Bio-Inspired Materials and 

Devices for Chemical and 

Biological Defense

James J. Valdes and Erica R. Valdes 

Center for Technology and National Security Policy 

National Defense University 

September 2010 
ii

The views expressed in this article are those of the authors and do not reflect the official 

policy or position of the National Defense University, the Department of Defense or the 

U.S. Government. All information and sources for this paper were drawn from 

unclassified materials. 

James J. Valdes is a Senior Research Fellow at the National Defense University’s Center 

for Technology and National Security Policy and the Army’s Scientific Advisor for 

Biotechnology. Dr. Valdes received a PhD in neuroscience from Texas Christian 

University and was a postdoctoral fellow at the Johns Hopkins Medical Institutes. He has 

published more than 120 papers in scientific journals and was a 2003 Presidential Rank 

Award winner. 

Dr. Erica R. Valdes holds a Masters Degree (Chemistry) and Ph.D. (Materials Science) 

from Johns Hopkins University. She has 27 years experience as a Research chemist with 

the U.S. Army, Edgewood Chemical Biological Research Center (ECBC) with research 

experience ranging from ultrafine particle studies and biologically-inspired materials to 

forensic investigation of hazardous materials. She currently works at the ECBC 

Chemical/Biological Forensic Analysis Center. 

Defense & Technology Papers are published by the National Defense University Center for Technology 

and National Security Policy, Fort Lesley J. McNair, Washington, DC. CTNSP publications are available 

at http://www.ndu.edu/ctnsp/publications.html. 
iii

Contents

Introduction......................................................................................................................... 1 

Background......................................................................................................................... 2 

Current Technologies.......................................................................................................... 5

Cell Membranes and Lipid Bilayers................................................................................ 6 

Stabilized Lipid Layers and Bilayers .............................................................................. 8 

Liposomes ..................................................................................................................... 10 

Microcapsules and Polymersomes ................................................................................ 12 

Hydrogels ...................................................................................................................... 13 

Aerogels ........................................................................................................................ 16 

Porous Inorganics.......................................................................................................... 20 

Chitosan......................................................................................................................... 21 

S-layer Proteins ............................................................................................................. 23

Molecular Imprints........................................................................................................ 25 

Nanotubes...................................................................................................................... 27 

Conclusions and Recommendations ................................................................................. 30 
Introduction 

Living systems are capable of manufacturing processes, molecular recognition and other 

complex functions which cannot be replicated by synthetic chemistry or other industrial 

technologies. Cells routinely manufacture monodisperse nanoscale structures and 

assemble molecular machines, carry out biochemical reactions and production processes 

of great complexity, and interact with the environment in an adaptive and emergent 

manner. Biotic (i.e., living) systems can be labile and, by their nature, difficult to 

precisely control. The ability to elucidate key metabolic pathways and to replicate their 

functional properties in a synthetic (i.e., abiotic) format will ultimately permit the design 

of completely artificial systems with abilities similar to those of a biotic system but with 

the advantages of precise process control and enhanced ruggedness. This will have 

profound implications for the many and varied missions of the Department of Defense 

(DOD) which include, but are not limited to, small-scale power and energy, lightweight 

flexible armor, on-demand manufacture of high-value products such as pharmaceuticals, 

low observable materials and—the subject of this paper—chemical and biological 

defense (CBD). 

Transformational advances in CBD are expected to depend heavily on biologically 

inspired technology. A leading vision in this direction is a conceptual platform known as 

the abiotic networked threat system (ANTS). ANTS is based on lessons learned from 

biology, incorporating abiotic homologues to biological recognition events and metabolic 

pathways to provide programmable capabilities to sense and respond to environmental 

threats. Further, it is a complete platform architecture with intelligent preprocessing and 

network capability and is envisioned to be embedded in all aspects of CBD including 

detection, communication, activation of response systems, and protection systems. The 

six major components in ANTS are: 

1. Molecular recognition elements for xenobiotic threats 

2. Materials for synthetic matrices 

3. Signal transduction in biotic, hybrid and synthetic systems 

4. Power generation at the nanoscale 

5. Computational tools for design of abiotic units, devices and modules 

6. Combinatorial design and deployment of synthetic systems 

On a very simplified level, the objective is a collection of self-sustaining micro- or mesoscale devices that mimic the xenobiotic (i.e., from outside the cell or tissue) threat 

recognition, response and adaptation of living cells. These units would operate as a 

decentralized network, communicating with one another, processing information, and 

triggering responses which may exhibit emergent properties. 

Recognition elements, power, signal transduction, and networking are generally 

recognized as essential and challenging areas for research in this concept. Less obvious 

are the challenges associated with the physical support matrix required to support and 

shelter the units. This report addresses area 2, materials for synthetic matrices. 
2

Background 

The problem of materials selection and development for membranes and support 

structures cannot be addressed independent of the problems of recognition, transduction, 

amplification, and communication. In fact, the materials need to interface with, support, 

and complement the functional aspects of the platform as a whole. As such, the approach 

to the materials problem should be agile and holistic, addressing materials for future 

technology while providing appropriate materials for intermediary technologies. 

In the natural living cell, the membranes and support structures (e.g., microtubules) are 

integral parts of the functional activities of the cell, and it could be argued that the inverse 

is equally valid: that the many functional activities of the cell are integral parts of the 

membrane. Proteins and lipids in the membrane are responsible for transporting nutrients, 

products, and information through and across the membrane, and the biomolecules 

involved in these processes are usually constituents of the membrane. The extent and 

nature of this interdependence varies with the specifics of the biochemical pathway in 

question. Thus, the role of membrane materials in abiotic cell homologues depends 

largely on the biochemical pathways being simulated and on the abiotic simulation 

technology being addressed. By extension, the type of support and interaction required 

from the membranes will be dictated by the nature and maturity of the pathway 

simulations. This gives four scenarios of environmental or mobility dependence that will 

affect membrane development, summarized below. In Table 1, a fifth scenario is included 

in which the functional elements are so environmentally stable, and their function is so 

elegantly engineered, that they require no protection or mobility control from the 

membrane. This attractive, though unlikely, scenario is the only one in which the 

membrane material serves a purely support function. Research toward the realization of 

an abiotic threat sensor platform and its integration into myriad CBD equipment and 

systems can be expected to progress through these scenarios, with different material 

technologies required at each step. What follows is a discussion of existing and emerging 

material technologies that will be relevant as such a concept progresses from initial 

investigations to the long-term vision. 

1. The functional components are subject to the same or equivalent 

environmental requirements as their biological homologues and require 

the same or equivalent through-membrane and intra-cellular mobility. 

2. The functional components are not dependant on environment but do 

require through-membrane and intra-cellular mobility. 

3. The functional components do not depend on the environment nor do they 

require through-membrane mobility, but they do require intra-cellular 

mobility. 

4. The functional components do not depend on the environment nor do they 

depend on intra-cellular mobility, but they do require through-membrane 

mobility. 
3

Table 1. Possible scenarios for environmental and functional element mobility 

requirements to be faced by support materials 

 Environmental Functional Element Mobility Dependence 

Dependence Intra-cellular Cross-membrane 

1 Y Y Y 

2 N Y Y 

3 N Y N 

4 N N Y 

5 N N N 

In the early research stages, the units are conceptual and research is aimed at making the 

functional components, that is, the recognition elements, transduction pathways, and 

homologues of associated biochemistry function. This constitutes a bread-board or 

bench-top capability, with a focus on: 

1. Understanding and identifying the functional components to be 

synthesized 

2. Recreating the components external to living cells 

3. Synthesizing abiotic homologues for the functional components 

4. Understanding and identifying the interactions between the components 

and between the components and environment 

5. Recreating these interactions 

Functional components would be investigated separately and in discrete combinations to 

establish proof-of-principle and for optimization. At this stage, materials requirements 

would be associated with localizing, immobilizing, and stabilizing individual elements, as 

well as possible facilitation of interoperability of components. 

To facilitate these early developments, support materials need to provide means for any 

environmental interaction required. Ideally, any requisite environmental protection would 

also be provided, though this may not be necessary for all intermediate proof-of-principle 

prototypes. This is akin to situation 1 from Table 1; something very close to a natural cell 

membrane will be required. Clearly this is not a desired end state for a fieldable product. 

However, in the research stages it will be likely that one or more investigational 

functional element will have these requirements. As technologies targeted for functional 

elements move deeper into the abiotic realm, the levels of environmental and mobility 

requirements will shift. 

