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Rocket Science at the Nanoscale Jinxing Li, Isaac Rozen, and Joseph Wang* Department of Nanoengineering, University of California San Diego, La Jolla, California 92093, United States S Supporting Information *
ABSTRACT: Autonomous propulsion at the nanoscale represents one of the most challenging and demanding goals in nanotechnology. Over the past decade, numerous important advances in nanotechnology and material science have contributed to the creation of powerful self-propelled micro/nanomotors. In particular, micro- and nanoscale rockets (MNRs) offer impressive capabilities, including remarkable speeds, large cargo-towing forces, precise motion controls, and dynamic self-assembly, which have paved the way for designing multifunctional and intelligent nanoscale machines. These multipurpose nanoscale shuttles can propel and function in complex real-life media, actively transporting and releasing therapeutic payloads and remediation agents for diverse biomedical and environmental applications. This review discusses the challenges of designing efficient MNRs and presents an overview of their propulsion behavior, fabrication methods, potential rocket fuels, navigation strategies, practical applications, and the future prospects of rocket science and technology at the nanoscale. KEYWORDS: nanomachines, nanoscale rockets, nanomotors, propulsion, motion control, nanomedicine, drug delivery, environmental remediation, biodefense, active nanomaterials
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possess extremely high velocities (over 1000 body length/sec), remarkable design flexibility, and surface functionalization with different nanomaterials and biomolecules. Considerable efforts have thus been made to use these impressive capabilities of MNRs to develop nanoscale machines for diverse applications. The development and applications of these powerful nanoscale rockets draw many parallels to conventional rocketry. Similar to their macroscale counterparts, the MNRs generate remarkable thrust and force from chemical reactions to move forward at ultrafast speeds (Figure 1A,B). The propulsion of MNRs can be enhanced through the rational optimization of their structure and surrounding environment, including the rocket material composition and morphology as well as the fuel and fuel additives.15−17 Additionally, precise motion control and advanced navigation capabilities can be achieved by using external stimuli, such as magnetic, heat, light, or acoustic fields, which can modulate their propulsive behavior reversibly. The bubble propulsion of MNRs enables them to operate in a wide range of salt-rich real-life environments and highly viscous biological media.15,18 Unlike large rockets which must carry their functional components internally, the outer surface of MNRs can be designed with different materials tailored for accomplishing specific missions and transporting their cargo.
he rapid development of rocket science during the 20th century laid the foundation for the Space Age and led to significant industrial and scientific advances. Largescale rockets consist of powerful engines that utilize chemical reactions to generate immense thrust for rapid acceleration and propulsion.1 The remarkable thrust generated by these large engines can overcome both gravity and pressure drag to break past Earth’s atmosphere, enabling the delivery of important payloads (e.g., satellites or astronauts) into and beyond Earth’s orbit. Likewise, nanotechnologyintroduced in 1959 by the visionary physicist Richard Feynman2has inspired scientists and engineers to design and manufacture nanoscale devices and microscale systems capable of complex operations in previously inaccessible locations. Over the past decade, the development of self-propelled nanoscale devices has become one of the most demanding goals of nanotechnology.3−11 Within this regime, propulsion poses its own set of unique challenges, where the forces of gravity and pressure drag at the macroscale have been replaced by low Reynolds number viscous drag and Brownian motion at the nanoscale.12 Inspired by the sophisticated propulsion strategies of natural biomolecular motors, scientists have developed self-propelled synthetic micro/nanoscale rockets (MNRs)also termed micro/nanoengines and micro/ nanojetsthat consume locally supplied chemical fuels to generate thrust, analogous to how large rocket engines ignite onboard propellants (Figure 1).3,6,13,14 MNRs offer tremendous advantages as self-propulsive nanodevices. Such tiny rockets © 2016 American Chemical Society
