Abstract

To explore novel materials graded for biological functions is one of the grand challenges and ambitions of robotics. In this study, the design, development, and external guidance of micron-sized hair-derived robots (hairbots) are shown as autologous cargo carriers for guided drug delivery, untethered osteogenesis, and sonographic contrast agents. Having biogenic origin, the hairbots show excellent biocompatibility, as demonstrated with cell adhesion, spreading and proliferation. External magnetic fields are used to enhance differentiation of mesenchymal stem cells (MSCs) into bone like cells, which can be used as magnetic therapy for bone healing. Effect of external magnetic forces was simulated by COMSOL Multiphysics® modelling software. The action of hairbots as osteoconductive material triggering osteogenic differentiations of MSCs is studied via calcium signaling by fluorescence microscopy. Further, by exploiting the hollow medullary region, the proposed hairbots are designed to perform theranostic dual functions (therapy + diagnostic) – as Doxorubicin drug delivery vehicle, and for Ultrasound contrast imaging. Harnessing sensing and actuation due to magnetic capabilities of hairbots, for enhanced biological functionality shown herein, provides a novel material in the search of new multifunctional microrobots.

Introduction

The future ambitions of medical robotics research ride on the development of novel and multifunctional robotic platforms, which will enable scientific discoveries for future biomedical applications. Among the grand challenges of robotics, the quest for novel materials is at the top of underpinning technologies, which would have a wider impact on all application areas of robotics [1]. In this context, compliant, power efficient, bio-mimicking, and synthetic multifunctional materials with superior actuation and sensing have been discovered [2,3]. However, complex fabrication schemes, and the use of untested biomaterials, impedes the biocompatibility, biodegradability, and load-carrying capacity of microrobot [3]. Synthetic and compliant bio-inspired soft robots and actuators are proposed to overcome this scenario and to function as the next generation biomedical robots [4]. However, many applications of microrobots face basic hurdles of large-scale fabrication, demanding substantial investment of cost, and time [5].

Despite substantial progress, the use of synthetic materials for advance biomedical applications is limited. Following this objective, here we propose biogenic hair, sliced into discs, as a unique multifunctional material for microrobotics (hairbots). This novel scientific term aims at an emerging area of research, identifying compounds of natural origin. Hair is a natural protein filament that grows from hair follicles in the skin of numerous biological species including humans, as represented in Fig. 1(a). It has the potential to be used in several applications like flexible microelectrodes [6], liquid fertilizers [7], pollution control [8], concrete reinforcement [9], etc. Such applications convert hair from environmental problem causing bio-waste into environmentally useful material. Along with its sociocultural roles, hair possesses several unique physicochemical properties. For instance, cuticular surface, cysteine rich keratin composition, high elastic recovery, mechanical (tensile) strength, slow degradation, and thermal insulation make them suitable materials for biocompatible microrobotics applications [[10], [11], [12]]. Being abundantly available makes it easy to establish economically efficient routes for the batch production of microrobots. As shown in this work, it could also be used for drug-delivery and targeted therapy. A big advantage of the proposed hairbots is that due to the inherent nature of the preparation method, and contrary to elaborate and time-consuming clean-room fabrication techniques, these hairbots can be produced in very large quantities in a short time. Not only is the process fast, but it has lower cost as compared to other means of fabrication. To the best of our knowledge, there has been no study or report in the literature on direct applications of hair as microrobotics material. Further, when loaded with Superparamagnetic iron oxide nanoparticles (SPIONs), the hairbots become responsive to magnetic fields and gradients. Iron-oxide nanoparticles are used because of their potential to be used in diagnostics as probes in magnetic resonance imaging (MRI), positron emission tomography (PET) and near-infrared fluorescence (NIRF) imaging [[13], [14], [15]]. As the hair and the hairbots contain a central medullary hollow space, they have the additional functionality of acting as Ultrasound contrast agents. Such nanoparticles have also been proposed to be used as biosensors for the detection of glucose, proteins, urea and uric acid [[14], [15], [16], [17]]. SPIONs are also currently undergoing clinical trials for biomedical applications [15]. Their unique properties such as low toxicity, biocompatibility, potent magnetic and catalytic behavior and superior role in multifunctional modalities enable them to be used in biomedical applications [14]. Along with the drug carrying capability of hairbots demonstrated in this paper, SPION coated hairbots are well suited for targeted and minimally invasive therapies.

In the present work, we report one such potential application of magnetic hairbots – the preferential differentiation of mesenchymal stem cells into bone cells. Fine-tuning the stem cells’ differentiation into specific cell lineage and clinical diagnosis is still limited to bioengineered scaffolds [18]. Actuating the hairbots using external magnetic fields, the rigidity of local microenvironment around mesenchymal stem cells can be manipulated. This is significant because it is this rigidity which determines which cell type the mesenchymal stem cells will differentiate into. Engler et al. [19] discovered that the mesenchymal stem cells are extremely sensitive to the elasticity of the surrounding environment. Soft surroundings (mimicking brain) produce neurons, stiffer surroundings (mimicking muscle) produce muscle cells, and rigid surroundings (mimicking bone) produce bone cells [19]. Therefore, it is important to produce the right local niche for the differentiation of mesenchymal stem cells into bone tissue. Considering the hairbot biocompatibility, a local rigid microenvironment could be created around mesenchymal stem cells attached to the magnetically responsive hairbots using external magnets (Supporting Fig. S3), as reported in this paper.

Section snippets

Ultramicrotome sections and SPION coating

For the hairbot samples, chemically untreated and healthy hair shafts were taken from a 30 year old female. Strands of hair, dehydrated in a graded series of alcohol, are folded to make rope like structures which are further embedded in Embed resin Epon 812 (Sigma Aldrich). Sectioning was carried out using an Ultracut UCT Ultramicrotome (Leica, Germany) fitted with a 35° diamond knife (Diatome, Switzerland). After sectioning, hairbots were washed with prewarmed (50 °C) Phosphate Buffer Saline

Fabrication of magnetic hairbots

‘Hairbots’ are prepared using Ultramicrotome following standard procedure to preserve physical and chemical nature of naïve hair [25]. The thickness of the hairbots is around 10 μm and the lateral dimension is between 60 μm and 80 μm, as shown in laser scanned 3D reconstructed images in Supporting Fig. S1(a). Drug loading (Fig. 1(c)) enables the hairbots for in vitro testing (Supporting Fig. S7, Supporting Video S6); and in future could enable in vivo therapies. In Fig. 1 (d) and supporting 

Discussion

As quantified from DNA extorted from proliferated mMSCs (Fig. 2(a)), hairbots show enhanced proliferations compared to control on day 5 and onwards, though statistically unsupported (student t-test, p-value 0.07). This might be due to the potential medullary spaces and microrough surfaces on hairbot sustaining high proliferations compared to control groups, where cells rapidly adopt to contact inhibition as monolayer. Due to 10 μm thickness of hairbots, visually capturing the cell adhesion on

Conclusion

In conclusion, we presented a fabrication method for preparing the microrobots, which is much cheaper, faster, and simpler than elaborate conventional clean room methods. We demonstrate how these microrobots can be used for the differentiation of mesenchymal stem cells into bone tissue. We further demonstrate the drug carrying capability of these microrobots, and how they can be made responsive to magnetic fields, thereby creating dual-functionality microrobots for targeted drug delivery. The

https://www.sciencedirect.com/science/article/abs/pii/S0142961219304934