As abiotic technologies advance toward the vision of independent and stable units that 

recreate the relevant actions of biological homologues (i.e., imitate living systems), the 

materials requirements will evolve toward more hardened materials and become more 

tailored to support specific technologies. Because the architecture for an ANTS platform 

will be technology driven rather than predetermined, the final form of the units could 

follow any one of numerous directions. In the closest homologue to the living cell, the 

units would be several functional components housed within a structure that allows the 
4

components to move and interact in a programmable manner or in response to ambient 

conditions while also allowing one or more to traverse the enclosure and interact with the 

exterior environment. Some of the substrate and functional element interactions possible 

for viable ANTS products include: 

1. Hardened material carrying functional components on its surface 

2. Hardened material carrying functional components internally and 

providing interaction with the environment 

3. Hardened material carrying some functional elements on both its surface 

and interior with communication facilitated by substrate 

4. Functional material with other functional components embedded and 

interacting within the matrix 

5. Functional material carrying some functional elements on both its surface 

and interior with communication facilitated by substrate which participates 

in function. 

This presents a range of possibilities, each of which in turn presents unique challenges in 

material development. The simplest case of a hardened material, carrying surface 

elements would require surface properties amenable to functionalization with the 

elements and allow interactions among the elements. A hardened material with internal 

elements would require controllable permeability, pores, or through-channels to allow 

correct environmental interaction and possibly require the ability to precisely position 

and immobilize, or provide specific mobility to, functional elements. Hardened material 

with functional elements on both the interior and exterior would have the same 

requirements as a hardened material with only internal elements, but with the possibility 

of added positioning and immobilization requirements to allow correct relative 

positioning and interaction of internal and external moieties. Situations 4 and 5 would 

pose similar requirements to 2 and 3, with the added requirement of any specific 

positioning or motility required to facilitate the various functional activities. 

Clearly, there is a broad range of possible material requirements that may be faced by 

functional abiotic aggregates. The extent of these requirements changes with the level of 

complexity of each functional element and of the interactions among the elements. As 

ANTS and similar technologies mature, these requirements will evolve, both from the 

perspective of increasingly complex functional interactions and from the decreasing 

dependence of functional elements on the environment. 

The evolution of functional interactive elements will require the simultaneous evolution 

of matrix materials for support and facilitation of elements and element function. What 

follows is an overview of candidate materials that can be expected to play a role in the 

support, protection, and facilitation of functional elements in an abiotic platform. 
5

Current Technologies 

Starting with a purely biotic (i.e., living) system, the target material is the outer 

membrane of a living cell. Structurally, the prime constituents of the cell membrane are 

two layers of phospholipid molecules. These molecules align themselves such that polar 

head groups are in contact with the surrounding environment (extracellular) and with the 

interior of the cell, both aqueous environments, while the hydrophobic tails associate 

themselves with hydrophobic lipid tails from the facing layer. This establishes an 

arrangement of hydrophilic-hydrophobic-hydrophilic regions as you pass between the 

interior and exterior of the cell. The hydrophobic region effectively blocks unwanted 

movement of aqueous solution between the interior and exterior. A number of 

transmembrane structures and proteins exist to facilitate desired movement of material 

and information between the cytoplasm and extracellular fluid. Thus the structural 

material most closely resembling the natural cell is a lipid bilayer, either as a film or in 

the vesicle form of a liposome. Within the bilayer itself, rafts of phospholipids move 

laterally to relocate functional molecules and information. Within the lipid layers of a cell 

membrane, heterogeneities arise from immiscibility among various lipids. Segments 

known as detergent resistant membrane fractions (DRM), or rafts, have been found to be 

more tightly packed than the surrounding bilayer and to float freely within the membrane. 

This rafting is suspected to be instrumental in a variety of cell activity, including receptor 

signaling. As early as 1956, Thomas Chang was working on the first concepts of artificial 

cells1. Even at that time, Chang recognized that the term “artificial cell” need not refer to 

a specific physical entity, let alone any given size or shape, but rather to the capability of 

a synthetic structure of approximately cellular size to carry out one or several activities 

normally attributed to cellular function. Early investigations in the area considered a wide 

range of materials such as cellulose nitrate, synthetic rubber, proteins, polysaccharides, 

lipids, and combinations, and were typically made via interfacial precipitation or 

interfacial polymerization. The goals of these early artificial cells were almost 

exclusively to function as a cell in environments typically inhabited by living cells, i.e., 

they were designed for use in aqueous media and for encapsulation of aqueous 

environments, and applications were primarily medical. 

Artificial cell technology remains a very active research area in the medical arena. 

Applications range from therapy delivery and tissue engineering to blood substitutes and 

gene therapy. The list of realized and investigational applications is extensive.2

 

1

 Chang, T.M.S. Artificial Cells, Charles C. Thomas: Springfield, IL, 1972. 2 Artificial Cells, Cell Engineering and Therapy. Prakash, S., Ed.; Woodhead Pub. Ltd., Cambridge, 

England, 2007.
6

Cell Membranes and Lipid Bilayers 

In this approach, a lipid bilayer similar to 

the liposome layers or cell membrane is 

constructed across a small pore or opening 

much like a microscopic soap bubble film.3

These bilayers are used to study functional 

activities requiring biochemical activity that 

crosses the membrane. This could include 

either biological macromolecules that bridge 

through the membrane or functions that 

involve small molecules traversing the 

membrane. Lipid bilayers can also be 

supported on solid surfaces; however, this generally poses problems with bridging 

macromolecules that need to be free on both sides of the bilayer membrane. One solution 

to this involves suspending the bilayer on porous substrates.4

 Successes in this area point 

to the possibility of linking surface roughness factors to functionality in suspended 

bilayers. 

Studies into the function of cell membranes led scientists to develop extracellular lipid 

bilayer membranes. It is typical for these to be supported across a gap in an impermeable 

material and held between two aqueous solutions. In this way the transmembrane activity 

can be probed. Although these approaches have been known since the 1960s, use of such 

“black lipid membranes” (BLM) has expanded and become more sophisticated with the 

explosive growth of biotechnology and nanotechnology. 

Lipid bilayers are often supported on solid nonporous 

surfaces for a variety of applications.5 In these cases, 

membranes are integrated with a layer covalently linked 

to the support, separated from the support by thin films 

of water or polymer, or cushioned by a film of 

hydrophobic molecules. Supported membranes are 

typically assembled on the substrate by monolayer 

transfer according to Langmuir-Blodgett techniques or 

by vesicle spreading. A wide range of methods for 

creation of supported lipid membranes have been 

reported and many are used for incorporation and 

subsequent studies of membrane proteins.6

 These 

approaches generally retain the structural and thermodynamic properties of free bilayers 

and can be functionalized, allowing valuable studies on the properties and behavior of the 

 

3

 Ichikawa, T.; Aoki, T.; Takeuchi, Y.; Yanagida, T. and Ide, T., Immobilizing Single Lipid and Channel 

Molecules in Artificial Lipid Bilayers with Annexin A5, Langmuir (2006) 22, 6302-6307. 4

 Worsfold, O., Voelcker, N.H., Nishiya, T., Biosensing Using Lipid Bilayers Suspended on Porous Silicon, 

Langmuir (2006)22, 7078-7083. 

5

 Sackmann, E. Supported Membranes: Scientific and Practical Applications. Science 1996, 271, pp 43-48. 6

 Lindhom-Sethson, B. Supported Lipid Membranes for Reconstitution of Membrane Proteins. Focus on 

Biotechnology 2007, 7, pp 131-165. 

Figure 1. Conceptual drawing of cell 

membrane. 

Figure 2. Lipid Bilayer 
7

film and functional elements. However, the space between the bilayer and the substrate is 

inadequate for transmembrane studies, which require a true free standing membrane. 