Received: April 13, 2016 Accepted: May 24, 2016 Published: May 24, 2016 5619
DOI: 10.1021/acsnano.6b02518 ACS Nano 2016, 10, 5619−5634
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PROPULSION MECHANISM Motion at the nanoscale requires unconventional approaches to achieve efficient propulsion because of the remarkably small Reynolds numbers associated with tiny swimmers. The Reynolds number is defined as the ratio of inertial to viscous forces and considerably varies with the characteristic length, that is, the size of the object in motion. For macroscale rockets moving at high velocities through the atmosphere, the Reynolds number ranges from 105 to 108, a regime where the inertial forces dominate and the flow is highly turbulent. Consequently, large rockets must be able to overcome significant pressure drag, on top of lifting over one million pounds of weight, including the rocket fuel and payload.1 The engines of these large rockets are highly efficient and have been optimized to maximize their thrust for lightweight propellants. Figure 1A details a common bipropellant rocket propulsion system, with both the fuel and oxidizer stored separately in liquid or solid forms. Ignition of the fuel within the combustion chamber generates hot exhaust gases, which are expanded and accelerated at the rocket nozzle to supersonic speeds for remarkable thrust. Such conventional rockets thus require internal tanks, pipes, and valves to cool, store, and transport the fuel and oxidizer to the combustion chamber, which add significant weight. After completing their missions and burning through their fuel, these rocket booster stages are typically discarded and are difficult to recover and reuse, adding significantly to the cost of orbital launches (e.g., the Atlas V launch; Figure 1C). However, at the micro- and nanoscale, the extremely small dimensions and velocities contribute to very low Reynolds numbers ranging from 10−1 to 10−5. Viscous forces heavily dominate inertial forces within this regime, giving rise to Purcell’s Scallop theorem, which states that in low Re environments, reciprocal motion which is invariant under time reversal cannot achieve propulsion.12 Hence, at these tiny length scales, swimmers must be able to exert a nonreversible thrust against the medium for propulsion. Figure 1B details how micro- and nanoscale rockets satisfy this requirement to achieve efficient motion. These microrockets draw in fuel from their surrounding medium through their smaller end for chemical reactions within their catalytic or reactive hollow interior. These reactions produce gas bubbles, which nucleate and are ejected out of the larger end of the rocket. This bubble ejection is accompanied by a powerful thrust, which propels the rocket forward by a discrete step length. At high ejection frequencies, the motion becomes nearly continuous,15 as displayed in the magnetically guided movement of multiple microrockets shown in Figure 1D. Other common catalytic micro- and nanomotors, such as spherical Janus particles and bimetallic nanorod swimmers, generate a local product gradient by breaking down similar chemical fuels and creating a selfdiffusio- or electrophoretic flux to achieve directed motion.8,9 However, this phoretic propulsion efficiency is significantly weaker than that observed using the thrust-based bubble recoil propulsion mechanism at equivalent fuel levels, only reaching relative peak speeds of 70 body lengths/s compared to 1400 body lengths/s for MNRs.15,24 Unlike their macroscale counterparts, nanoscale rockets have the advantage of harvesting their chemical fuel directly from their surrounding environments. These tiny catalytic rockets thus continue to move as long as the fuel is present. This eliminates the needs for fuel storage or refueling and allows for
Figure 1. Comparison of macroscale rocket propulsion to microrocket propulsion. Both large-scale and nanoscale rockets rely on chemical fuels to achieve motion. (A) Traditional rockets carry on-board fuel and oxidizer for reactions within the combustion chamber, and the hot exhaust provides thrust. (B) Microrockets harvest their fuels directly from the local environment at the narrow opening, which reacts along the inner cavity to produce bubbles that emerge out of the wider exterior opening for thrust. (C) Atlas V rocket launches with the Juno spacecraft payload from Space Launch Complex 41 at Cape Canaveral Air Force Station in Florida on Friday, August 5, 2011. Photo credit: NASA/Bill Ingalls. (D) Catalytic microrockets propelled by oxygen bubbles (from Supporting Information Video S1).
