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Crack engineering for the construction of arbitrary hierarchical architectures, Li, Yu, etc., Wang, Ren, PNAS, 2019

Novelty/impact/significance:

A conventionally detrimental phenomenon, cracking (leads to material failure), is used as a beneficial aspect to develop a powerful replication technique (two curing stages for the elastomer mold) to construct bioinspired complex hierarchical structures for interesting functions and properties, ingenious and effective.

Compared with current approaches that suffer from limited materials selection, high cost, and low throughput, this new replication method produces reliable, accurate, and economical hierarchical structures with diverse materials for mass production of  biomimetic materials, representing one step further to translate them into real-world applications.

Scientific question:

How to manufacture the complex, 3D hierarchical structures of biological materials in a material choice-flexible, cost-effective, and high-precision manner? 

Key of how:

The new replication technique leverages on the two-step curing, elastomer mold with exceptional elastic deformation and controllable cracking (to enable easy detachment while keep master and replica structures intact) and re-configurability (for reliable creation of replicate structures).

Such a replication process involves two stages, one is in lower-temperature curing to prepare the intermediate, soft mold (allow peeling from mater structure with controlled cracking), and secondly is higher-temperature curing (seal the cracks and stiffen with inverse structures intact) to become a rigid mold to create the replicate structures. The cracks reopen to ease the detachment of replica.

The polymerization and curing of the mold need to be carefully controlled to induce desired crack formation, in certain location, direction and degree within the mold.

  

Major points:

1. Three-dimensional hierarchical structures of many biological surfaces accounting for the interesting functionalities such as liquid and mass transport, adhesion, etc., have been extensively mimicked for practical applications. Current techniques can hardly duplicate the structural hierarchy, material diversity, and functional sophistication in a facile, accurate, durable, and reproducible manner.

2. With previous finding of crack formations in replication process and inspired by studies controlling cracks for desired functionalities, a technique, configurable elastic crack engineering (CECE), is developed which turns detrimental cracking into a beneficial feature of the mold to duplicate the delicate structures with deadlocking regions such as closed-loop and high-aspect-ratio rod structures.

3. A normal conventional replicating process includes: having the master, making the mold (with inverse structure of the master), and obtaining the duplicated structure (by formation on/in the mold), in which the mold is cured once and the detrimental crack formation usually leads to failure in replication.

Here the replication technique with beneficial cracking has two phases (for a master with closed-loop structures): the first stage consists of making the soft PDMS mold (phase 1 mold) in lower-temperature cure and detaching the master with controlled crack formation in the soft mold; the second one involves higher-temperature cure of the mold (phase 2 mold) with the cracks self-sealed, molding the replica, and the replica detaching from the reopened cracks.

Both stages are indispensable; without the first stage, random cracks form in the mold, damaging the mold and the master, while the second one the cracks is too soft to self-seal, affecting the accuracy in replicated structures in the replica.

4. The cracking within the mold needs to be carefully controlled for easy detachment (phase 1 mold from the master and replica from phase 2 mold) and accurate structural retention (for the mater, the mold, and the replica), e.g., the reproducible formation, propagation direction, and configuration of cracks in both stages.

A simple analytical model, based the closed-loop array structure, produces a phase diagram (geometric parameters versus cracking behavior) that are consistent with experiments and FEA analysis. A further analysis of the crack configurability with geometrical features in terms of the pulling force for replica detachment generates a plot, which guides the selection of a variety of replica and mold materials (fulfilling criteria in mechanical properties).

5. This replication technique is used to duplicate various complex, hierarchical structures with great success, e.g., a multi-beaded probe, multi-cycle helix, a bull, which all reproduce the exquisite nano- to micro-scale and large geometry contrast (high aspect ratio) structural details. These are extremely challenging for existing fabrication approaches, and the masters of these structures are made by expensive and time-consuming prototyping methods.

6. One this mold can be used to create replicas more than 50 cycles, still showing identical structures with the master, superior robustness and great potential for mass production of true 3D complex structures.

7. This replication technique successfully re-produces the complex, hierarchical structure (pillar arrays with doubly reentrant structures) of springtail skin, at much lower-cost than standard 3D prototyping; the replicas show exceptional dynamic wetting property and preservation in structural integrity and liquid repellence during/after repeated mechanical loading. And this technique duplicates many structural-delicate and functional-robust structures mimicking biological surfaces on other materials for diverse functional devices.

 

With virtues of high accuracy, rapid processing, and low cost, such a technique is simple in idea but impressive in output. Here is the link of the fulltext: https://www.pnas.org/content/116/48/23909

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