Ferromagnetic soft continuum robots

本文来自机器人领域顶刊，Science Robotics。介绍了一种铁磁软连续体机器人，主要贡献为1）介绍了一个基于磁性微粒和聚合物的软连续体机器人的制造方法，在机器人外表面涂了一层水凝胶，极大地降低了摩擦阻力；2）并研究了在磁场下，不同粒子体积分数下，此机器人的驱动性能；利用手动操控的磁铁，进行了许多实验证明该机器人性能的优异性。

铁磁软连续体机器人
Ferromagnetic soft continuum robots [1]
Paper Link
Authors: Kim, Yoonho, et al.
2019, Science Robotics

0.摘要 Abstract

小尺度的软连续体机器人能够在远程可控方式下主动转向和导航，它们在各种各样的领域都有巨大的潜力，特别是医疗应用方面。现存的连续体机器人，然而，经常受限于毫米级或厘米级尺度因为传统驱动机理中固有的小型化挑战，比如拉动机械线，给气动或者液压腔室充气，或者内置刚性磁铁用于操纵。另外，在导航过程中连续体机器人经受的阻力对于它们的应用来说是另一个挑战。这里，我们提供一个半毫米尺度，自润滑软连续体机器人，有基于磁驱动的全向转向和导航能力，这通过编程在它软体内的电磁区域使能，同时增长它表面的水凝胶。机器人的身体由一个软体聚合物基质的同质连续体和均匀分布的铁磁微型颗粒组成，能够小型化到低于直径几百微米，并且水凝胶皮肤减小了超过十倍的阻力。我们演示导航通过复杂和受限制的环境的能力，比如例如多发性动脉瘤的曲折脑血管仿体。我们进一步演示其他功能，比如通过一个嵌入机器人身体的功能核心进行可转向激光给药。鉴于其紧凑，自成一体的驱动方式和直观的操作方式，我们的铁磁软连续介质机器人可能会为微创的机器人手术开辟途径，以解决以前无法接近的病变，从而解决医疗保健方面的挑战和未满足的需求。
Small-scale soft continuum robots capable of active steering and navigation in a remotely controllable manner hold great promise in diverse areas, particularly in medical applications. Existing continuum robots, however, are often limited to millimeter or centimeter scales due to miniaturization challenges inherent in conventional actuation mechanisms, such as pulling mechanical wires, inflating pneumatic or hydraulic chambers, or embedding rigid magnets for manipulation. In addition, the friction experienced by the continuum robots during navigation poses another challenge for their applications. Here, we present a submillimeter-scale, self-lubricating soft continuum robot with omnidirectional steering and navigating capabilities based on magnetic actuation, which are enabled by programming ferromagnetic domains in its soft body while growing hydrogel skin on its surface. The robot’s body, composed of a homogeneous continuum of a soft polymer matrix with uniformly dispersed ferromagnetic microparticles, can be miniaturized below a few hundreds of micrometers in diameter, and the hydrogel skin reduces the friction by more than 10 times. We demonstrate the capability of navigating through complex and constrained environments, such as a tortuous cerebrovascular phantom with multiple aneurysms. We further demonstrate additional functionalities, such as steerable laser delivery through a functional core incorporated in the robot’s body. Given their compact, self-contained actuation and intuitive manipulation, our ferromagnetic soft continuum robots may open avenues to minimally invasive robotic surgery for previously inaccessible lesions, thereby addressing challenges and unmet needs in healthcare.

1.介绍 Introduction

半毫米精度的内置微型磁铁的软聚合物被提出
a submillimeter-scale device (500 $\mu m$ in diameter) with tiny magnets embedded in a soft polymer rod has been proposed

然而，用完全软体连续体机器人导航通过脑血管结构还没有被实现。
However, navigating through cerebrovascular structures with fully soft-bodied continuum robots has not been realized so far.

本质的限制与刚体磁铁的使用有关。
the inherent limitations associated with the use of such rigid magnets.

端部微型磁铁可能崩解，这导致不期望的临床问题。
tiny magnets at the tip could break off, which may lead to undesired clinical problems.

首先，现存的基于气动或者液压的软机器人都有很重的尾绳，这限制在真实医疗应用中的使用，那都需要无尾绳驱动。第二，大部分软机器人基于定量模型是很难精准控制的，主要因为它们的驱动机理总是依赖于高度非线性变形或不稳定性。第三，传统软机器人是很难小型化到低于毫米尺度的，因为它们的制造工艺流程不能支持如此小的尺度。
First, existing soft robots based on pneumatic or hydraulic actuations are mostly heavily tethered, which limits their use in realistic medical applications that typically require tether-free actuation. Second, most soft robots are difficult to accurately control based on quantitative models, largely because their actuation mechanisms often rely on highly nonlinear deformation or instabilities. Third, conventional soft robots are difficult to miniaturize below millimeter scales, because their fabrication schemes are often unfavorable to such small size.

