Traditionally, when it comes to high-tech self-assembled microscopic structures for drug delivery and refined and delicate grippers for robotics, there has been a dearth of efficient and cost-effective options. Although some options exist, they are rarely as effective as desired, with microscopic drug delivery mechanisms, for example, not having the optimal porosity. Similarly, in so-called soft robotics, many compromises had to be made.
A promising technology here involves manipulating flat structures in a way that allows them to either self-assemble into 3D structures or non-destructively transform into 3D structures with specific features such as clamps that could be useful both microscopically and macroscopically. applications, including robotics.
Perhaps the most interesting part is how much of this technology borrowed from the Japanese art of origami and associated kirigami.
Apartment is better
Rather than trying to build three-dimensional structures, it is much more efficient if one can coax sheets of a material or even individual molecules into self-assembling into the desired shape. In the field of biomedical engineering, for example, there are strong use cases for everything from drug delivery mechanisms that can precisely and efficiently deliver certain drugs to where they are needed, to microscopic surgical tools such as forceps. which can be controlled using external triggers.
In a review article by Randall et al. (2012), potential applications and the state of the art at the time were explored, focusing on the use of self-folding hinge mechanisms. The exciting notion here is that it would allow us to create tiny mechanisms using two-dimensional lithography methods and similar common 2D fabrication mechanisms.
As shown in the image on the right, Randall et al. were able in previous research to produce 2D structures that, when released from a substrate, automatically folded into a 3D structure using built-in hinges. This essentially turns these structures into a type of self-assembled origami by designing the positioning of the hinges.
They note that the use of lithography as commonly used in the semiconductor industry is not optimal for this type of assembly, due to a preference for using organic materials and others that are not commonly encountered in semiconductor lithography. The use of soft lithography methods that shape biopolymers and the like into the required shape was considered promising.
In an article by Felton et al. (2014) (PDF), a method similar to Randall et al is used, except on a much larger scale with self-assembled robots. The essential idea here is to use the concept of computer origami to create what is essentially a flat circuit board with on-board electronics. Upon activation, the shape memory composites along the integrated hinges are activated
Their demonstration robot uses a sandwich of layers of pre-stretched polystyrene (PSPS), paper and a PCB. PSPS is a shape memory polymer which, when heated to around 100°C, contracts. When the joint has completed its rotation, the heat source is removed and as the PSPS hardens, the new orientation is permanent until heated again.
Of the five steps needed to fold the robot from a flat shape to the final 3D shape, three are self-folding, with the motors handling the remaining two steps. After connecting a power source to the flat assembly, it takes approximately four minutes for the bending steps to be completed and the joints to cool. After folding, the robot can then continue to move in its new three-dimensional configuration.
However, not all of their robots folded successfully: as they note in the diary, they needed three attempts to successfully self-assemble. Apparently precision with the hinges to get them into the desired orientation is an issue there. Nevertheless, given the low cost of materials, one could imagine that flat, self-assembled robots like these would be mass-produced.
As noted, this technique could be rather useful for rapid prototyping and to facilitate the self-assembly of everything from robots to satellites once the intended location is reached. Their heat-sensitive polymer temperature of 100°C is a quality of the selected material, and depending on the intended operating environmental conditions, different materials could be chosen to suit a different temperature range.
A delicate touch
The aforementioned studies essentially involved high-tech origami types, as their 3D shape was derived from their 2D surface using only a certain number of folds. This is distinct from kirigami (切り紙), as the name clearly indicates: 切り (kiri) meaning “cut” and 紙 (kami) meaning “paper”. Instead of folding the paper creating the final shape, with kirigami it’s the initial cutouts made in the paper that determine the 3D shape it will take.
A well-known western example of kirigami can be found in so-called pop-up books, where opening a page will cause a variety of shapes to form from the flat paper due to the way the paper was cut, sometimes helped by a guide. to fold. Depending on the level of sophistication, the most delicate shapes can be created this way.
This was also the guiding principle of a recent study by Hong et al. (2022) in Nature Communication, with an attempt to create a type of computer kirigami that would translate three-dimensional shapes into a series of cuts in a flat surface. A simple example of this is demonstrated in the article with three basic shapes:
At the heart of their approach to computer kirigami is the Gauss-Bonnett theorem. In the field of differential geometry, this covers the relationship between surfaces, relating its geometric curvature to its Euler characteristic (topological curvature). This applies for example to the geodesic curvature of the Earth and its Gaussian curvature. In effect, this provides a mathematical way of describing transformations as they move from different representations
Using Finite Element Method (FEM) simulation and analytical modeling, the morphology changes were compared to the theoretical model, establishing the correlation between the limiting curvature of the 2D kirigami sheet and the Gaussian curvature of the 3D shape .
Using the model thus developed, Hong et al. moved to create a flexible gripper that could exert precisely defined forces, allowing delicate objects to be gripped and released without damage.
This structure essentially consists of two flaps along a central region, on which an external (tensile) force is exerted. Due to the precisely calculated slots in the structure, the force exerted causes a non-permanent deformation of the gripper thus created. Due to the large amount of control, this simple structure can then be used to gently grab, hold and release anything from a raw egg yolk to a live fish. It also has enough strength to pick up and hold a human hair, as shown in the embedded video:
Go off the beaten track
Perhaps the most intriguing aspect of previous studies is the extent to which they can already be applied today. Rather than going the obvious route of focusing on finding ways to build complex three-dimensional structures directly – whether on a macro or micro scale – we can instead make the box assemble itself. .
While it’s hard to say at this point how much of this research will find real-world uses and what will hit other hurdles along the way, there seems to be a lot of promise in these 2D transformation-focused approaches. . Just as with general research on self-assembled nanostructures, there seems to be a trend towards engineering systems that can handle assembly on their own.
While the medical applications of self-folding surgical nanorobots are probably still a long way off, that doesn’t mean we can’t already make self-assembling, flatpack-like robots, as well as soft robotic grippers and everywhere else our imagination and our mathematics. take us.