Such remarkable capabilities and versatility of MNRs have paved the way toward multifunctional and intelligent micro/ nanomachines. These sophisticated nanoscale rockets have advanced rapidly since their introduction nearly a decade ago and have already proven to be promising for performing diverse biomedical tasks, ranging from drug delivery to isolation of cancer cells. Recent discoveries have shown that MNRs can efficiently function and actively deliver payloads in vivo, with no toxic byproducts or tissue damage.19 These developments indicate that nanoscale rockets may play a key role in the “moon-shot” for curing diseases and toward realizing the “Fantastic Voyage” vision for medical treatment.20 These MNRs have also found successful use in diverse environmental and defense applications, including water monitoring, remediation, and detoxification,21−23 with the latter involving the transport and dispersion of remediation agents. The success in realizing powerful and functional nanoscale rockets represents a critical step toward the future of intelligent nanovehicles and nanomachines. This review aims at discussing these advances and capabilities, provides a survey of the fabrication methodologies, potential rocket fuels, navigation strategies, practical applications of MNRs, and presents an outlook for the future of rocket science at the nanoscale. 5620
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Figure 2. (A) Body-deformation model for the stepwise movement mechanism of tubular microengines. (B) Calculated speed v in the form of body length/s as a function of microrocket radius and length. Reproduced with permission from ref 25. Copyright 2011 The Royal Society of Chemistry.
long, sustained propulsion throughout the medium, a particularly attractive feature for use in biological or environmental media. Furthermore, microengines that catalyze the bubble-thrust reaction are highly reusable because none of the rocket components is consumed during the propulsion and can thus be easily recovered (such as through simple filtration or magnetic collection techniques). Other microrockets based on reactive materials (e.g., Zn) stop their motion once their reactive layer is completely dissolved. Open tubular microrockets were introduced in 2009 by Solovev et al., who fabricated such microengines and discussed their propulsion mechanism.14 The basic structure and propulsion mechanism offered by Solovev et al. are still integral to current microrocket design and performance.6 In particular, the microrocket must contain an inner catalytic or reactive surface around a hollow core to provide a favorable nucleation site for the thrust-providing bubbles essential for giving these rockets their momentum. Such thrust, which is the force propelling the microrocket, is the result of pressure exerted by the bubble ejection. This catalytic layer cannot be exposed to the exterior, so an outer unreactive stabilizing tubular shell must be included. Furthermore, the microengine must be mildly conical or asymmetric in shape to favor the fuel intake at the smaller opening and bubble ejection out of the other larger opening. The exact mechanism of the bubble release accompanying each step length has been studied in detail.25−30 Li et al. treated the microrocket−bubble system as an asymmetric cycle (Figure 2), where each bubble released would propel the rocket a step length l forward.25 The average velocity would thus be closely related to the product of l and the ejection frequency f. Accordingly, they built a body-deformation model to quantitatively describe the dynamics of hydrogen peroxide fueled microrocket propulsion. In this model, the microrocket and a microbubble are treated as one system that changes its configuration in each step from “microrocket with a bubble inside” to “microrocket with a detached bubble” (Figure 2A). Considering the drag forces along the center axis of the microrocket, they expressed the moving distance l of the microrocket in each step as
l=
∫
t0
t1
vj(t )dt =
6R b2 ⎛ ⎛ 2L ⎞ ⎞ 3R b + L /⎜ln⎜ R ⎟ − 0.72⎟ ⎝ ⎠ j ⎝ ⎠
(1)
where Rb is the bubble radius and L and Rj are the length and radius of the tubular cavity of the microrocket, respectively. In a continuous motion, the average speed v of the microrocket is then given by v=
9nC H2O2R jL
⎛ ⎛ 2L ⎞ ⎞ 3R b2 + LR b/⎜ln⎜ R ⎟ − 0.72⎟ ⎝ ⎠ j ⎝ ⎠
(2)