进一步阐述在材料，制造，铁磁软机器人理论的最近的进展后，我们展示半毫米尺度的铁磁软连续体机器人，它能够导航穿梭在高度受限的环境中，比如狭窄的曲折的血管结构中，基于磁驱动的主动型全向的转向。
Further elaborating on the recent progress in materials, fabrication, and theory for ferromagnetic soft robots, we present submillimeter-scale ferromagnetic soft continuum robots that can navigate through highly constrained environments, such as narrow and tortuous vasculature, based on active, omnidirectional steering upon magnetic actuation.

2.结果 Results

2.1 铁磁复合油墨 Ferromagnetic composite ink

软磁性材料容易失去磁化一旦外加磁场被移除
soft magnetic materials easily lose the induced magnetization once the external field is removed

我们的软连续机器人的主体是由一个弹性体复合物组成，它包含可磁化微型颗粒。机器人身体的软聚合物由PDMS或TPU组成。
The main body of our soft continuum robot was made of an elastomer composite that contains magnetizable microparticles (5-$\mu m$-sized on average) of a NdFeB alloy. The soft polymer matrix of the robot’s body was composed of either silicone [polydimethylsiloxane (PDMS)] or thermoplastic polyurethane (TPU) elastomers, depending on desired mechanical properties.

2.2 打印/注射成型 Printing/injection molding

软连续体机器人能通过打印或者注射成型被制造
The soft continuum robot could be fabricated by either printing or injection molding

2.3 水凝胶皮肤 Hydrogel skin

测量显示在所有给定条件下润滑的水凝胶皮肤在阻力系数上有十倍的下降。
The measurements showed a 10-fold decrease in the friction coefficient as a result of the lubricious hydrogel skin in all given conditions.

2.4 包裹粒子的二氧化硅外壳 Silica shells around the particles

2.5 最优驱动设计 Design for optimal actuation

由铁磁软合成物制作的机器人的磁和机械性质随粒子加入浓度变化。这里，我们描述我们的材料设计策略来优化所提出铁磁软连续体机器人的驱动表现。
Both magnetic and mechanical properties of the robot’s body made of ferromagnetic soft composites varied with the particle loading concentration. Here, we describe our material design strategy to optimize the actuation performance of the proposed ferromagnetic soft continuum robot.

因为我们的软连续体机器人的磁反应迅速的端子是轴向磁化的，端子趋向弯曲沿施加磁场方向因为从内置磁粒子生成的磁体力矩。为了找到最优粒子浓度，在给定条件和形状下达到最大弯曲，失去一般性，我们考虑一个长度L直径D的棒在均匀磁场B下，B是正交于M的。另外，为了使用易处理的解析解，我们进一步假设磁主动端子经历小弯曲，偏转低于端子长度L的10%
Because the magnetically responsive tip of our soft continuum robot is axially magnetized (i.e., M along the axial direction), the tip tends to bend along the applied magnetic field B due to the magnetic body torques generated from the embedded magnetized particles. To find the optimal particle concentration that yields the largest bending under given conditions and geometry, without loss of generality, we consider a beam of length L and diameter D under uniform magnetic field B that is being applied perpendicularly to M. In addition, to use a tractable analytical solution, we further assume that the magnetically active tip undergoes small bending, where the deflection is below 10% of the tip length L. Then, we can reach the following analytical expression for the deflection of the magnetically active tip

对于小的弯曲，棒的形变与无量纲数MB/G正比，二次依赖于比L/D。

给定依赖于粒子体积分数的M和G，公式一暗示可能存在一个最优点，归一化形变是最大的。

二氧化硅涂层对剪切模量没有明显改变。

我们确定浓度，在这个浓度下形变最大，在给定条件下（场强和几何）。临界体积分数为20.7%。
we can identify the critical concentration at which the deflection is maximized for given conditions (field strength B and the geometric factor L/D). The critical volume fraction is calculated to be 0.207 (or 20.7 volume %)

对于大的弯曲，对于不同的L/D比例，仿真和实验结果都显示驱动角度随着归一化场强的增加。
For large bending, the simulation and experimental results indicate that the actuation angle monotonically increases as a function of the normalized field strength MB/G for different aspect ratios L/D.

