Concept. Functional surfaces fail due to surface contamination or damage. Instead of trying to make “better” functional materials, in this project we proposed a fundamental change in the design of such materials. In contrast to established concepts of self-healing and self-regeneration, we wanted to create a material system that can shed a contaminated or damaged top layer, like a reptile shedding its skin, and present a new, functional one. This was to be achieved by stacking discrete, non-interpenetrating polymer multilayers on top of each other. Thus, functional layers and sacrificial layers were stacked alternatingly, and the functional layers were to be shed by selective disintegration of the topmost sacrificial layer.
In the beginning of the project, we focused on the molecular functionalities of the individual layers in the stack, which each had a thickness on the tens to hundreds of nanometer scale. As the project advanced, we learned that we cannot just consider molecular interactions between the adjacent layers. Instead, we have to treat these stack interfaces, even though they are relatively thin, as a macroscopic system. Thus, for successful layer shedding, it is not sufficient to simply consider the molecular mechanisms of sacrificial layer dis¬integration – instead, the simultaneity of the entire process must be ensured. Thus, in addition to the molecular design of the stack materials, the disintegration kinetics of each sacrificial layer need to be considered. If these are too slow, layer shedding is prevented because the adjacent polymer layers, which are soft and flexible, can adapt to slow interfacial changes without delamination. However, if the disintegration kinetics are sufficiently fast, solvent influx into the disintegration zone and disintegration product transport out of that zone prevents layer re-attachment and thus enables layer shedding.
Aims: Initially, we wanted to demonstrate the feasibility of our concept with two research objectives: Objective 1: Sequential regeneration of a functional surface property, exemplified by antimicrobial activity; Objective 2: Regeneration of the activity of a functional device, exemplified by a glucose sensor. We initially focused on sacrificial layers made from degradable polymers to reach these aims, which should have a degradability gradient for sequential shedding. Within the amendment of the action, these aims were adjusted to include the study of faster disassembly mechanisms, including dissolution and triggered depolymerization of self-immolative polymers (additional Work package 1.4). This additional work package was prioritized over the original objective 1 (i.e. work packages 2.1 and 2.2).
Summary and outlook: We presented a fundamentally new and important concept to regenerate a functional surface via selective shedding of polymer layers from a multi-stack. In the beginning of the project, it was anticipated that shedding could be achieved by simply removing any kind of sacrificial layer. We extensively studied the hydrolytic degradation mechanism and kinetics of hydrolytically degradable polymer thin films and established analytical methods for such systems, which were previously rather scarce in the literature. With these tools, we made enormous progress in understanding the molecular and systemic origins of layer shedding, i.e. we learned that in order to selectively shed a macroscopic object like a polymeric film entirely, even though it is only a few tens of nm thin, we need to consider the forces at the macroscopic level instead of focusing on the degradation reactions solely. Also, learned that the kinetics of disintegration of the sacrificial layer are essential for successful shedding, as polymer layers are elastic and can rearrange upon non-immediate interfacial changes. Understanding these factors both at the molecular and systemic level is ground-breaking for functional material systems and their applications. In future work, we want to continue to apply these findings create a polymer multi-stack with conductive layers for sensor applicaitons