where CH2O2 is the fuel concentration and n is the O2 production rate. Crucially, the MNR speed generally increases with the concentration of fuel and decreases with the size of the ejected bubble. The authors also found that the initial nucleation of the bubble at the front end is followed by its growth upon moving outward toward the rear of the rocket. The bubble can expand to a larger size than the original cavity radius after exiting the cavity, and the detachment thrust force is proportional to this ejected bubble radius. This bubblerelease cycle is strongly dependent on the rocket geometry because the bubble recoil force depends on the ejected bubble size. It was found that MNRs with larger exit radii tend to release larger bubbles but are then limited by a slower ejection frequency, which is inversely proportional to the ejected bubble size.25 These trade-offs indicate that large exit radii of the rockets lead to discrete stepwise propulsion, while smaller exit radii contribute to continuous high-speed motion. In another model, Li et al. proposed that the conical angle should also be considered when selecting radii size, as the nanorocket drag coefficient tends to decrease with larger conical angles.27 Fomin et al. examined the mechanism of the bubble release, proposing that the geometric asymmetry of the microrocket leads to the development of a capillary force which drives the bubble out of the larger tube end, causing a momentum transfer to the fluid responsible for the recoil propulsion.28 This model is complementary to the available models of the self-propulsion of catalytic tubular microrockets. Other geometry considerations examined by the authors included the effect of the rocket length on the average velocity. Generally, the speed of the rocket improves with an increased length, reflecting the larger surface area of the inner catalytic layer, and hence improves the 5621
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Figure 3. Common methods for fabricating micro/nanoscale rockets. (A) Strain-engineered self-rolling of thin films into microtubes. Reproduced with permission from ref 14. Copyright 2009 Wiley-VCH. (B) Template-assisted electrodeposition of conical microtubes. Reproduced from ref 31. Copyright 2011 American Chemical Society. (C) Template-assisted electrodeposition of bimetallic nanorockets. Reproduced with permission from ref 38. Copyright 2013 The Royal Society of Chemistry. (D) Polymer electrospinning and cryo-sectioning process for hollow microtube microrockets. Reproduced with permission from ref 40. Copyright 2016 Wiley-VCH.
bubble-ejection frequency.25 However, for long lengths, the increased speed is limited by the larger viscous drag on the rockets. In addition, at excessive lengths, the ultrahigh bubble production rate will cause bubbles to exit from both ends, which significantly impedes the thrust. Wang et al. illustrated that microrockets with a smaller opening need a higher fuel concentration to be activated, while microrockets with larger tubes can possess a higher absolute speed than smaller tubes but do not necessarily have a higher relative speed in units of body length/s.29 These geometric and size considerations leave considerable room for optimizing the rocket speed for different dimensions and MNR aspect ratios. Beyond these geometric effects, the chemical environment also plays a significant role in the propulsion of these microrockets. The choice of fuel and catalytic layer has a profound effect upon the reaction kinetics and propulsion efficiency, as will be discussed in-depth in the following sections. Generally, the speed of peroxide fuel microrockets increases in a nearly linear relationship with the fuel concentration.31 However, the exact dependence may depend on the specific reaction kinetics. The rate of these catalytic reactions of the fuel can also be increased upon raising the temperature of the medium.31,32 The bubble propulsion can be improved by adding fuel additives or low concentrations of surfactants.15,17 The latter lower the surface tension and stabilize the microbubbles to dramatically improve the bubble evolution and their ejection frequency. The movement and bubble evolution of tubular microengines have been shown to induce significant fluid transport, as was illustrated from the enhanced diffusion of passive microsphere tracers.33
pressure drag, which dictate the overall form of the rocket exterior, and stability concerns demand careful attention to the rocket shape and center of mass. The material and structural composition of large rockets is also important for heat resilience, drag reduction, and strength against stresses when the rocket undergoes maximum dynamic pressure.1 Likewise, careful geometric considerations must be made to enhance the performance of rockets at the nanoscale. The conical shape is crucial for achieving effective unidirectional bubble ejection, and control of the length, entry, and exit radii is also crucial for optimizing the speed of small-scale rockets, as discussed above. The choice of outer layer is important to reduce the viscous drag on the microrocket, and additional functionalities can be implemented through the careful design of these outer layers. Current microfabrication processes are still predominantly planar techniques, which makes three-dimensional tubular structures, let alone with structures with hollow inner cores and conical geometries, difficult to realize. Here, we detail the main approaches for fabricating commonly used microrockets. Roll-up Nanotech. The initial tubular microjets were massfabricated by Schmidt and co-workers using strain-engineered two-dimensional roll-up nanotechnology.13,14 This fabrication