Development of Nanoenergetic Micro-fluidic Jet Injectors

by Clay S. Staley, Christopher J. Morris, and Luke J. Currano

ARL-TR-5872 January 2012

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Army Research Laboratory

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ARL-TR-5872 January 2012

Development of Nanoenergetic Micro-fluidic Jet Injectors

Clay S. Staley, Christopher J. Morris, and Luke J. Currano

Sensors and Electron Devices Directorate, ARL

Approved for public release; distribution unlimited. 
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4. TITLE AND SUBTITLE

Development of Nanoenergetic Micro-fluidic Jet Injectors

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6. AUTHOR(S)

Clay S. Staley, Christopher J. Morris, and Luke J. Currano

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14. ABSTRACT

We present the fabrication, assembly, and packaging of first generation nanoenergetic fluidic jet generators. The generators 

consist of an energetic material chamber, elastic piston, vaccine reservoir, and convergent nozzle cap all tightly bonded in a 

self-contained device using micro-electromechanical system (MEMS) processing techniques. Maximum deformation lengths 

of the polydimethylsiloxane (PDMS) membranes are characterized as a function of PDMS diameter, PDMS thickness, 

energetic material composition, and energetic material mass using high-speed photography and compared with theoretical 

calculations. Assembled nanoenergetic fluidic jet generators are analyzed to extract critical operation parameters such as 

fluidic jet velocity, ejected jet diameter, penetration depth, and fluidic dispersion using in air jet velocity measurements and 

ballistic gel penetration characterization. Performance scaling laws have been established to determine design parameters for 

smaller future generation devices.

15. SUBJECT TERMS

Nanoporous silicon, nanothermites, needle-free drug deliver

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Christopher J. Morris

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iii

Contents

List of Figures iv

Acknowledgments v

Student Bio vi

1. Introduction and Background 1

2. Experiment and Calculations 2

3. Results and Discussion 5

4. Summary and Conclusions 9

5. References 10

List of Symbols, Abbreviations, and Acronyms 11

Distribution List 12
iv

List of Figures

Figure 1. Illustration of needle-free transdermal injection process (3)..........................................1

Figure 2. Energy density vs. power density of various energy sources compared to 

nanoenergetic materials (4–6)....................................................................................................1

Figure 3. Illustration of elastic membrane modeling: (a) unstressed elastic membrane 

previous to pressure input and (b) elastic membrane deformation in response to uniform 

pressure q. ..................................................................................................................................3

Figure 4. PDMS mechanical modeling: PDMS membrane deflection (left) and 

tension/flexture edge stress (right) as a function of CuO/Al mass and PDMS diaphragm 

thickness.....................................................................................................................................4

Figure 5. CuO nanorod synthesis flow sheet. .................................................................................5

Figure 6. Cross-sectional illustration of micro-fluidic injector assembly......................................6

Figure 7. Photograph of fabricated and diced ignition substrate. ..................................................7

Figure 8. Two piece partial assembly of a nanoenergetic micro-fluidic jet injector: ignition 

substrate bonded with a material chamber (left) and fluid reservoir bonded with a nozzle 

(right). ........................................................................................................................................8

Figure 9. Assembled nanoenergetic micro-fluid jet injectors: without nozzle (left) and with 

nozzle (right)..............................................................................................................................8
v

Acknowledgments

We wish to acknowledge technical guidance from Dr. Luke Currano and Dr. Shubhra 

Gangopadhyay. We would also like to acknowledge Mr. Brian Isaacson for technical assistance 

during wafer processing and the U.S. Army Research Laboratory cleanroom staff for providing 

the resources necessary to accomplish this research.
vi

Student Bio

Mr. Clay Stephen Staley is currently pursuing a doctoral degree in electrical engineering at the 

University of Missouri-Columbia under the advisory of Dr. Shubhra Gangopadhyay. He 

acquired a bachelor of science in electrical engineering from the University of Missouri on 