To provide access to both sides of a bilayer, traditional BLMs are supported over 

openings on the order of 0.1 mm, which allows fluidity but has limited stability. At the 

Paul Scherer Institute in Switzerland, researchers are building nanochips for functional 

assays. Supporting lipid bilayers across nanopores in ultra thin SiN membranes and 

placing this composite film between two compartments allows cross-membrane activity 

while improving on the stability of the film.7

 Other groups have developed alternate 

methods based on the same general concept. The Posner group at Arizona State 

University8 suspends lipid bilayers across 40 nm pores in silicon and inserts a protein ion 

channel to conduct current from one side of the chip to the other. Specific applications of 

this are in the area of single-molecule detection. Following a similar principle, 

investigators9 have inserted α-hemolysin nanopores into lipid bilayers such that the pore 

is positioned to pass through the membrane. These membranes, when placed between two 

salt solutions, allow current to pass through the nanopore. Attaching molecular 

recognition agents to the inner surface of the pore renders the pore selective, such that 

when an analyte binds with the agent the current is blocked for a length of time dictated 

by the chemistry of the binding event. This approach has been shown to be effective for 

selective sensing of metal ions, proteins, drugs, and oligonucleotides. 

The lipid bilayer approach to transmembrane studies has become prevalent enough that 

analyzers and artificial bilipid membranes developed specifically for this purpose have 

become commercially available.10 In addition to studying transmembrane conduction, 

lipid bilayers have been used to study lateral diffusion of single molecule lipids and 

proteins in artificial bilayers, and methods of reducing the lateral diffusion by a factor of 

200 with the use of annexin A5 have been reported.3 Another approach to the use of lipid 

bilayers involves arrays of supported bilayers, which are synthesized by stamping. An 

agarose hydrogel patterned for a desired array is “inked” with lipids and used to 

selectively place an array of spots of supported bilayers.11 These stamps have been used 

in studies of membrane properties and in the screening of drug-membrane and proteinmembrane interactions.12 

Fully supported and pore-suspended bilayers provide a very useful approach to studying 

the individual activity of functional elements, but are severely limited by the fragility of 

the bilayers. The obvious next step is to stabilize the membranes either for more 

demanding studies or for applications. 

 

7 http://sb.web.psi.ch/projects/bionanotechnology.html (accessed June 2008) 8

. Bayley, H.; Cremer, P.S. Stochastic Sensors Inspired by Biology. Nature 2001, 413, pp 226-230. 9

 Martin, C.R.; Siwy, Z.S. Learning Nature’s Way: Biosensing with Synthetic Nanopores. Science 2007, 

317, pp 331-332. 10 http://www.membrasense.com (accessed June 2007. 11 Membrane Printing Made Easy. Nature Chemical Biology [Online] 2005, 1(6), p 323. 12 Majd, S.; Mayer, M. Hydrogel Stamping of Arrays of Supported Lipid Bilayers with Various Lipid 

Compositions for the Screening of Drug-Membrane and Protein-Membrane Interactions. Angew. Chem. Int. 

Ed. 2005, 117(41), pp 6855-6858. 
8

Stabilized Lipid Layers and Bilayers 

In order to put lipid bilayers to practical use, it is often desirable to stabilize the bilayer. 

Several approaches to this have been reported, some based on substrate support, on 

anchoring, or on chemical modification. In a number of applications, usually associated 

with surfaces to be used in contact with blood for medical purposes, a bilayer is not 

necessary and the desired product is a single, properly oriented layer of phospholipid. 

In the preceding section, BLM spanning nanopores in silicon and silicon nitride 

membranes were discussed. This approach provides significant improvement in stability 

over BLMs supported over larger (~0.1 mm) pores.7 A number of other approaches to 

supporting and stabilizing lipid layers and bilayers have been reported. Continuing with 

support on nanoporous substrates, lipid bilayers have been successfully suspended on 

porous silicon and used to demonstrate single cell biodetection.4 The approach uses 

oxidized aminosilanized porous silica which is further functionalized by biotin, which 

allows for a biotin-streptavidin layer that distances the bilayer from the substrate, helping 

to maintain fluidity as well as providing access to the bottom of the bilayer. In this case, 

fluidity is required for lateral mobility of peptides. 

Lipid bilayers are supported on nonporous substrates by a variety of methods.5 Typical 

approaches involve tethering the bilayer to a substrate using lipids either alone or in 

combination with more complex chemistries. Early supported lipid bilayers were put on 

substrates by simple physical adsorption, examples being vesicle fusion and LangmuirBlodgett techniques. In vesicle fusion, a substrate is immersed in an aqueous solution of 

lipid vesicles. When vesicles contact the substrate surface they adhere, break and spread 

to form a lipid monolayer, in the case of hydrophobic surfaces, or bilayer, in the case of 

hydrophobic surfaces. Langmuir-Blodgett techniques involve spreading solutions of 

lipids in volatile organics across an air/water interface and transferring the lipid layer to a 

substrate after the solvent evaporates. 

More sophisticated versions of these methods are often aimed at patterning the bilayer. 

Urisu et al13 looked at the formation of lipid membranes on surfaces modified with 

monolayer islands. Placing self-assembled monolayer islands of octadecyltrichlorosilane 

(OTS) on silica surfaces and using these surfaces for subsequent vesicle fusion 

techniques showed a dependence on the relative size of the vesicles and the monolayer 

islands. With dipalmitoylphosphatidylcholine (DPPC) vesicles, silica domains larger than 

the vesicles were covered by bilayers of DPPC while the OTS domains formed 

DPPC/OTS bilayers. Vesicles larger than the silica domains formed only DPPC/OTS 

bilayers with no DPPC bilayers on the silica. 

As work in the area of supported bilayers becomes more complex, these physical 

approaches are being joined by a wide range of chemical tethering approaches. Gilchrist 

 

13 Tero, R.; Takizawa, M.; Li, Y.-J.; Yamazaki, M.; Urisu, T. Lipid Membrane Formation by Vesicle 

Fusion on Silicon Dioxide Surfaces Modified With Alkyl Self-Assembled Monolayer Islands. Langmuir 

2004, 20, pp 7526-7531. 
9

et al14 have used biotin-PEG3400-bacteriorhodopsin conjugates to tether lipid bilayers to 

streptavidin-coated microspheres, providing more stability and control of the membranes 

and versatility in the attachment of membrane proteins. 

Phospholipid polymers have also been successfully produced and employed. Amphiphilic 

phospholipid polymers have been applied to gold electrodes and used to immobilize 

antibodies. The active ester groups of the polymer are very effective in immobilization of 

biomolecules, and the attached antibodies have been shown to remain stable for 60 

days.15 The same group16 developed phosphorylcholine surface technology (PCST) for 

assembling phospholipids and phospholipid polymers. Applied to material surfaces, this 

places an artificial cell membrane surface on a material, an approach which improves 

biocompatibility of surfaces and is generally useful for biointerfaces. 

 In a number of cytomimetic (cell mimics) approaches, a phospholipid monolayer is 

desired on the substrate. Some of the simplest approaches to this include immobilization 

by poly(ethylene glycol) (PEG), cholesterol mixing, chemical anchoring and chemical 

anchoring to a PEG-covered surface,17 though stability of these layers is often an issue. 

One approach to addressing stability is in situ polymerization of the lipid layer. Initial 

work in this area used thermal initiation of polymerization on alkylated substrates, though 

this precludes applications that incorporate thermally sensitive molecules in the lipid 

layer. More recently18,19,20 the same in situ polymerization processes have been realized 

used photo-initiation. In this series of studies, Chaikof et al used self-assembly of 

acrylate-functionalized phospholipid on alkylated glass and silica supports and the 

resulting layer was photo polymerized in situ via irradiation of eosin Y/triethanolamine. 

Resultant poly(phosphatidylcholine) was more stable than products of thermally initiated 

polymerization and showed stability in air, water and flow conditions. In order to 

improve stability of polymerized lipid layers in surfactant and organic solvents, Byun et 

al21 polymerized PEG-covered phospholipid monolayers on methacroyl-terminated 

surfaces of silanized Si wafers. In general, these monolayer approaches are suitable for 

medical applications in which it is desirable for the material to be compatible with blood, 

yet not exhibit unwanted adherence to proteins and cells. 

 

14 Sharma, M.k.; Jattani, H.; Gilchrist, M.L.Jr. Bacteriorhodopsin Conjugates as Anchors for Supported 

Membranes. Bioconjugate Chem. 2004, 15, pp 942-947. 15 Park, J.; Kurosawa, S.; Takai, M.; Ishihara, K. Antibody Immobilization to Phospholipid Polymer layer 

on Gold Substrate of Quartz Crystal Microbalance Immunosensor. Colloids Surf. B Biointerfaces 2007, 

55(2), pp 164-172. 16 http://aiche.confex.com/aiche/2006/techprogram/P51540.HTM (accessed June 2008) 17 http://lcmt.snu.ac.kr/research/research4.php (accessed May 2008) 18 Orban, J.M.; Faucher, K.M.; Dluhy, R.A.; Chaikof, E.L. Cytomimetic Biomaterials 4. In situ 

Photopolymerization of Phospholipids on an Alkylated Surface. Macromol. 2000, 33(11), pp 4205-4212. 