小角度形变分析在大角度情形时依然有效。
the small-deflection analysis will remain effective for large bending cases as well

当由磁驱动产生的机械功比大角度形变重要时，我们能够以能量密度的名义优化驱动表现，能量密度对应于从连续体机器人中获取的功（单位体积）。
When producing mechanical work out of magnetic actuation is of greater importance than the large deflection, we can optimize the actuation performance in terms of the energy density, which corresponds to the amount of work (per unit volume) that one can extract from the continuum robot.

能量密度达到最大值当粒子体积分数是29.3%时，给定作用场B和形状L/D。
the energy density reaches its maximum when the particle volume fraction is 29.3 volume % under given conditions in terms of applied field strength B and geometry L/D

当弯曲变大，然而能量密度最大化的顶点移动到右侧，达到了更高的体积分数。
As the bending becomes larger, however, the peak at which the energy density is maximized shifts to the right, toward the higher volume fractions

定性的，这能被理解为考虑这指数增加的刚性，这主要贡献了能量密度当变形级别保持几乎不变的时候。
Qualitatively, this can be understood by considering the exponentially increasing stiffness, which dominantly contributes to the energy density when the deformation level remains almost unchanged

这暗示了高尺寸比的良好性质，如同纤毛软连续体机器人，需要极小的场强来诱导弯曲驱动。
This implies that fine features with high aspect ratios, such as cilia-like soft continuum robots, would require significantly lower field strength to induce the bending actuation.

2.6 主动转弯和导航 Active steering and navigation

我们使用一个圆柱永磁铁在一定距离施加驱动磁场。
we used a cylindrical permanent magnet (diameter and height of 50 mm) to apply the actuating magnetic fields at a distance.

原型机是通过注射成型制造的，直径0.6毫米。为了提供演示任务所需的机械支持和可推动性，我们将镍钛合金（镍钛合金）芯集成到了机器人的身体中。 因为镍钛合金芯是来自商用导丝的尖端，所以磁响应尖端自然地连接到商用导丝。我们也基于打印TPU+磁微粒合成物不带核心制造了另一个原型机，直径0.81毫米。
第二个原型机粗一些。也运动慢一些。选择基于PDMS的聚合物用于进一步的探索。
The demonstrated prototype was fabricated through injection molding of the PDMS + NdFeB composite ink and was designed to be 600 $\mu m$ in diameter. To provide mechanical support and pushability required for the demonstrated task, we incorporated a nickel-titanium alloy (nitinol) core in the robot’s body. Because the nitinol core was from the tip of a commercial guidewire, the magnetically responsive tip was naturally connected to the commercial guidewire. We also fabricated another prototype based on printed TPU + NdFeB composite without a core. The printed segment was also connected to the commercial guidewire. Because the printed segment does not contain any core for additional support, this prototype was designed to be thicker in diameter (810 $\mu m$) to ensure sufficient bending rigidity required for the demonstrated tasks. we chose to use the PDMS-based composite for further exploration

为了能急剧转弯，我们改变端子的弯曲刚度。
To enable making sharp turns and hence navigating through a tortuous path, we introduced a variation in the bending stiffness of the magnetically responsive part of our soft continuum robot.

有一短的较软的部分再磁主动部分的远端。这较软的部分只包含PDMS+磁微粒聚合物，因而比其他部分软，其他部分有刚性镍钛合金核心。更刚性部分的杨氏模量是较软部分的十倍。
the continuum robot (diameter of 600 $\mu m$) has a short (3-mm-long), softer segment at the distal end of the magnetically active portion. This softer segment was composed of the PDMS + NdFeB composite only and thus substantially softer than the remainder, which contained a stiff nitinol core. The effective Young’s modulus of the stiffer segment (14 MPa) was calculated to be 10 times that of the softer segment (1.4 MPa)

在导航过程中，推动（机器人的）近端来推动机器人的磁操纵式远端。（就是手拿着往里怼，让机器人深入。）
The proximal end was pushed to advance the magnetically steered distal tip of the robot during the navigation.

接下来在多种复杂、曲折、狭窄的血管结构仿体中做了很多有意思的实验。

2.7 Steerable laser delivery 可转弯激光给药

结合一个光纤（制造的时候插在内部）
incorporated an optical fiber

[1]: Kim, Yoonho, et al. “Ferromagnetic soft continuum robots.” Science Robotics 4.33 (2019): eaax7329.