December 2009. The crux of his research is the integration of nanoenergetic materials with 

micro-device fabrication and applied technologies. Mr. Staley has worked on nanoenergetic 

materials synthesis and characterization, micro-thruster propulsion systems, shockwave-induced 

drug transfection, chip disruption information security systems, micro-initiator platforms, 

environmentally friendly secondary propellant initiators, and nanoenergetic needle-free vaccine 

injection. In the future, Mr. Staley will continue basic scientific research using nanoenergetic 

materials packaged in micro-devices to develop innovative approaches towards solving

fundamental challenges present in the scientific community. He hopes to facilitate the transition 

of his research to field readiness levels in an effort to satisfy real-world commercial and military 

application demands.
1

1. Introduction and Background

Needle-free liquid jet injectors were invented more than 60 years ago for the delivery of proteins 

and vaccines (1, 2). The concept of operation behind needle-free injectors is to provide 

transdermal injection by forcing liquid through a tiny orifice that is held against the skin. This 

creates a very fine, high-velocity stream of medication that penetrates the skin. The process of 

needle-free injection in illustrated in figure 1.

Figure 1. Illustration of needle-free transdermal injection 

process (3).

Typically, needle-free injection systems are driven by compressed gas or a spring-loaded piston. 

An alternative to traditional liquid jet generators is a device driven by the rapid combustion of 

nanoenergetic material, defined as an exothermic reaction between particles having characteristic 

dimensions of 1–100 nm. Such a device is similar to one driven by compressed gas in that the 

impulse arises from the rapid release and expansion of gases. However, a pellet of solid 

nanoenergetic material harbors greater energy and power density than an equally sized 

compressed gas cylinder, and greater than many other energy storage technologies, as shown in 

figure 2. 

Figure 2. Energy density vs. power density of various energy 

sources compared to nanoenergetic materials (4–6).

0.001

0.01

0.1

1

10

100

10 100 1000 10000 100000 1000000 1000000

0

1E+08

Power Density (W/kg)

Energy Density (MJ/kg) 10-3

10-2

10-1

100

101

102

101 102 103 104 105 106 107 108 Energy Density (MJ/kg)

Power Density (W/kg)

Fuel Cells

Batteries

Capacitors

Ultra Capacitors

Nanoenergetics
2

Nanoenergetic materials can be integrated with micro-electromechanical system (MEMS) and 

solid-state electronics to fabricate millimeter- or micrometer-scale liquid jet generators. Ideally, 

by fabricating micro-injection devices, traditional pitfalls associated with commercially available 

needle-free jet injectors, such as the causation of bruising and abrasions, inconsistent vaccine 

dose delivery, and the requirement of a trained administrator, may all be addressed. The 

fundamental device design to accommodate nanoenergetic actuation incorporates an energetic 

material chamber and fluid reservoir separated by an elastic piston membrane. The reaction and 

expansion of the energetic material pushes the membrane and applies pressure to the fluid 

chamber ejecting fluid from the reservoir. The elastic membrane prevents reactants from 

interfering with the fluid or the injection recipient, and also provides a thermal barrier preventing 

the transfer of heat and, if necessary, light to the fluid. Output fluidic jet power can be 

modulated by adjusting the energetic material formulations and quantities, and reaction rates can 

be modulated to minimize any heat transfer to the surrounding system.

2. Experiment and Calculations

Prior to designing and fabricating nanoenergetic micro-fluidic jet injectors, device volumes, 

nanoenergetic mass loaded, and elastic membrane thickness need to be considered. To establish 

design parameters for device fabrication, first-order approximations were conducted to determine 

maximum elastic diaphragm deflection and mechanical stresses incurred in response to applied 

transient pressures. Polydimethylsiloxane (PDMS) was chosen as the elastic membrane material 

owing to a high elasticity and maturity of use in MEMS processing. Copper oxide-aluminum 

(CuO/Al) nanothermite was selected as the nanoenergetic material for this study. First, the 

volume of gas liberated from the combustion of a CuO/Al mass at standard temperature and 

pressure (273.15 K and 101.325 kPA) is extracted through the ideal gas law expressed as