19. Marra, K.C.; Winger, J.M.; Hanson, S.R.; Chaikof, E.L. Cytomimetic Biomaterials I. In situ 

Polymerization of Phospholipids on an Alkylated Surface. Macromol. 1997, 30, pp 6483-6488. 20 Marra, K.G.; Kidani, D.D.A.; Chaikof, E.L. Cytomimetic Biomaterials 2. In situ Polymerization of 

Phospholipids on a Polymer Surface. Langmuir 1997, 13, pp 5697-5701. 21 Kim, K.; Shin, K.; Kim, H.; Kim, C.; Byner, Y. In situ Photopolymerization of a Polymerizable 

Poly(ethylene glycol) Covered Phospholipid Monolayer on a Methacroyl -Terminated Substrate. Langmuir 

2004, 20, pp 5396-5402. 
10

Figure 3. Micelle 

Although lipid layers and bilayers provide excellent mimicry of cell membranes and offer 

a range of parametric control, they are not expected to develop to a level of hardiness and 

stability of use in practical field applications. They are, however, well-characterized 

systems that are very useful in the study of biological or biologically inspired activity, 

and are good choices for early studies on the various functional elements of abiotic 

materials. 

Liposomes 

In the early stages of development, membranes and support materials would look to 

optimize existing technologies to provide substrates or 

material environments in which research on functional 

components can be conducted. By far the most common 

approach to this is the liposome, typically a spherical lipid 

bilayer consisting of phospholipid and cholesterol layers 

which serve to isolate the interior of the “cell” from the 

exterior while allowing some molecular interchange 

between the interior of the “cell” and its environment. In 

general terms this could be considered a double layer 

micelle (micelles are single layer vesicles of amphiphilic 

molecules). Typically a micelle aligns hydrophilic head 

groups out toward an aqueous environment and entraps 

nonpolar materials inside the cell. The reverse situation, in 

which nonpolar tail groups align toward a nonpolar 

environment trapping polar materials inside, also occurs. 

In a liposome, polar head groups align both inward and 

outward, creating a hydrophobic barrier within the wall of 

the vesicle which separates two different aqueous 

environments, one inside and one outside the liposome. 

This is the closest available homolog to the membrane of a 

living cell. Liposomes have been heavily investigated by the drug delivery community as 

vesicles for encapsulating and delivering therapeutic agents and are used in early 

nanopharmaceutical products. There is therefore a rich literature background covering 

encapsulation and release of drugs using liposomes.22,23 

Going beyond clinical applications, investigators have been studying phospholipid 

vesicles as bioreactors to act as artificial cells for protein expression. Moving protein 

expression from bulk solution to bilayer vesicles and isolating the transcriptiontranslation of plasmid genes extends expression by more than a factor of two. However, 

the expression is limited by the energy and material limitations imposed by the vesicle. 

Noireaus and Libchaber24 addressed this by expressing the α-hemolysin pore protein in 

 

22 Schiffelers, R.; Storm, G.; Bakker-Woudenberg, I., Liposome-encapsulated Aminoglycosides in Preclinical andClinical Studies, Journal of Antimicrobial Chemotherapy (2001) 48, 333-344. 23 Vemuri, S.; Rhodes, C.T. Preparation and Characterization of Liposomes as Therapeutic Delivery 

Systems: A Review. Pharmaceutica Acta Helvitae (1995) 70, 95-111. 24 Noireaux, V.; Libchaber, A. A Vesicle Bioreactor as a Step Toward an Artificial Cell Assembly. PAS

2004, 51, pp 17669-17674. 

Figure 4. Reverse 

micelle. 
11

the vesicle, providing permeability and allowing 

nutrients to enter the vesicle. These vesicles extended 

expression to as long as four days, opening the door to 

viable cell-free bioactivity. Eventually, this approach 

could lead the way to complete pathways encoded by 

“minigenomes” expressed without living cells. In an 

alternate approach Monnard25 capitalized on the fact 

that, at temperatures approaching the melting 

temperature of the phospholipids, natural defects arise 

in the bilayer. Similar to the α-hemolysin approach this 

provides pores that allow ADP and ATP exchange 

while retaining DNA within the vesicle. Ha et al26 used 

these techniques to study single molecule activity within vesicles tethered to quartz slides 

via a PEG-neutravidin-biotinylated lipid immobilization scheme. 

Liposomes are fragile, environmentally unstable, generally used and studied in an 

aqueous suspension, and susceptible to coalescence. As a practical matter, this makes 

them unsuitable as stand-alone abiotic cell membranes but useful for functional studies 

and potentially useful as subunits in otherwise stabilized abiotics. 

As with lipid bilayers, there is great interest in stabilizing liposomes and several 

approaches toward this end have been considered. Liposomes in suspension have a 

tendency to fuse together, affecting the size, monodispersity and leakage of the vesicles. 

Several approaches to this problem include steric stabilization with block copolymers, 

polymerization of either the vesicle lipids or surface-adsorbed molecules, and 

incorporation of cholesterol. In a novel approach, Granick et al27 have demonstrated 

stabilization of liposomes by the simple incorporation of charged nanoparticles. Using 

200-nanometer liposomes of 1,2-dilauroyl-sn-glycero-3-phosphocholine they added 20-

nanometer carboxyl-modified polystyrene with negatively charged hydrophilic surfaces 

to adsorb on 25 percent of the liposome surface. This extended the stability of the 

liposomes against fusion for up to 50 days. 

O’Brien and Liu28 used cross-linking to liberate liposomes from the aqueous 

environment. Using the heterobifunctional lipids Acryl/DenPC16,18 and Sorb/DenPC18,20

they made large unilamellar liposomes which they subsequently polymerized. The 

resulting particles were freeze-dried and later rehydrated. 

 

25 Monnard, P. J. Membr. Biol 2003, 191, pp 87-97. 26 Cisse, I.; Okumus, B.; Joo, C.; Ha, T. Fueling Protein-DNA Interactions Inside Porous Nanocontainers. 

PNAS 2006, 104(31), pp 12646-12650. 27 Zhang, L.; Granick, S. How to Stabilize Phospholipid Liposomes (Using Nanoparticles). Nanoletters 

2006, 6(4), pp 694-698. 

28 Liu, S.; O’Brien, D.F.; Stable Polymeric Nanoballoons: Lyophilization and Rehydration of Cross-Linked 

Liposomes. JACS 2002, 124, pp 6037-6042. 

Figure 5. Lipid Bilayer Vesicle 
12

Although bilayer vesicles are generally considered to be phospholipid, this is not the only 

viable approach. Nakamura et al29 have demonstrated spherical bilayer vesicles based on 

fullerenes. Attaching polarizable phenyl groups to fullerene provides a hydrophilic tail to 

the otherwise hydrophobic molecule, an example being penta-substituting fullerene with 

phenyl groups to form a potassium salt of pentaphenyl-fullerene. The amphiphilic nature 

of these constructs allowed the formation of bilayer vesicles similar to those of lipid 

bilayers. However these fullerene-based analogs differ from their lipid counterparts in 

that the fullerene “head” is the hydrophilic part while the flexible hydrocarbon “tail” 

associated with the surrounding solution, while lipid bilayers generally involve flexible 

hydrophobic tails and hydrophilic heads aligned toward the water. 

Microcapsules and Polymersomes 

The obvious step beyond liposomes is a rugged analog that retains the size and material 

transport properties of the liposome while reducing or eliminating environmental fragility 

and adding permeability and transport control; in other words, a microcapsule. 

