Study On 4D Printed Structure Responsive To Light Stimuli

4D printing is something pretty new in the world of adhesive printing.A structure called 4D printed will have properties that allow itself to react to another dimension.Such innovative structures can be really useful in the future as they could change shape without the interaction of humans and using 3D printing to create those is also usually a gain of time and money as the production can be fully automated.These fields offer not only tremendous opportunities for engineers but also for designers as many new applications will arise.Smart materials will solve design/engineering problems,which often arise from limitations of current materials.Applications can range from new materials for shoes and wearables,to applications in robotics and new type of sensors.To that extend,structures reactive to temperature or humidity have already been created with success.Immersion into water of a structure that can then fold by itself was created by a laboratory at the Massachusetts Institute of Technology.In the present thesis,the goal is to create a structure that is responsive to light stimuli as this has never been done before by anyone.The interest of having a structure able to react to light exposure could lead to improvements in the fields of solar energy for example or even in the medical sector.A structure printed through additive manufacturing and then able to bend to follow the path of the sun without the use of electricity would be for example a great way to improve the productivity of solar panels.To that extend the 3D printing process that will be used in this thesis is the stereolithography,process which polymerizes a resin after exposure to light.Several printings were done to achieve it.Those printings were done using a resin containing fluorescent pigments inside.After an introduction on the available lecture concerning stereolithography and the two processes that are going to be used in this thesis,already existing 4D printed structure and fluorescence,a presentation of all the process and way of thinking through this thesis is explained.The first idea in this thesis was to create a structure that could react mechanically to light stimuli and could then bend when exposed to it,using a multi material printing process.To that extend,the azobenzene compound was needed as this one is responsive to light exposure.After the abandonment of this idea due to lack of resources and safe environment to handle the azobenzene,new ways of making this structure responsive to light had to be found.It was then decided to create a structure that could react to light in a color way using fluorescent pigments knowing that printing these types of pigments using stereolithography was something never done before.Those thoughts then lead to simulations using the software COMSOL.Mechanical response to light stimuli was studied in a theoretical way even though the experimental part could not be done.As for the fluorescent response to light stimuli,parameters affecting the composition of the resin such as the density or the thermal conductivity were studied to know if they could affect the quality of printing.Then several experiments were done with two different types of printing methods: right-side up stereolithography process and bottom-up reverse stereolithography process.Moreover,the fluorescent pigments that were introduced in the curable resin for later printing were present in two different types: one is a fluorescent liquid and the other a fluorescent powder.Influence of the curable resin color was also studied.Comparison of those processes is then done to tell which one is better.The results of all those experiments then lead to the conclusion that the bottom-up reverse stereolithography apparatus is the best fit to achieve the printing of a fluorescent structure.Finally,it is concluded that the best fluorescent media is the powder,as the concentration can be adjusted in a better way than the fluorescent liquid and it is also noticed that using a curable resin with a light color such as white,yellow or even transparent(like in this thesis)is also better as it will not prevent the fluorescent pigments from glowing in the dark.