1 1

1

1

n RT V

P

 , (1)

where V1 is the volume of gas generated in an unconfined environment, n1 is the amount of gas 

in moles calculated from the theoretical gas evolved during CuO/Al decomposition equal to 

0.0054 mol/g, R is the universal gas constant, T1 is standard temperature, and P1 is standard 

pressure (7, 8). Assuming an isochoric combustion process, the uniform pressure exerted on an 

energetic material reservoir of volume V2 is given as

1 1 2

2

2 1

PV T P

V T

 , (2)

where P2 is the material reservoir pressure and T2 is the adiabatic flame temperature of the 

CuO/Al nanothermite given as 2570 °C (7, 8). By evaluating equations 1 and 2 for a range of 
3

CuO/Al mass contained in a chamber volume of 1.57 mm3at 20% Theoretical Maximum 

Density (TMD), a linear relationship is extracted to determine pressure generated versus material 

mass expressed as

P m   81.26 MPa/mg 7.105 MPa  , (3)

where P is reaction pressure and m is mass of CuO/Al (in mg) loaded into the containment 

volume. The density for slurry deposited nanothermite materials as prepared in this work is 

approximately 20% TMD and is accounted for here to ensure an experimentally practical range 

of CuO/Al mass is used in the calculation. 

To approximate maximum PDMS diaphragm deflection as a function of applied pressure, the 

mechanical behaviors of the diaphragm are modeled as a disc with clamped edges as shown in 

figure 3. 

Figure 3. Illustration of elastic membrane modeling: (a) unstressed 

elastic membrane previous to pressure input and (b) elastic 

membrane deformation in response to uniform pressure q.

The maximum deflection of the membrane resulting from the application of a uniform pressure is 

calculated using the expression

3 4

4 1 2

qa y y K K

Et t t

 

     

, (4)

where q is the pressure applied, a is disc radius, E is Young’s modulus, t is disc thickness, and y 

is the maximum deflection distance at the center of the disc. K1 and K2 are parameters calculated 

from the following equations (9):

1 2

5.33

1

K 

 

, (5)

2 2

2.6

1

K 



, (6)
4

where is Poisson’s ratio of the disc material. To evaluate equations 4, 5, and 6 a Young’s 

modulus and Poisson’s ratio for Sylgard 184 PDMS (prepared using 10 parts silicone base to 1 

part curing agent) of 1.8 MPa and 0.45, respectively, were used (10). With a known maximum 

deflection distance, the mechanical flexure and tension stresses combined are evaluated against 

the ultimate yield strength for PDMS equal to 8 MPa (11) to determine if the diaphragm will 

rupture during actuation. The combined diaphragm stresses are calculated from the equation

2 2

2 3 4

a y y K K

Et t t

  

     

, (7)

where  is the combination of tension and flexure stresses (9). K3 and K4 are constants for a 

Poisson’s ratio of 0.45, given as 3.64 and 0.98, respectively, for stress calculation at the center of 

the disc or as 7.27 and 1.73, respectively, for calculating stress at the edges of the disc. 

Equation 7 was evaluated for 1.57 mm3devices under the conditions of edge stresses as the edge 

stress magnitude will always be higher than the stresses at the center of the disc in this particular 

physical configuration. PDMS deflection and tension/flexure edge stress versus CuO/Al 

nanothermite mass for varied thickness PDMS membranes with a 1-mm radius are plotted in 

figure 4.

Figure 4. PDMS mechanical modeling: PDMS membrane deflection (left) and tension/flexture edge stress (right) 

as a function of CuO/Al mass and PDMS diaphragm thickness.

From this analysis, devices were designed to operate at maximum tensile and flexure stresses of 

approximately 4 MPa, which is half the value of the ultimate tensile strength for PDMS. The 

maximum deflection of the PDMS membrane will be engineered to be as large as possible 

without breaching membrane stresses of greater than 4 MPa. Material chamber and fluid 

reservoir diameters of 2 mm, 2.5 mm, and 3 mm were subsequently used for this study. 

