Yi Zhang 张翼

Magnetic Soft Millirobots 3D Printed by Circulating Vat Photopolymerization to Manipulate Droplets Containing Hazardous Agents for In Vitro Diagnostics, Advanced Materials, 2022, 2200061

3D printing via vat photopolymerization (VP) is a highly promising approach
for fabricating magnetic soft millirobots (MSMRs) with accurate miniature 3D structures; however, magnetic filler materials added to resin either strongly interfere with the photon energy source or sediment too fast, resulting in the nonuniformity of the filler distribution or failed prints, which limits the application of VP. To this end, a circulating vat photopolymerization (CVP) platform that can print MSMRs with high uniformity, high particle loading, and strong magnetic response is presented. After extensive characterization of materials and 3D printed parts, it is found that SrFe12O19 is an ideal magnetic filler for CVP and can be printed with 30% particle loading and high uniformity. By using CVP, various tethered and untethered MSMRs are 3D printed monolithically and demonstrate the capability of reversible 3D-to-3D transformation and liquid droplet manipulation in 3D, an important task for in vitro diagnostics that are not shown with conventional MSMRs. A fully automated liquid droplet handling platform that manipulates droplets with MSMR is presented for detecting carbapenem antibiotic resistance in hazardous biosamples as a proof of concept, and the results agree with the benchmark.

微型软体机器人可以主动改变自身的形状以实现更加灵活的运动和操作,因而十分擅长在小尺寸且空间受限的环境中工作，这一独特的优势将使得他们被广泛地运用在生物医学领域。磁驱动是目前控制软体机器人最流行的方法之一，因为磁驱动具备可以远程无线控制、响应快速且易于实现的优势。然而由于制造技术受限，目前大多数磁性软体机器人是由翻模铸造，通常需要繁琐及耗时的制作步骤且局限于2D结构，因而功能简单且单一。

3D打印技术为打破这种局限提供了可能。立体光固化3D打印技术(Vat Photopolyerization，简称VP)具有高精度、高分辨率、快速成型等优点。通过立体光固化技术，3D打印添加了磁性颗粒填料的弹性树脂，可以实现微型磁性软体机器人的快速整体成型。然而在普通的立体光固化平台上，如果使用纳米尺度的磁性颗粒填料，磁性颗粒会对树脂基质的光吸收造成很大的干扰。如果使用粒径较大的微米尺度磁性颗粒填料，虽然对光吸收干扰较小，但是磁性颗粒会快速沉降到容器底部，这将导致磁性填料在打印部件中的分布不均匀，甚至导致打印无法完成。因此，大多数前期研究只使用纳米磁性颗粒作为磁性填料，同时保持较低的载量(<1%) 来确保磁性复合材料能够成型，然而此类材料打印的微型机器人不能产生大的磁力用以驱动微型结构的形变，这些机器人大多只能实现刚性移动，通常也需要在液体环境中，利用液体的润滑特性与浮力来减少运动的阻力。

    本文提出了一种新型的循环立体光固化3D打印技术(Circulating Vat Photopolymerization, 简称CVP)来解决上述问题。CVP在普通立体光固化平台中整合了一套树脂循环系统，该系统在打印过程中能不断地对磁性复合材料进行混合，从而保证颗粒在树脂中的均匀分布。本文发现并解释了磁性颗粒的材料和尺寸对光固化3D打印过程的影响，论证了锶铁氧体(SrFe₁₂O₁₉)磁性颗粒作为磁性填料的优越性，其作为磁性填料在磁性树脂中载量可高达30%。通过使用CVP，本文展示了各种一次成型的栓系和非栓系微型磁性软体机器人以及其通过3D形变操纵液滴的能力。通过合理的自动化控制实现不同的磁性软体机器人通力合作，成功地展示了针对碳青霉烯类抗生素耐药性的检测，这是目前其他类型的磁性微型软体机器人难以实现的。

Control of liquid droplet and liquid container by magnetic soft claw.

磁性软体机械爪控制液滴及液体容器。

Control of liquid droplet by untethered magnetic soft “caterpillar” and “rocking horse”.

磁性软体“毛毛虫”和“摇摇马”控制液滴

Biomimetic magnetic soft millirobots. 放生磁性软体机器人。

Magnetic soft millirobots-empowered automated magnetic digital microfluidic platform for in vitro diagnostics of carbapenemase-producing enterobacteriaceae.

磁控软体机器人操控的自动化磁性数字微流体体外诊断平台用于检测碳青霉烯类抗生素耐药菌。

VeroWhitePlus is a hard and rigid material at room temperature with a glass transition temperature of 58 °C, whereas TangoBlack-Plus is soft and rubbery at room temperature with a glass transition temperature of −10 °C. In the programming stage, the printed part with a bilayer structure is pro-grammed into the second shape in heated ethanol that acts as the first stimulus for rapid contactless shape-setting. The elastomer (TangoBlackPlus (TBP)) is swelled via the diffusion of ethanol. The ethanol is heated to speed up the rate of diffusion. The high temp-erature also heats the transition material (VeroWhitePlus (VWP)) above its glass transition temperature (Tg)to reduce its stiffness for easy reshaping. When soaked in ethanol, only the elastomer is swelled. This asymmetric swelling results in a difference in strain between the elastomer and the transition material. The induced strain exerts a force on the transition material. As the entire printed part is heated above Tg, the transition material becomes rubbery and morphs into a different shape under the exerted strain.In the recov-ery stage, the printed part is dry heated using an air gun or in a convection oven to recover its original shape. Dry heating is used to speed up the evaporation of ethanol from the elastomer and to heat the transition material above its Tg for easy reshaping.

VeroWhitePlus （VWP）和TangoBlackPlus(TBP) 都是相变材料，其相变温度分别为58℃和-10℃，常温下VWP表现出高硬度状态，达到相变温度后表现出弹性且柔软的状态，TBP在常温以及VWP相变温度范围内则一直为弹性状态。同时，TBP还是吸湿膨胀材料，当TBP置于99%酒精中时，体积膨胀率最高可达4.506。根据这两种材料的特性以及上文提到的多层材料膨胀原理，团队设计了双层结构并通过按一定顺序施加特定的激发条件，实现了结构的双向转化。具体流程如图文展示2a所示，当双层结构需要从平直状态转化为弯曲状态时，首先需要将整个结构浸入到60℃的酒精中，此时VWP达到相变温度并转化为弹性且柔软的状态，TBP则吸收酒精分子致使体积膨胀，因而结构发生弯曲。弯曲结束后，结构被迅速放入冰水中使VWP温度迅速降到相变温度之下，尽管TBP会因酒精的蒸发而体积收缩，但其产生的弹性势能并不足以将VWP拉回平直的状态，整个结构因此能在常温下保持弯曲状态，弹性势能则被储存在这个阶段。当再次加热整个结构时，TBP受热达到相变温度变柔软，VWP储存的弹性势能得到释放，部件恢复到初始状态。

Contactless Reversible 4D-Printing for 3D-to-3D Shape Morphing, Virtual and Physical prototyping, 2020, 15, 481-195
Preliminary Investigation of the Reversible 4D Printing of a Dual-Layer Component, Engineering, 2019, 5, 1159-1170