Microencapsulation is the general term for enclosing one substance within another for a 

variety of purposes: to retard degradation, enhance handling properties such as powder 

flow, control release time, etc. For most applications microcapsules are larger than 

liposomes, typically in the millimeter range, and can be made by a variety of processes 

depending on the materials involved and the results desired. Some of the more common 

include pan coating, air suspension coating, coaxial jets or centrifugal extrusion, spraydrying, and polymerization. All of these have been widely applied in commercial 

applications ranging from common consumer products to advanced pharmaceutical and 

medical products, while electrified coaxial jets provide particularly good control over 

sub-micrometer particle size, size range, and coating thickness.30 

A number of encapsulation approaches have been considered in association with artificial 

cell and related technologies. As early as 1964, in association with his artificial cell work, 

Thomas Chang encapsulated 1-100 micron droplets of aqueous solutions of proteins in 

polymer membranes. This was achieved by interfacial polycondensation or interfacial 

coacervation of collodion, polystyrene and copolymers around emulsified droplets, and 

the result was aqueous suspensions of particulates enclosed in semipermeable 

microcapsules.31

The previous section mentioned cross-linked phospholipid bilayer vesicles, which fall 

into the category of micro- or nano-capsules. Discher et al32 have coined the term 

“polymersome” to describe a similar but more durable and controllable construct. 

Whereas phospholipid molecules have a molecular weight on the order of 750, polymers 

 

29 Zhou, S.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U.E.; Isobe, H.; 

Nakamura, E. Spherical Bilayer Vesicles of Fullerene-Based Surfactants in Water. Science 2001, 291, pp 

1944-1947. 

30 Loscertales, I.G.; Barrero, A.; Guerrero, I.; Cortijo, R.; Marquez, M.; Gan<n-Calvo, A.M. Micro/Nano 

Encapsulation via Electrified Coaxial Liquid Jets. Science 2002, 295, pp 1695-1698. 31. Chang, T.M.S. Semipermeable Microcapsules. Science 1964, 146, pp 524-525. 32 http://science.nasa.gov/headlines/y2003/29may_polymersomes.htm (accessed January 2008). 
13

can have much higher molecular weights which along with the high level of cross-linking 

available using polymers, allows construction of much more durable vesicles. These have 

been targeted for medical applications including possible dehydrated blood supplies and 

other artificial cell constructs.33 The distinction between polymerosomes and other 

durable polymer capsules is the bilayer architecture, and the amphiphilic nature of the 

polymers used provides properties similar to those of liposomes, with the added 

advantage that they are much more controllable. 

In the specific case referred to in Discher’s original work, polyethyleneoxidepolyethylethylene diblock copolymers, which self-assembled in water to form ordered 

lamellar phases, were formed into vesicles by an electroformation process. The resulting 

vesicles were 10 times less water permeable and 10 times tougher than analogous 

phospholipid vesicles, and are also robust and less likely than liposomes to leak 

encapsulated small molecules. More recently, polymersomes have been prepared by 

reverse evaporation techniques as well as electroformation and they are under 

investigation as microreaction containers.34 

The University of Pennsylvania team of Hammer, in collaboration with the University of 

Minnesota,35 have used polymersomes for optical signaling by incorporating near 

infrared fluorophores in the lamellar membranes of polymersomes for the purposes of 

deep-tissue medical imaging and drug-delivery. Polymersomes were uniquely effective in 

uniformly distributing the fluorophores and facilitating an exceptionally intense optical 

signal for penetration through tissue.36 

Although the vast majority of research and suggested applications for polymersomes 

involve spherical or near-spherical vesicles, the technology isn’t limited to this shape. 

Scientists at the National Institute of Standards and Technology have successfully used 

both optical tweezers and micropipettes to pull nanotubes from polymersomes37, and 

subsequent cross-linking of the polymers stabilizes the nanotubes. These methods were 

capable of forming, in addition to single stand-alone nanotubes, networks of polymer 

nanotube nanotubes and nanotube networks connecting multiple vesicles to one another. 

Thus, the relatively simple concept of enclosing one material within another has 

expanded to the ability to connect various vesicles via controlled networks of nanotubes 

to net a submicron microfluidic network. 

Hydrogels 

Hydrogels represent a class of materials which has been employed extensively in contact 

lenses, medical implants, a wide variety of medical device studies dependent on tissue 

 

33 http://science.nasa.gov/headlines/y2003/29may_polymersomes.htm (accessed January 2008). 34 www.physics.nist.gov/Divisions/Div842/Gp4/Tweezers/research.html (accessed March 2008). 35 http://www.eurekalert.org/pub_releases/2005-02/uop-snp020705.php (accessed June 2008). 36 Ghoroghchian, P.P.; Frail, P.R.; Susumu, K.; Blessington, D.; Branna, A.K.; Bates, F.S.; Chance, B.; 

Hammer, D.A.; Therien, M.J. Near-Infrared-Emissive Polymersomes: Self-Assembled Soft Matter for in 

vivo Optical Imaging. PNAS 2005, 102(8), pp 2922-2927. 37 Reiner, J.E.; Wells, J.M.; Kishore, R.B.; Pfefferkorn, C.; Helmerson, K. Stable and Robust Polymer 

nanotubes Stretched from Polymersomes. PNAS 2006, 103(5), pp 1173-1177. 
14

ingrowth, and familiar consumer products such as diaper 

absorbents and moisture retainers for agriculture. The term 

“gel” is fairly broad, referring to any dense array of crosslinked material generally with liquid filling the pores, the 

most common being the hydrogel, where the cross-linked 

material is filled with water and the material, if dehydrated, 

is highly absorbent. Familiar hydrogels are made from high 

molecular weight natural polymers such as gelatin, fibrin 

and polysaccharides, or high molecular weight synthetic 

polymers from, for example, acrylic acid, ethylene oxide, 

or vinyl alcohol. All hydrogels are amphiphilic, with 

networks of self-assembled high aspect ratio (ratio of 

length to width) nanofibers arranged to form high surface area hydrophilic cavities. 

Because hydrogels are adept at absorbing and retaining water, they are ideal materials for 

many biological applications. They have recently received attention from a number of 

sectors interested in materials bridging the biology-materials gap from the standpoint of 

functioning biological materials ex-vivo as well as the more common standpoint of using 

nonbiological materials in biological environments, i.e., biocompatibility. 

Many biological molecules such as proteins require water in order to carry out their 

functions. In the case of proteins this is associated with the fact that their functions 

generally include specific folding and unfolding sequences for which the molecules 

require a freedom of motion not available in the dry state. Hamachi et al38,39 addressed 

this problem by constructing glycosylated amino acetate hydrogels and incorporating 

peptide/protein arrays into the hydrophilic cavities. The environment within the cavities 

allows the proteins to function unhindered. 

Hydrogels are being used for microenvironmental control, primarily in applications 

associated with encapsulated arrays of living cells. Albrecht et al,40 recognizing the need 

for a three dimensional environment for cells in “lab-on-a-chip” style applications, 

encapsulated living cells in PEG-based hydrogels. They used photo-patterning to crosslink hydrogels into 100μm features and electro-patterning to position cells within these 

hydrogel structures with micron level resolution. Recently Schaus, Grinstaff et al41 have 

immobilized arrays of hydrogel chambers, each measuring approximately 200 microns, 

on glass slides for use in biological event recognition. Hydrogel chambers containing 

biotin, goat IgG and antisense RNA were probed with Cy5-labeled streptavidin, Cy5-

labeled protein G and Cy5-labeled aRNA, respectively. These experiments successfully 

demonstrated recognition of small molecule-protein, protein-protein and nucleic acidnucleic acid binding events. 

 

38 Kiyonaka, S.; Sada, K.; Yoshimura, I.; Shinkai, S.; Kato, N.; Hamachi, I. Semi-Wet Peptide/Protein 

Array Using Supramolecular Hydrogel. Nature Materials 2004, 3, pp 58-64. 

39 Zhang, S. Hydrogels: Wet or Let Die. Nature Materials 2004, 3, pp 7-8. 40 Albrecht, D.R.; Tsang, V.L.; Sah, R.L.; Bhatia, S.N. Photo- and Electropatterning of HydrogelEncapsulated Living Cell Arrays. Lab Chip 2005, 5, pp 111-118. 41 Dominguez, M.M.; Walthier, M.; Grinstaff, M.W.; Schaus, S.E. Immobilized Hydrogels for Screening of 

Molecular Interactions. Anal. Chem. 2007, 79, pp 1064-1066. 

Figure 3.2. Aerogel 

Supporting a Brick. 
15

Another area of investigation in hydrogels is gelation control, i.e., switching between 

viscous liquid and stable gel in response to external stimuli. Tirrell et al42 created triblock 

synthetic proteins, independently engineering the interchain binding and solvent retention 

segments of the molecules in order to allow independent investigation and control of the 

effects of these on gel formation, sol-gel transition temperature and reversibility. 