To prepare CuO/Al nanothermite, copper chloride (CuCl2)∙2H20 powder, polyethylene glycol

(PEG)-400, and sodium hydroxide (NaOH) flakes were purchased from Sigma Aldrich and used 

as received for CuO nanorod synthesis. The three chemicals were mixed and ground together 
5

following a weight ratio of 1.7 CuCl2∙2H20: 1 NaOH: 2.3 PEG-400 via mortar and pestle,

initiating the following reactions:

CuCl2+2NaOH →Cu(OH)2 + 2NaCl (8)

Cu(OH)2 → CuO + H2O + ΔH. (9)

The reaction of CuCl2 with NaOH yields copper hydroxide (CuOH) and sodium chloride (NaCl). 

Unstable CuOH then hydrolyzes in an exothermic process to yield CuO as a viscous, black paste. 

PEG-400 serves as a polymer surfactant in the process by coating the CuO nanorods to influence 

their growth geometry and prevent undesirable macro-structure formulations by occupying 

potential binding sites. A PEG-coated CuO slurry was then washed multiple times in 800 mL of 

deionized (DI) water through ultrasonic agitation for 1 h, left to settle, and drained until all PEG400 surfactant was removed from the mixture. Wet CuO nanorods were then extracted from the 

solution onto fine porosity filtration paper using a suction filtration assembly and left to dry 

under vacuum, yielding primed CuO nanorods. A flow sheet for CuO nanorod synthesis is 

shown in figure 5.

Figure 5. CuO nanorod synthesis flow sheet.

As prepared CuO was then placed in a scintillation vial and dispersed uniformly in 1.2 mL of 

isopropyl alcohol. Al nanoparticles purchased and used as received from Novacentrix (80 nm 

diameter, 80% active Al content, and a 2–3 nm aluminum oxide [Al2O3] passivation coating)

were added to the solution in a weight ratio of 1 Al: 1.22 CuO. The slurry was then agitated in 

an ultrasonic bath for 4 h to allow for the self assembly of the Al nanoparticles onto the exposed 

CuO nanorod surface areas, producing an intimately and homogenously mixed CuO/Al 

nanothermite slurry suspended in isopropyl alcohol.

3. Results and Discussion

Nanothermite actuated micro-fluidic injectors were assembled from four separately processed 

silicon (Si) wafers consisting of an ignition substrate (with an integrated electro-thermal heating 

unit), an energetic material chamber, fluid reservoir, and convergent nozzle cap with a 150-mdiameter orifice, as illustrated in figure 6.
6

Figure 6. Cross-sectional illustration of micro-fluidic 

injector assembly.

To begin device fabrication, a p-type [100] orientation Si wafer double side coated with 600-nmthick films of silicon nitride (Si3N4) was used for ignition substrate fabrication. Si3N4 from the 

backside of the wafer was stripped via plasma etching and a 1700-Å film of platinum (Pt) was 

sputtered to serve as a backside electrode during nanoporous Si electrochemical etching later in 

the fabrication process. Following Pt deposition, the front side of the wafer was patterned with 

photoresist and Si3N4 was etched away selectively to yield bare Si regions in preparation for 

nanoporous Si etching. Next, patterned thin-film heaters were deposited onto the wafer via ebeam evaporation comprising of 100, 1000, and 3800 Å of chrome (Cr), Pt, and gold (Au),

respectively. Following heater deposition, ProTEK, an electrical insulating polymer, was spun 

coated onto the wafer and patterned with photoresist. The wafer was then O2 plasma etched to 

leave only small square morphologies of ProTEK over two discrete areas of the thin-film igniter. 

Patterned square solder contact pads consisting of 100, 1000, 500, and 1000 thick films of Cr, Pt, 

Au, and copper (Cu) were then deposited on to the wafer. Following solder pad deposition, 

exposed Si was electrochemically etched in a solution of hydrofluoric acid (HF) (3 parts):

ethanol (EtOH) (1 part):H2O2 (0.5% v/v) to produce nanoporous Si pits approximately 10 m 

deep, which serve as a thermal insulating barrier between the thin-film heater and Si substrate. 