route utilizes the intrinsic strain gradients inside the nanomembranes to assemble three-dimensional micro/nanostructures, such as microtubes and microsprings (Figure 3A). This approach takes inspiration from the strain-engineered growth of rolled-up nanotubes by molecular beam epitaxy, which also allows strong control over the critical dimensional parameters but proved too slow for the mass production of nanorockets. As displayed in Figure 3A, microtube roll-up fabrication is achieved by the selective under-etching and release of a strained thin film from a sacrificial layer.14 The strain gradient perpendicular to the surface is introduced by the inherent variation of thermal expansion in different metallic layers through the sequential deposition processing. Upon release from the sacrificial layer,
ROCKET FABRICATION AT THE NANOSCALE The propulsion behavior of rockets at both extremes of the length scale is strongly dependent on the rocket geometry and composition. Macroscale rockets are subject to extremely high 5622
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Ni−Pt alloy, Zn), upon the movement of such bilayer microjets.31 The use of a graphene oxide outer layer was shown recently to offer highly efficient propulsion (up to 170 body lengths/s) and operation at extremely low peroxide levels (0.1%) due to deposition of a microporous Pt inner surface that offered enhanced catalytic activity.35 The morphology of template-prepared microrockets can be fine-tuned by adjusting these compositional and conditional parameters, including the monomer concentrations, solvent choice, and deposition charges and durations. The flexibility and versatility in the design of microengines by template-assisted electrodeposition allow for the integration of additional intermediate layers (e.g., magnetic layer for steering control). Additionally, a gold layer could be sputtered on the outer layer, enabling the confinement of different surface functionalities and self-assembled monolayers for diverse applications ranging from cargo delivery to chemical detection. While the outer layer of most micromotors is inert, it is possible to deposit polymeric layers with “built-in” functionalities, such as recognition capability, as was illustrated using a boronic-acid-based polymer or a molecularly imprinted polymer outer layer.36,37 An alternative template-assisted fabrication approach, described by Pumera’s group, involved longitudinal deposition of bimetallic tubular nanotubes capable of bubble propulsion (Figure 3C).38 This route was illustrated for preparing Pt/Au bimetallic nanotubes capable of reaching velocities of 40 body lengths/s in the presence of 15% H2O2, comparable to speeds obtained by conical microjets. The specific longitudinal placement of these bimetal layers is a consequence of the deposition of the sacrificial conductive layer which enables electrochemical deposition only along the template walls. Hence, longer depositions primarily affect the segment length, as opposed to layer thickness in the aforementioned templateassisted deposition processes. The advantage of this longitudinal deposition is that the individual metallic segments can be specifically functionalized, resulting in different surface chemistries along the length of the microrocket. Wu et al. described the fabrication of polyelectrolyte multilayer rockets by nanoporous template-assisted layer-by-layer assembly, with Pt nanoparticles assembled within the inner surface. Besides control of the length, thickness, and diameter, this technique also confers the ability to vary the membrane wall properties with corresponding molecular components for further functionalization.39 Sitt et al. described a different electrodeposition route for preparing tubular microengines through the bulk fabrication of hollow fibers.40 Here, co-electrospinning was used to generate long hollow strands of phase-separated polymer layers, specifically composed of a poly(lactic-co-glycolic acid) (PLGA) and poly(ethylene oxide) (PEO) core and shell, respectively (Figure 3D). Subsequent cryo-sectioning dices these fibers with lengths ranging from 10 to 200 μm, yielding hollow polymer microtubes which retain their original bilayer structure. These chemically inert layers can be mixed with functional derivatives during the electrospinning process. Crucially, branched polyethylenimine mixes well with the PEO inner core, enabling selective decoration of the core with Ag/Pt nanoparticles or catalase for highly efficient propulsion. A similarly appropriate polymer mixed into the outer PLGA shell can be used for other surface chemistries (e.g., receptors). Other polymer-based fabrication methods have been used recently to develop bubble-propelled nanomotors that could be adapted for preparing future MNRs. Efficient bubble-thrust
