# Geometric Tiles and Powers and Limitations of Geometric Hindrance in Self-Assembly

## TL;DR

This paper explores the capabilities and limitations of geometric hindrance in tile-based self-assembly, demonstrating how geometric features can simulate complex systems but also establishing boundaries where such methods cannot replicate certain cooperative behaviors.

## Contribution

It introduces a model with geometric bumps and dents, showing how it can simulate complex glue functions and weakly cooperative systems, and proves limitations of geometric hindrance in simulating deterministic cooperative systems.

## Key findings

- Geometric bumps and dents can simulate complex glue functions.
- The model can simulate weakly cooperative systems with duples.
- Geometric hindrance cannot simulate certain deterministic cooperative systems.

## Abstract

Tile-based self-assembly systems are capable of universal computation and algorithmically-directed growth. Systems capable of such behavior typically make use of "glue cooperation" in which the glues on at least $2$ sides of a tile must match and bind to those exposed on the perimeter of an assembly for that tile to attach. However, several models have been developed which utilize "weak cooperation", where only a single glue needs to bind but other preventative forces (such as geometric, or steric, hindrance) provide additional selection for which tiles may attach, and where this allows for algorithmic behavior. In this paper we first work in a model where tiles are allowed to have geometric bumps and dents on their edges. We show how such tiles can simulate systems of square tiles with complex glue functions (using asymptotically optimal sizes of bumps and dents), and also how they can simulate weakly cooperative systems in a model which allows for duples (i.e. tiles either twice as long or twice as tall as square tiles). We then show that with only weak cooperation via geometric hindrance, no system in any model can simulate even a class of tightly constrained, deterministic cooperative systems, further defining the boundary of what is possible using this tool.