Detailed analysis of the nanoporous Si etching procedure can be found in other works (12, 13). 

An optical microscope photograph of a fabricated and diced ignition substrate chip is shown in 

figure 7.
7

Figure 7. Photograph of fabricated and 

diced ignition substrate.

To verify the capacity of the heaters to reliably ignite a confined quantity of nanothermite, 

electrical characterizations of the electro-thermal heating elements used in these devices was 

conducted presented in other works (14).

The material chamber and fluid reservoirs are fabricated using p-type Si double polished wafers

and starts with depositing a patterned solder pad with equivalent dimensions and positioning to 

the solder pad region located on the ignition substrate wafer. Cleave lines, ports for wire 

bonding, and material chamber/fluid reservoir cavities are then deep reactive ion etched (DRIE) 

in the wafer. 

The nozzle caps were fabricated on a p-type [100] orientation Si wafer double side coated with 

600-nm-thick films of Si3N4. Solder pads were initially deposited on the wafer to provide 

alignment marks for subsequent processing and a method to bond nozzles onto the fluid 

reservoirs. Si3N4 was etched from the top side of the wafer selectively to leave bare Si pits for 

potassium hydroxide (KOH) etching. The wafer was then submerged in a 45% solution of KOH 

heated to 90 °C for 8 h to etch approximately 250-m-deep cavities with angular side walls. 

Following KOH etching, the wafer was inverted and subjected to backside DRIE to produce 

150-m-diameter ejection ports.

Following wafer processing, single dice from each wafer were cleaved to facilitate individual 

device assembly. Material chambers and fluid reservoirs were submerged in a heated solution of 

ethylene glycol containing a molten 47 °C melting point solder alloy. The molten solder 

preferentially wetted the metallic features on each dice resulting in a uniform solder coating on 

to the exposed solder pads. Following solder coating, the material chamber and fluid reservoir 

were brought into contact with the ignition substrate and nozzle dice, respectively, in a heated 

solution of ethylene glycol to bond the two chips together. Optical microscope photographs of 

the assembled ignition substrate/material chamber chip and fluid reservoir/nozzle chip are shown 

in figure 8.
8

Figure 8. Two piece partial assembly of a nanoenergetic micro-fluidic 

jet injector: ignition substrate bonded with a material chamber 

(left) and fluid reservoir bonded with a nozzle (right).

PDMS membranes were prepared by mixing Sylgard 184 Si base (10 parts) with a curing agent 

(1 part) thoroughly in a glass beaker. The mixture was then degassed under vacuum for 1 h and 

spun coated at 1000 RPMs for 90 s on Kapton taped glass substrates. Kapton was used as 

opposed to glass to minimize chemical interactions between the PDMS and substrate as it cures, 

facilitating an easy film transfer. The PDMS was allowed to cure at room temperature for 24 h

and individual sections were measured, and then cut and peeled from the Kapton substrate to use 

for assembling complete devices. The PDMS membranes were manually aligned and brought 

into contact with the material chamber/ignition substrate chip and then subsequently with the 

fluid reservoir/nozzle chip. Van der waals forces provided a PDMS reversible bond, which 

successfully assembled complete devices together. A photograph of two assembled 

nanoenergetic micro-fluidic jet injectors next to a U.S. dime is shown in figure 9.

Figure 9. Assembled nanoenergetic micro-fluid 

jet injectors: without nozzle (left) and

with nozzle (right).
9

Although the PDMS bonding procedure used to assemble these devices results in relatively weak 

interfacial bonds, more robust device bonding can be achieved in the future by using a 

combination of O2 plasma treatments (to produce hydrophilic hydroxyl radicals on the surface of 

PDMS) and epoxy bonding. 

