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Title of the thesis: 4D printing of smart co extruded wire fibre polymer composites.

Thesis defender: S. Siddharth Kumar
Opponent: Prof. Sepideh Ghodrat, TU Delft, Netherlands 
Custos: Prof. Mika Salmi, Aalto University School of Engineering

Standard 3D printing creates a static object. With 4D printing, the printed object can transform itself after being printed. The objects can bend, move or even change their colour when applying thermal or electrical input. The fourth dimension is time because the transformations take place after the object has been printed. The printing process explored in this thesis involves embedding a continuous fibre or even a thin metal wire into a thermoplastic polymer during printing. For most applications previously reported in literature, the embedded continuous fibre using such a method has been for reinforcement purposes only. Printing an object that is robust, deforms reversibly and senses its own state, all at the same time, remains an open challenge. This thesis investigates the possibility in combining the extrusion-based manufacturing with embedded filaments and wires, and studies whether this combined approach can introduce multiple functionalities such as mechanical shape-memory, electrical heating, colour change, reversible bending and self-sensing. The effects of the material, orientation, location and adhesion of the embedded element on the shape-memory behaviour, Joule heating, thermochromism, bending characteristics and self-sensing ability were studied. 

It was found that the architecture of the embedded fibres dictates how the printed object can be programmed to obtain a certain shape, memorize it, and recover to its original shape when heated. It was also found that good adhesion between the embedded fibres and the polymer matrix, as well as good impregnation of the polymer into the fibre bundle, is required for load transferring and stable actuation. Embedding metal wires offered additional multifunctional capabilities. By passing an electric current through a resistance wire, Joule heating could be used to induce colour change of a thermochromic polymer and recover its original colour once the current was turned off. Shape-memory alloy wires, which change their shape upon heating, were used to demonstrate reversible bending of a printed structure in both directions. The change in electrical resistance of the wires when deformed allowed estimating the curvature of the printed structure, without the need of an additional sensor. 

The findings show that the same co-extrusion based printing method can offer reinforcement, local heating, bending actuation, colour change and self-sensing capabilities in diverse materials systems. To the best of the author's knowledge, this study demonstrates for the first time a single printable platform offering shape programming, reversible thermochromism and bidirectional bending while containing self-sensing capabilities. 

Such multifunctionality can enable a wide range of applications from compact actuators, soft robotics and morphing structures to reconfigurable display technology and smart manufacturing systems. However, further research is needed before multifunctional and large-scale 3D printed composite components can be manufactured. Future work should focus on stronger bonding between the metal wires and the polymer, better process control, routing the wires in three dimensions, and long-term durability and environmental stability.

Thesis available for public display 7 days prior to the defence at . 

Contact information: ssiddharth.kumar@aalto.fi