the free nanomembrane layers spontaneously bend and roll up unevenly due to this strain gradient, leading to conically shaped microstructures with hollow cores. Sequential deposition is used to implement a nonreactive external shell at the bottom layer, a catalytic inner shell at the top layer, and other functional intermediate shells, including adhesive and magnetic layers within the rocket structure. Here, the geometry is controlled by both the pretreated sacrificial layer as well as the engineered stresses within the nanomembrane layers. The sacrificial layer is generally composed of a thin photoresist layer, which can incorporate outer patterns to control the rolling direction and the microrocket length after the etching treatment. On the other hand, the crucial microrocket diameter is controlled by the thicknesses of the deposited layers, along with the strength of the stress gradient within the layers. The nanomembrane grain sizes, which are responsible for the internal stresses, depend on the chamber temperature, pressure, and deposition rate, which can be individually adjusted for a well-controlled stress state.13 However, the interplay between these parameters and the stress state is not as well-described, so careful strain engineering is thus required for the usage of different layers and layer combinations. While offering extremely attractive performance and boasting improved fabrication speed over molecular beam epitaxy,34 these rolled-up microengines require complex topdown photolithographic fabrication processes and related facility (clean room) costs. The main advantage of roll-up nanotech is its versatility and flexibility in design, by enabling nanorockets to be fabricated with nearly any type of inorganic material as a functional layer. Template Electrodeposition. Wang’s group developed a greatly simplified template-membrane-based electrosynthesis of highly efficient mass production and smaller high-performance tubular microrockets (Figure 3B).31 The template-assisted electrosynthesis of bimetal conical microengines was based on the sequential electrodeposition of metallic layers on a sacrificial silver wire template, which was then diced and dissolved for precise length control.18 Although effective, it was limited by a low total yield and slow motion. Subsequently, the San Diego group developed an efficient template-directed electrosynthesis that relies on the sequential electrodeposition of tubular polymeric and catalytic metal layers within commercial microporous membranes consisting of conical pores (Figure 3B). This template electrodeposition approach utilizes the growth of a conducting polymer outer layer along the pore wall due to electrostatic and solvophobic interactions at the wall, followed by the galvanostatic deposition of inner catalytic tubular layer within the polymeric shell. Although this fabrication method sacrifices some of the material flexibility of roll-up fabrication, its main advantage is that the template obviates both clean-room expenses as well as strain engineering for a conical structure, simplifying the geometric considerations to the choice of template and thickness of the deposited layers. Compared to conventional rockets, where the single-use rocket booster stages comprise the bulk of the launch cost, the template-assisted method provides large-scale, cost-efficient production of reusable microrockets. The influence of the composition and electropolymerization conditions upon the propulsion of these template-patterned microrockets has been examined.15 This study assessed the effect of the polyaniline (PANI), polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene) (PEDOT) outer polymeric layers and of various inner catalytic metal surfaces (Ag, Pt, Au, 5623
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ACS Nano Table 1. Different Fuels Used for Nanoscale Propulsion fuel
rocket materials
reaction
bubble
refs
hydrogen peroxide acid water water
Pt, Ag, MnO2, catalase Zn Al−Ga Mg
2H2O2 → 2H2O + O2 2H+ + Zn → H2 + Zn2+ 2Al + 6H2O → 2Al(OH)3 + 3H2 2H+ + Mg → H2 + Mg2+
oxygen hydrogen hydrogen hydrogen
14−24, 46, 47 19, 50 51 52−54
speed beyond adjusting the fuel concentration have been studied. Fuel additives, such as hydrazine, were shown to markedly enhance the catalyst activity.15 The addition of low concentrations surfactants, such as sodium cholate, sodium dodecyl sulfate (SDS), and Triton-X, can also dramatically enhance bubble evolution and speed by increasing the wettability of the inner surface for improved fuel−surface interactions.17 These surfactant molecules adsorb at the inner catalytic or reactive surface to decrease the interfacial free energy, which greatly enhances the bubble evolution. Generally, anionic surfactants like SDS are the most suitable to improve propulsion because they most readily adsorb to the positively charged inner microengine surface, compared to neutral or cationic surfactants like Tween 20. Alternative Biocompatible Fuels. For operations where using peroxide fuel is impractical, such as in vivo biomedical applications or within large bodies of natural water, different microrockets have been designed to creatively use different fuels in conjunction with different catalytic or reactive