• 4D printing
• conductive materials
• medical devices

Additive Manufacturing (AM), commonly known as three-dimensional (3D) printing or rapid prototyping, has been introduced since the late 1980s. Considerable amount of progress has been made in this field. Recently, multi-material 3D printing combining of “smart materials” has led to the development of a new fabrication paradigm called four-dimensional (4D) printing, where fourth dimension is given by controlled evolution in time of 3Dprinted object. Smart materials have the ability to change their shape or properties under the influence of external stimuli. 4D printing has the prospective to simplify the design and manufacturing of different products and the potential of automating the actuation of devices that react under the application of mechanical, chemical, thermal and other stimuli. Simplifying the design of the 3D printed products could help to decrease the logistic cost as the printed products can be stored as compactly as possible before activated to full volume and functionality. 4D printing can also provide an alternative solution to the fabrication of structures that are difficult to produce by conventional technologies. Products can be made more durable as well as they can be designed to adapt to environmental changes such as humidity level or moisture content, temperature, altitude and pressure. 4D printing could find applications in the biomedical field, allowing prototyping of interactive or sensing devices for patient rehabilitation. Devices that normally need electrical motors could be simplified maintaining their functionalities. This thesis was carried out in the context of a collaboration between the Research Centre “E. Piaggio” of the University of Pisa and the Universidad Politécnica de Madrid. The aim of this thesis was the complete engineering design of electrically controlled, shape-memory actuators and sensors for biomedical applications. The design principle is based on the combination, by FDM 3D printing of passive elements (made of Polylactic Acid (PLA) or thermoplastic elastomer (TPE)) and active parts (made of conductive thermoplastic polyurethane rubber (cTPU) or conductive Polylactic Acid (cPLA)). The conductive materials were characterized from mechanical, electrical and electromechanical point of view. The fabrication was accomplished in a single printing step, using a dual extrusion 3D printing mode, which minimizes post-operations and the use of additional heating elements such as resistors, Peltier cells or knitted or glued thermal patches. Geometries were designed by a Computer Aided Design (CAD) software and optimized with the support of a Finite Element Modeling (FEM) software. Pressure sensing devices were fabricated by integrating in the printing process sensing elements connected by 3D printed paths. A “smart” keyboard that react at finger pressure, a sensorized orthopedic insole and three different actuators (a hexagon-shaped robotic claw or gripper, a finger-like actuator composed of seven phalanxes and a of active “snake” toy), were manufactured and presented.4D printed actuators were based on the Joule effect experienced by the conductive thermoplastic material when voltage is applied, and on the shape-memory property of passive materials. The FEM models, the heating process of manufactured prototypes and the consequent shape recovery process were validated with the support of infrared thermography. These tests helped to assess and validate the proposed engineering design methodology and to put forward some current challenges when compared with alternative approaches for the development of complex shape-memory actuators and sensing devices.