Speaker
Description
Smart Materials, broadly defined here as any biomaterial that alters its shape and material properties over a time period ranging from minutes to years in response to externally applied (light, heat, etc) or host (fluid, cells, etc.) stimuli hold tremendous promise for fabricating resorbable, implanted devices for pediatric reconstruction applications. Regulatory approval requires understanding how these effects depend on material composition, 3D printing modality, host clinical site application, ability to meet design requirements and how material alterations occur in a pre-clinically and clinically. This paper reports the affect of printing modality on polycaprolactone (PCL), a widely used clinical material, and Poly Glycerol Dodecanedioate (PGD), a shape-memory biomaterial developed by our group. It also reports in vivo responses in devices 3D printed from both of these smart biomaterials.
We first demonstrate that elastic and post-elastic mechanical properties (i.e. plastic and damage behavior) depend on both the raw material source and the 3D printing modality (e.g. extrusion, DLP, or laser sintering) used to make a device. PCL shows elastic-plastic behavior that depends both on medical grade material supplier and the 3D printing modality (extrusion vs laser sintering) used to print the device. PGD exhibits shape memory nonlinear elastic post-elastic damage behavior whose whose shape memory, elastic and failure stress properties also depend on 3D printing modality (extrusion vs DLP). PGD This is critical information for iterating device design.
Second, we show that that shape memory and mechanical properties change significantly in vivo due to fluid penetrating the polymer matrix that itself depends on tissue coverage of the device. Suprisingly, both PCL and PGD become stiffer (increased elastic properties) and demonstrated increased yield and failure stress during degradation before complete resorption. PCL demonstrated a transition from a highly ductile material after 3D printing to a completely brittle material with no plasticity after degrading 2 years in vivo. This process has been termed "hydrolic embrittlement" and is demonstrated here in a large pre-clinical animal model.
Third, in pre-clinical and clinical implantation, we demonstrate that clinical outcomes in pediatric applications depend on a complex interaction between smart biomaterial property changes during degradation (especially elastic stiffening and yield/failure stress changes), host tissue remodeling, and host anatomic growth. Tissue growth over the device limits mass transport, thereby inhibiting polymer chain exudation from the device. Polymer chains reorganize into lamellar structures stiffening the device. In turn, a stiffnened device places greater stress on growing tissue, potentially affecting pediatric growth and development after device placement. The tissue growth in turn stresses the implanted device and can lead to the fracture. The key is designing for eventual fracture after the device has fulfilled its function. Rapid fractures is more likely in rapidly growing pre-clinical models than in patients, as shown for a 3D printed tracheal repair device. Such complex interacting phenomena (smart biomaterial property changes couple with in vivo tissue remodeling and growth) must be better understood and incorporated in long term (months to years) pediatric device design and simulation to improve 3D printed biofabricated pediatric device development and translation.