4D printing: From the “ink” to the multi-functional medical device.

The impressive progress made in the 3D printing field in recent years has made possible the engineering of a myriad of new objects in a diversity of areas, including the medical devices arena. That said, only a new generation of 3D printed constructs, combining pioneering concepts and novel tailor-made materials will permit further progress. As derived from this conceptual framework and contrary to the vast majority of the 3D structures printed to date, which are static in nature, our work aims at engineering dynamic 3D printed architectures that change “on command”. By judiciously tailoring the “inks”, responsive architectures actuated by different environmental stimuli were 3D printed and their behavior investigated.

After briefly reviewing this expanding field, this contribution will first report the development of a series of environmentally responsive 3D printable polymers, and the generation of 3D printed constructs responsive to temperature and pH. In this case, the basic building blocks of these printable polymers are polyethylene oxide-polypropylene oxide-polyethylene oxide dimethacrylates and acrylic acid, responsible for the temperature and pH responsiveness, respectively. The cyclic behavior of the structures as they fluctuate between the two stimuli was also studied. 3D printed structures comprising more than one temperature and pH-responsive polymer were engineered and their behavior was studied as a function of their respective composition.

The ability to engineer custom-made medical devices and to implant them following minimally invasive procedures are two important trends in modern surgery. While the personalization of the device is achieved by 3D printing it, the capacity to deploy it minimally invasively harnesses the shape memory behavior displayed by the “inks” used. In this study, we introduce a 3D printed, shape memory-displaying tracheal stent based on specially designed, flexible "inks" comprising polypropylene glycol/polycaprolactone triblocks, end-capped with photo-polymerizable double bonds.

Magnetically responsive 3D printed structures will also be described. By adding magnetic nanoparticles to the “ink” being printed, shape-memory displaying structures able to be remotely actuated by an alternate magnetic field were 3D printed and their no-contact actuation will be discussed.