layers. For example, for use in highly acidic environments, Gao et al. substituted the Pt catalytic layer of their template-deposited PEDOT microrockets with a reactive Zn layer to utilize the following redox reaction: Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g).50 In such acidic environments, these microengines can directly draw fuel from the surrounding environment, without requiring any other additives. The gradual dissolution of the Zn reactive layer limits the operational lifetime of these motors. Depending on the specific acid used and its pH, such lifetime may range from 30 s to over 2 min. The ability to propel in the strongly acidic gastric fluid is of considerable importance for in vivo use (discussed below). Such Zn-based microengines can reach high speeds of 92 μm/s through simulated biological media. As desired for in vivo operation, the Zn2+ byproducts are fully biodegradable, and the entire rocket is composed of nontoxic materials. The formation of hydrogen bubbles with Zn is critical to the operation of these microrockets in acidic media (except in nitric acid, where the formation of nitrous oxide is more favorable than the hydrogen-generating reaction). Changes in the speed of these zinc microengines at different H+ concentrations can also be used to quantitatively determine the local pH of the environment or induce motion changes depending on a pH gradient. These acid-powered microengines are capable of directly harvesting fuel from the environment without any additives. This approach of drawing/harvesting fuel directly from the surrounding environment has been used also for developing of water-driven micromotors, based on Mg or Al that react directly with water to form hydrogen gas.51−54 So far, these efforts have focused on water-driven microsphere motors and have not led to hydrogen thrust for powering microrocket engines. The byproduct formation of metal oxide layers passivates the bubbling reaction, an issue that can be addressed by incorporating an Al−Ga or Mg−Ga alloy to penetrate and remove this passivating layer. Passivating layers can also hinder the lifetime of catalytic microrockets and require effective
propulsion was demonstrated by platinum-loaded stomatocytes and nanoparticle-assembled polymer crystals.41,42 Very recently, bubble-propelled fish-like and tadpole-like micromotors have been fabricated by 3D printing or screen-printing methods.43,44 It is expected that 3D microprinting could be adapted for largescale manufacturing of complex microrockets in the future.7,45
ROCKET FUEL The bubble-propulsion mechanism of chemically powered microrockets relies on the controlled bubble evolution out of their wider rear conical end, as described above.25−30 At the macroscale, this is analogous to the exhaust of high-velocity mass through the rocket nozzle, produced by the reaction of solid or liquid fuel with an oxidizer in a combustion chamber. The bubble nucleation in microscale rockets occurs as a result of the decomposition reaction of a chemical fuel, present in the aqueous environment, which enters the rocket at the narrower end and reacts at the inner cavity. The reaction must be moderately energetic, enough to produce a sufficiently sized bubble at an appreciable frequency but without excessive bubbling that might produce a counter-thrust by bubbles exiting the front end. Furthermore, the reaction must be capable of continuous operation, as reactions that rapidly degrade the inner layer or render it chemically inert are incapable of sustaining a moderate operation lifetime. Hydrogen Peroxide Fuel. Hydrogen peroxide has been the most commonly used fuel for powering microrockets. In most cases, the peroxide fuel is combined with an inner Pt catalytic layer, which catalyzes its decomposition to produce gaseous oxygen: 2H2O2 → 2H2O + O2. Other catalysts for this reaction, such as Ag, MnO2, Pd, and Ir, have been examined for their stability, low costs, or resistance to catalyst poisoning.13,46,47 Biocatalytic layers have also been reported for efficient hydrogen peroxide decomposition and propulsion.48,49 Such peroxide-based biocatalytic microrockets are based on the immobilization of the enzyme catalase on an inner gold layer. The enzyme is anchored to the gold layer via a common carbodiimide/N-hydroxysuccinimide chemistry through a binary mixed self-assembled monolayer of 11-mercaptoundecanoic acid/6-mercaptohexanol alkanethiols. Using such efficient catalysts enables the use of lower peroxide fuel concentrations while retaining the high-speed motion through highly viscous media, which is highly desirable for diverse practical applications. The bubbling frequency and hence the microrocket velocity increase upon increasing the fuel concentration. Solovev et al. reported speeds of up to 2 mm/s with rolled-up prepared Pt microengines,14 while Gao et al. reported speeds of up to 3 mm/s at similar peroxide concentrations (∼10% w/v) for the template-electrodeposited Pt microengines with similar dimensions.31 At lower concentrations (