Similarly Pochan, Schneider et al43 took advantage of the self-assembling of 

biomolecules to create hydrogels based on the intramolecular folding of designed 

peptides. The hydrogelation is itself defined by the intramolecular folding and selfassembly of monomeric β-hairpin peptides. These proteinaceous polymer gels provide 

additional control of responsiveness to chemical and stress environments but, unlike most 

protein gels, rely entirely on self-assembly in the absence of extra cross-linking agents or 

steps. In subsequent work the same group designed a peptide that forms a self-assembled 

hydrogel that is thermally responsive. The system has an amphiphilic β-hairpin that, upon 

heating, folds unimolecularly providing a conformation that self-assembles into a 

hydrogel that dissolves when temperature is again reduced.44 

The biocompatibility of hydrogels is also being exploited by the biomaterials community. 

Providing the possibility of controlled stiffness, swelling, degradation and molecular 

binding and release make them attractive materials for many applications. Alginate 

hydrogels, derived from a linear polysaccharide copolymer naturally occurring in algae 

and other marine plants, have been used in drug delivery and as extracellular matrices for 

basic research. In the quest for transplantable islets of Langerhans to treat Type I 

diabetes, cells have been entrapped in alginate hydrogels that were subsequently crosslinked with polylysine to provide permeaselectivity.45 In further refinements of this 

approach, the lysine has been replaced by chitosan to improve biocompatibility.46 In 

general terms, hydrogels have been effectively used in the encapsulation of living cells. 

Mooney and Augst47 have demonstrated alginate hydrogels as effective scaffolds for 

osteoblast transplantation and improved bone formation. From a different angle on 

hydrogels and cells, Nakajima et al48 used 1 percent strontium alginate hydrogel as an 

artificial cell wall, generating hydrogel beads that housed the production of invertase by 

protoplasts of Saccharomyces cerevisae. 

 

42 Petka, W.A.; Harden, J.L.; McGrath, K.P.; Wirtz, D.; Tirell, D.A. Reversible Hydrogels from SelfAssembling Artificial Proteins. Science 1998, 281,pp389-392. 43 Schneider, J.P.; Pochan, D.J.; Ozbas, B.; Rajagopal, K.; Pakstis, L.; Kretsinger, J. Responsive Hydrogels 

from the Intramolecular Folding and Self-Assembly of a Designed Peptide. JACS 2002, 124, pp13030-

13037. 

44 Pochar, D.J.; Schneider, J.P.; Kretsinger, J.; Ozbas, B.; Rajagopal, K.; Haines, L.. Thermally Reversible 

Hydrogels via Intramolecular Folding and Consequent Self-Assembly of a de Novo Designed Peptide. 

JACS 2003, 125, 11802-11803. 45 Lim, T.; Sun, A.M. Microencapsulated Islets as a Bioartificial Endocrine Pancreas. Science 1908, 210, pp 

908-910. 

46 http://www.swri.edu/3pubs/ttoday/spring97/cell.htm (accessed 14 May 2008). 47 Augst, A.D.; Mooney, D.J. Alginate Hydrogels as Biomaterials. Macromol. Biosci. 2006, 6(8), pp 623-

633. 

48 Tanake, H.; Kamogawa, T.; Aoyagi, H.; Kato, I.; Nakajima, R. Invertase Production by Saccharomyces 

cerivisae Protoplasts in Strontium Alginate Gel Beads. J. Biosci. Bioeng. 2000, 89(5), pp 498-500. 
16

Sidorenko et al49 have focused on using hydrogel activity to control embedded materials. 

By controlling the stress fields within a hydrogel, achieved by humidity control, they 

demonstrated controlled orientation of silicon nanocolumns that had been integrated into 

the gel. They went on to demonstrate various complex micropatterns that could be 

reversibly actuated in this manner. The result is akin to a hydrogel “muscle” reacting to 

external stimulus. 

Aerogels 

Although hydrogels can be very versatile and have many applications to biologically 

inspired systems, by definition they are dependent on high water content and are 

susceptible to disablement via dehydration. This places severe limits on them in 

unattended environmental applications or in desert climates. A viable environmentally 

hardened alternative may be aerogels. 

As the name implies, an aerogel can be envisioned as a hydrogel in which the water has 

been replaced by air. Typically, this is achieved by step-wise replacement of the water 

with a more volatile polar solvent, usually an alcohol to form an alcogel, and subsequent 

supercritical drying which removes the liquid in the absence of surface tension, 

eliminating the shrinkage and collapse that occurs when gels are dried under ambient 

conditions. This retains most of the network structure of the gel, and the result is a dry, 

environmentally stable material with very high surface area, very low density, low 

thermal conductivity (0.014 W/mK), low dielectric permittivity and unique porosity. The 

volume of an aerogel monolith is largely void and, in an architectural metaphor coined by 

Rolison and Long,50 can be viewed as spaces that can house functions and be decorated 

with functional elements hung on the supporting walls and scaffolds. Along with the 

common silicas and aluminas, other commonly discussed metal oxide aerogels include 

titania, zirconia, stannic oxide and tungsten oxide. On a weight basis aerogels are very 

efficient and repeatable adsorbents for water, typically exceeding 1 kilogram water per 

kilogram aerogel for 25 cycles without degradation. However, due to the very low 

density this does not translate to high adsorption on a volume basis.51

The first aerogels were primarily silicas and aluminas derived from the sol-gel processes 

common in ceramic synthesis, and were dried in difficult high-pressure autoclave 

procedures. However, there are now a broad range of materials that have been used to 

produce aerogels and related porous materials, and recently there has been progress in 

alternate synthetic approaches. 

One of the difficulties in synthesizing aerogels is the supercritical drying stage. During 

the intense investigation of aerogels in the 1980s, this generally involved cumbersome 

 

49 Sidorenko, A.; Krupenkin, T.; Taylor, A.; Fratzl, P.; Aizenberg, J. Reversible Switching of HydrogelActuated Nanostructures Into Complex Micropatterns. Science 2007, 315, pp 487-490. 50 Long, J.W.; Rolison, D.R. Architectural Design, Interior Decoration, and Three-Dimensional Plumbing 

En Route to Multifunctional Nanoarchitectures. Acc.Chem.Res. 2007, 40, pp 854-862. 

51 Knez, .; Novak, Z. Adsorption of Water Vapor on Silica, Alumina and Their Mixed Oxide Aerogels. 

J.Chem.Eng.Data 2001, 46, pp 858-860. 
17

operations in high pressure autoclaves. More recently, critical point dryers such as those 

used in electron microscopy preparations have been successfully used for this purpose. In 

this approach the alcohol in the intermediate alcogel is replaced by liquid carbon dioxide 

which is removed at conditions supercritical for carbon dioxide. Others have had success 

with freeze-drying methods. Schiraldi et al52 have recently reported a freeze-dried clay 

aerogel and clay/aerogel polymer composite reinforced with natural fibers, a process 

which removes both the alcogel step and the super critical drying. The product exhibited 

compressive moduli and strengths increased significantly over non-reinforced materials, 

demonstrating not only vastly simplified processing techniques but also reduced fragility. 

Metal aerogels are less common and are typically difficult to manufacture, with 

conventional approaches yielding lower surface area and larger pores than oxide aerogels. 

Son et al53 have successfully made aerogel-like metal foams with density down to 0.011 

grams per cubic centimeter and surface area up to 270 square meters per gram. They 

achieved this using self-propagating combustion synthesis of metal (Fe, Cu, Co, Ag) 

foams from metal complexes of bistetrazolamine (BTA). 

In the search for more readily processed aerogels, Guadalupe and Guo54 made composite 

gels of Bis[(3-triethoxysilyl)propyl] tetrasulfide (SIS) and sodium dodecyl sulfate (SDS). 

Extraction of the SDS by alcohol yielded silica aerogels with no requisite supercritical 

drying. The proposed mechanism suggests that the surfactant facilitates formation of SDS 

vesicles enclosing SIS and that the SIS polymerizes while the vesicles aggregate. 

Removal of the SDS by alcohol extraction produced aerogels, aided by the 

hydrophobicity of the polymerized SIS, indicating a novel mechanism for aerogel 

formation. 