4. Summary and Conclusions

We have demonstrated the fabrication of first generation nanoenergetic micro-fluidic jet injectors 

assembled from the cleaved dice of four separately processed Si wafers. The devices may be 

integrated with nanoenergetic material and previous experiments have verified the capacity of 

the electro-thermal heater to provide controlled and repeatable ignition. In the future, tightly 

bound devices will be filled with CuO/Al nanothermite and the mechanical deflection of the 

PDMS membrane will be evaluated against modeled behaviors to locate an ideal PDMS 

membrane thickness. Following these experiments, the fluid reservoir/nozzle chip will be filled 

with dyed water and subjected to various characterization methods to extract critical device 

operating parameters such as jet ejection velocity, jet diameter, fluid dispersion, and penetration 

depth.
10

5. References

1. Mitragotri, S. Current Status and Future Prospects of Needle-free Liquid Jet Injectors. Nat 

Rev Drug Disc. 2006, 543–548.

2. Stachowiak, J. C. Piezoelectric Control of Needle-free Transdermal Drug Delivery. Journal 

of Controlled Release 2007, 124, 88–97.

3. Arora, A.; Prausnitz, M. R.; Mitragotri, S. Micro-scale Devices for Transdermal Drug 

Delivery. International Journal of Pharmaceutics 2008, 364, 227–236.

4. Morris, Christopher; Currano, C. J.; Zakar, E. Nanoenergetic Materials for Energy 

Conversion Applications. European MRS Symposium, Strasbourg, France, 2009.

5. Currano, L. J.; Churaman, W. Energetic Nanoporous Silicon Devices. JMEMS 2009, 18, 

799–807.

6. Shende, Rajesh et al. Nanoenergetic Composites of CuO Nanorods, Nanowires, and AlNanoparticles. Propellants, Explosives, Pyrotechnics 2008, 33, 122–130.

7. Fischer, S. H.; Grubelich, M. C. Theoretical Energy Release of Thermites, Intermetallics, 

and Combustible Materials. Sandia National Labs (Albuquerque, NM, USA), 1998.

8. Cooper, P. W.; Kurowski, S. R. Introduction to the Technology of Explosives, 1996.

9. Young, W. C.; Budynas, R. G. Roark’s Formulas for Stress and Strain, 2001.

10. Schneider, F. et al. Mechanical Properties of Silicones for MEMS. Journal of 

Micromechanics and Microengineering 2008, 18, 065008.

11. Mata, A.; Fleischman, A. J.; Roy, S. Characterization of Polydimethylsiloxane (PDMS) 

Properties for Biomedical Micro/Nanosystems. Biomedical Microdevices 2005, 7, 281–293.

12. Becker, C. R.; Apperson, S. J.; Morris, C. J.; Gangopadhyay, S.; Currano, L. J.; Churaman,

W.; Stoldt, C. R. Galvanic Porous Silicon Composites for High-velocity Nanoenergetics. 

Nanoletters 2010, 11, 803–807.

13. Becker, C. R.; Currano, L. J.; Churaman, W.; Stoldt, C. R. Thermal Analysis of the 

Exothermic Reaction Between Galvanic Porous Silicon and Sodium Perchlorate. ACS 

Applied Materials and Interfaces 2010, 2, 2998–3003.

14. Staley, C. S. et al. Silicon-based Bridge Wire Micro-chip Initiators for Bismuth Oxidealuminum Nanothermite. Journal of Micromechanics and Microengineering article in press, 

2011.
11

List of Symbols, Abbreviations, and Acronyms

Al aluminum 

Al2O3 aluminum oxide

Au gold 

Cr chrome 

Cu copper 

CuCl2 copper chloride 

CuO copper oxide

CuOH copper hydroxide 

DI deionized 

DRIE deep reactive ion etched 

EtOH ethanol 

HF hydrofluoric acid 

KOH potassium hydroxide 

MEMS micro-electromechanical system 

NaCl sodium chloride 

NaOH sodium hydroxide 

PDMS polydimethylsiloxane 

PEG polyethylene glycol

Pt platinum 

Si silicon

Si3N4 silicon nitride 

TMD Theoretical Maximum Density 
12

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