to work closely to humans. However, existing cobots often prioritize <a href="" target="_blank">performance </a>aspects, such as precision, speed, and payload capacity, or prioritize safety, leading to a challenging balance between them. To address this issue, this dissertation introduces innovative concepts and methodologies for variable stiffness mechanisms. These mechanisms are applied to create easily fabricated cobot components to allow for controllable trade-offs between safety and performance in human-robot collaboration intrinsically. Additionally, the end-effectors developed based on these mechanisms enable the flexible and adaptive gripping of objects, enhancing the utility and efficiency of cobots in various applications.</p><p dir="ltr">This article-based dissertation comprises five peer-reviewed articles. The first essay introduces a reconfigurable variable stiffness parallel-guided beam (VSPB), whose stiffness can be adjusted discretely. An accurate stiffness model is also established, capable of leveraging a simple and reliable mechanical structure to achieve broad stiffness variation. The second essay discusses several discrete variable stiffness actuators (DVSAs) suitable for robotic joints. These DVSAs offer high stiffness ratios, rapid shifting speeds, low energy consumption, and compact structures compared to most existing variable stiffness actuators. The third essay introduces a discrete variable stiffness link (DVSL), applied to the robotic arm of a collaborative robot. Comprising three serially connected VSPBs, it offers eight different stiffness modes to accommodate diverse application scenarios, representing the first DVSL in the world. The fourth essay presents a variable stiffness gripper (VSG) with two fingers, each capable of continuous stiffness adjustment. The VSG is a low-cost, customizable universal robotic hand capable of successfully grasping objects of different types, shapes, weights, fragility, and hardness. The fifth essay introduces another robotic hand, the world's first discrete variable stiffness gripper (DVSG). It features four different stiffness modes for discrete stiffness adjustment in various gripper positions by on or off the ribs. Therefore, unlike the VSG, the DVSG focuses more on adaptability to object shapes during grasping.</p><p dir="ltr">These research achievements have the potential to facilitate the construction and popularize of next-generation collaborative robots, thereby enhancing productivity in industry and possibly leading to the integration of personal robotic assistants into countless households.</p>