Another approach to simplified synthesis of aerogels is freeze-drying, an approach used 

in the production of SEAgel,55 an aerogel made from agar and therefore non-toxic, 

biocompatible and biodegradable. The agar forms a hydrogel which, upon freeze-drying, 

results in SEAgel. In the related patents56,57 the products are called Biofoams. Biofoams 

are made from agar, agarose, gelatin, algin, alginates, gellan gum and cellulose which are 

added to a polar solvent, then either freeze-dried or emulsified with a non-polar solvent 

prior to gelation and freeze-drying. Composite-forming additives can be incorporated 

prior to gelling to modify properties of the aerogel product. 

Several groups have already looked at aerogels as supports for lipid bilayers. The 

collaborative effort led by Stanford and UC Davis was mentioned previously. In 

 

52 Finlay, K.; Gawryla, M.D.; Schiraldi, D.A. Biologically Based Fiber-Reinforced/Clay Aerogel 

Composites. Ind.Eng.Chem.Res. 2008, 47, pp 615-619. 53 Tappan, B.C.; Huynh, M.H.; Hishey, M.A.; Chavez, D.E.; Luther, E.P.; Mang, J.T.; Son, S.F. Ultra-Low 

Density Nanostructured Metal Foams: Combustion Synthesis, Morphology and Composition. JACS 2006, 

128. pp 6589-6594. 54 Guo, Y.; Guadalupe, A.R. Functional Silica Aerogel from Metastable Lamellar Composite. Chem. 

Commun. 1999, pp 315-316. 55 http://en.wikipedia.org/wiki/SEAgel (accessed January 2008). 56 Morrison, R.L. Biofoam. U.S. Patent 5,382,285, January 17, 1995.

57 Morrison, R.L. Biofoam 2. U.S.Patent 5,360,828, November 1, 1994.
18

association with this effort, Frank et al58 used critical point dried silica aerogels from 

tetramethoxysilane (TMOS) and xerogels from tetraethoxysilane (TEOS) spin-coated on 

quartz crystal microbalance crystals as substrates for a phospholipid bilayer formed from 

L-α-phosphatidylcholine. The bilayer fusion on the aerogel surface was slowed 

significantly compared to spreading on a smooth surface. However, their experiments do 

indicate a continuous bilayer membrane spanning the pores in the gel and provides 

promise for systems in which the aerogel support provides access to both sides of the 

bilayer. 

While the work by the Stanford group was successful in producing supported bilayers 

spanning the external surface of an aerogel surface, it raises the question of coating the 

entire aerogel surface to include internal pore surfaces. Zemanian et al59 have patented a 

method for coating aerogel surfaces with monolayers, potentially useful for control of 

internal surface chemistry as well as for improving monolith strength. Schubert and 

Hηsing60 have demonstrated organofunctional aerogels from mixtures of TMOS with 

organofunctional alkoxysilanes, indicating the feasibility of aerogel functionalization. 

Another material being investigated in aerogel form is carbon, which is typically 

achieved by production of an organic aerogel that is subsequently carbonized. RodriguezCastellon et al61 used resorcinol and formaldehyde for the initial organic aerogel and 

carbonized them at 500 or 900 degrees Celsius adding tetrabutyl orthotitanate in order to 

produce particulate titania inclusions. It was possible to load the aerogel with well 

dispersed ~4 nanometer titania particles up to a 70 percent loading. This is expected to 

capitalize on the synergistic effects of titania/carbon catalysts, for example for 

photodegradation in water purification. Other metal/carbon aerogels reported include Cu, 

Cr, Fe, Co, Ni, Ru, Ag, Pd and Pt, with applications in electrodes, supercapacitors, 

adsorbents, and catalysis.62 In this work they control the uniformity of the dispersion of 

metal dopant by replacing resorcinol with the potassium salt of 2,4-dihydroxybenzoic 

acid and replacing the potassium ions in the resultant gel with the desired metal ion, in 

this particular case Cu. 

Another interesting family of materials is aerogel composites, most often with nanotubes. 

Kaneko et al63 embedded single-wall carbon nanohorns into resorcinol-formaldehyde 

organic aerogels. Since the nanohorns were embedded in the aerogel after it was dried 

and the gel was not carbonized, the properties of the nanohorns remain intact and the 

 

58 Weng, K.C.; StΔlgren,J.J.R.; Duval, D.J.; Risbud, S.H.; Frank, C.W. Fluid Biomembranes Supported on 

Nanoporous Aerogel/Xerogel Substrates. Langmuir 2004, 20, pp 7232-7239. 

59 Zemanian, T.S.; Fryvell, G.; Ustyugov, O.A. Monolayer Coated Aerogels and Method of Forming. U.S. 

Patent 7,019,037, March 28, 2006. 

60 Husing, N.; Schubert, U. Organofunctional Silica Aerogels. J.Sol-Gel Sci.Tech. 1997, 8, pp 807-812. 

61 Morena-Castilla, C.; Maldonado-Hτdar, F.J.; Carrasco-MarΡn, F.; RodrΡquez-Castell∴n, E. Surface 

Characteristics of Titania/Cabon Composite Aerogels. Langmuir 2002, 18, pp 2295-2299. 62 Fu, R.; Yoshizawa, N.; Dresselhaus, M.S.; Dresselhaus, G.; Satcher, J.H.Jr.; Bauman, T.F. XPS Study of 

Copper Doped Carbon Aerogels. Langmuir 2002, 18, pp 10100-10104. 63 Tao, Y.; Noguchi, D.; Yang, C.-M.; Kanoh, H.; Tanaka, H.; Yudasaka, M.; Iijima, S.; Kaneko, K. 

Conuctive Mesoporous Single-Wall Carbon Nanohorn/Organic Aerogel Composites. Langmuir 2007, 23, 

pp 9155-9157. 
19

resulting composite materials showed high electrical conductivity and controllability of 

pore sizes. 

There has been very little investigation of aerogel systems based on chalcogenides; 

however the work of Kanatzidis et al64 demonstrates that such systems are possible and 

that they combine the electronic properties of the chalcogenides with the internal porosity 

of aerogels. They developed a strategy for forming semiconducting aerogels derived from 

chalcongenide clusters with platinum linking ions. They prepared chalcogels with the 

formula Pt2[M4Q10] where M = Ge, Sn and Q= S, Se. These materials are stable in humid 

environments, have high affinity for hydrophobic species, and absorb light in the visible 

and infrared. 

In a novel application of aerogel technology, Risen et al65 have generated hybrid aerogels 

of bioderived polymers and silica, working with chitosan, pectic acid and alginic acid. 

Their purpose has been to subsequently react the biopolymers to form new materials 

which can be harvested by dissolution of the aerogel scaffold. This work is mentioned 

here as it relates to chitosan, a high interest material in the ANTS program. 

 

64 Bag, S.; Trikalitis, p.N.; Chupas, P.J.; Armatas, G.S.; Kanatzidis, M.G. Porous Semiconducting Gels and 

Aerogels From Chalcogenide Clusters. Science 2007, 317, pp 490-493. 65 Wang, M.; Liu, X.; Ji, S.; Risen, W.M.Jr. A New Hybrid Aerogel Approach to Modification of 

Bioderived Polymers for Materials Applications. In Materials Research Society Symposium Proceedings 

2002, 702, pp 77-85. 
20

Table 2. Comparison of aerogel and related high surface area solids 

Density (g/cc) Surface area 

(m2/g) Pore sizes 

Traditional oxide 

aerogel 0.003-0.35 500-1000 <100 nm 

Clay 0.07 

Reinforced 

clay/composite 

Typical metal 

foam (Al, Mg) 0.04-0.08 

Self-propagating 

combustion Metal 

Foam 

0.011 270 

1-3 μm 

20-200 nm 

Silica/polymer 

hybrid 

0.357 766 11.5 nm 

Biofoam/SEAgel 0.0012-0.5 

Typical 0.3 

10 2-3 μm 

Chalcogel 0.12-0.17 117-327 

Porous Inorganics 

Nanoporous materials are not limited to aerogels and a number of relevant studies exist 

regarding nanoporous inorganic materials in biomimicry. Nanoporous alumina 

membranes from anodized aluminum have been used to imitate ion diffusion and ion 

pumping using experimental techniques similar to those used with bilipid membranes.66

Based on the honeycomb morphology of anodic alumina, Fukuda et al67 have used anodic 

porous alumina as a template for the production of morphological replicas in platinum 

and gold. 

Functionalized mesoporous silica has been used for entrapment of enzymes such that the 

kinetics of entrapped enzymes approached reaction rates in solution.68 In a related study, 

enzymes entrapped in functionalized mesoporous silica have been observed to retain 

activity that would have been lost in the absence of the mesoporous surroundings, 

 

66 Wang, G.-J.; Chang, W.C. Imitation of the Ion-Diffusion and Ion Pumping Mechanism of a Living Cell 

by Nanoporous Alumina Membrane. In ENS ’05, 14-16 December 2005, Paris, France. 67 Masuda, H.; Fukuda, K. Ordered Metal Nanohole Arrays Made by a Two-Step Replication of 

Honeycomb Structures of Anodic Alumina. Science 1995, 268, pp 1466-1468. 68 Lei, C.; Shin, Y.; Magnuson, J.K.; Fryxell, G. Lasure, L.L.; Elliott, D.C.; Liu, J.; Ackerman, E.J. 

Characterization of Functionalized Nanoporous Supports for Protein Confinement. Nanotech. 2006, 17, pp 

5531-5538. 
21

hypothetically because the crowding within the pores forces protein folding.69 As 

mentioned previously, Nishiya et a,l,4

 among others, used porous inorganics as substrates 

for lipid bilayers in biosensing. Lopez et al70 extended this to Mesoporous silica 

microbeads, which they used for bilipid supports for use with transmembrane proteins, 

while scientists at the Paul Scherrer Institut have found that BLMs on ultrathin 

nanoporous silicon nitride retain fluidity but not stability.7

 

In a novel twist on the idea of nanopores, Achim Muller et al have been studying 

polyoxomolybdate clusters.71,72 Placing protonated urea ions in the pores provides a 

molecular stopper that can be opened to allow ion uptake and subsequently closed. 

Although this specific technology may not be immediately relevant, the ability to build 

inorganic molecular constructs that can controllably behave in a manner homologous to a 

biological functional element is revolutionary. The possibility of using a single stoppered 

pore as an abiotic channel in an abiotic membrane remains to be investigated. 

Chitosan 

Another material of high interest due to its inherent 

biocompatibility is chitosan. Chitin, the precursor 

to chitosan, is a naturally occurring biopolymer, a 

polysaccharide found prevalently in the 

exoskeletons of arthropods, including crustaceans. 

While chitin is a partially deacetylated nacetylglucosamine, chitosan is a more extensively 

deacetylated form of the same material. Chitosan is 

commercially available in a range of degrees of 

deacetylation, typically between 60 and 100 

percent, and molecular weights. Alternately, it can 

be produced by enzymatic deacetylation of chitin 

from a number of natural sources. It is positively 

charged and soluble in acidic solutions. 

Commercial uses for chitosan are diverse. In the biomedical field it has been found to aid 

blood clotting and is used in field bandages; in agriculture it is used to enhance growth 

and protect against fungal infestations; in filtration and brewing it is used to scavenge 

sediment particles; in the weight loss industry it is sold with claims that it removes 

dietary fat; and claims have been made that it can reduce LDL cholesterol and increase 

HDL cholesterol. Recently, the properties of chitosan have drawn attention in the field of 

 

69 http://www.pnl.gov/topstory.asp?id=202 (accessed June 2008) 70 Buranda, T.; Huang, J.; Ramaro, G.V.; Ista, L.K., Larson, R.S.; Ward, T.L.; Sklar, L.A.; Lopez, G.P. 

Biomimetic Molecular Assemblies on Glass and Mesoporous Silica Microbeads for Biotechnology. 

Langmuir 2003, 19, pp 1654-1663. 71 Freemantle, M. Artificial Cells Allow Ion Entry. Chemical and Engineering News, 2005, 83(48), pg 10. 72 www.physorg.com/printnews.php?newsid=8079 (accessed June 2008) 

Figure 3.3. Chitosan from 

Deacetylation of Chitin. 
22

biofabrication for a variety of applications73 based on its film forming and electronic 

properties which allow for directed assembly and ease of biological functionalization. 

For purposes of protein immobilization, the acid environment of chitosan solution is 

harmful to the proteins while calcium alginate hydrogels, often used in the 

immobilization of enzymes, are damaged by phosphate ion in physiological buffer. Amiji 

et al74 addressed this with hybridized chitosan-alginate microcapsules. They prepared 

layered microcapsules of chitosan over alginate, adding enzyme to the process step in 

which the alginate cores are generated. This yielded enzyme-bearing alginate cores 

protected by outer layers of chitosan. The chitosan layer provided permeability selectivity 

and protection while the alginate protected the enzyme from the harsh environment of 

chitosan solution, with the overall structure stabilizing and extending the activity of the 

enzyme. 

There are many ways to fabricate chitosan microspheres for drug delivery. Sinha et al75

compared several of theses with respect to their mucoadhesive properties, which are 

related to electrical charge. In general, chitosan carries a net positive charge which 

impacts its interactions with biological systems. Sinha’s work indicates from zeta 

potential measurements that the charge is strongly affected by the production method 

chosen, as is stability at low pH. 

Groups at Southwest Research Institute and the University of Pittsburgh Medical Center 

have been developing methods to encapsulate insulin-producing cells for transplantation 

to fight diabetes. Their efforts led to alginate hydrogels cross-linked with polylysine. 

However, this faces immunorejection problems, and chitosan has been used as a 

replacement for the lysine, allowing for the selective permeability of the original 

materials without the immunorejection problems of the lysine. 

As a biocompatible and non-toxic material with antimicrobial properties, chitosan has 

also received attention for applications in the food industry. With food applications in 

mind, Kim et al76 and Nadarajah et al77 have studied the effect of pH and solvent on the 

properties of chitosan films, specifically water vapor permeability, tensile strength and 

elongation. In both studies the acid system chosen (formic, lactic, acetic, propionic, 

ascorbic, malic) had significant effect on the product, particularly in terms of strength, 

flexibility, hydrophilicity, and appearance. 

 

73 Yi, H.; Wu, L.-Q.; Bentley, W.E.; Ghodssi, R.; Rubloff, G.; Culver, J.N.; Payne, G. Biofabrication with 

Chitosan. Biomacromolecules 2005, 6(6), pp 2881-2894. 74. Taqieddin, E.; Lee, C.; Amiji, M. Perm-Selective Chitosan-Alginate Hybrid Microcapsules for Enzyme 

Immobilization Technology. Pharmaceutical Engineering 2002, 22(6), pp 1-3. 75 Dhawan, S.; Singla, A.K.; Sinha, V.R. Evaluation of Mucoadhesive Properties of Chitosan Microspheres 

Prepared by Different Methods. 

76 Kim, K.M.; Weller, C.L.; Hanna, M.A. Properties of Chitosan Films According to pH and Types of 

Solvents. [Online] IFT Annual Meeting, 12-16 July 2004, Las Vegas, NV, Session 83C. 77.Nadarajah, K.; Prinyawiwatkul, W.; No, H.K. Phisicochemical Properties of Chitosan Edible Films 

Developed From Crawfish Shell Waste. [Online] IFT Annual Meeting, 15-20 July 2005, New Orleans, LA. 
23

Another area that has received a great deal of attention is composites of chitosan and 

carbon nanotubes. Tong et al78 used simple solution-evaporation to prepare composites of 

multiwalled carbon nanotubes in chitosan. Distribution of nanotube was found to be 

homogeneous even with this simple process and other reports79 have suggested that this 

can be attributed to the polymer chains wrapping around the individual nanotubes. With 

very low(<1 percent) nanotube loading the tensile modulus was improved by 93 percent 

and strength increased by 99 percent. Chitosan facilitates the stabilization of carbon 

nanotubes, a property exploited by Gorski et al79 to deposit chitosan-carbon nanotube 

composite films on glassy carbon electrodes. They subsequently immobilized enzyme in 

the film, another process facilitated by chitosan and the resulting electrodes were 

successfully used as glucose sensors. This approach should be readily adaptable to other 

analytes. Chen et al80 used a similar philosophy to fabricate chitosan-carbon nanotube 

composite films on gold electrodes. In this work the film was electrodeposited, 

capitalizing on the pH dependence of chitosan solubility. This device was also 

demonstrated as a glucose sensor. Ruzgas et al81 used single wall carbon nanotubes 

(SWCNT) in chitosan to form films on glassy carbon electrodes. In this case the films 

were further stabilized by cross-linking with glutaraldehyde which also presented free 

aldehyde groups for further functionalization. Immobilizing galactose oxidase on these 

surfaces provided an effective electrochemical galactose sensor. 

The biocompatibility and fabrication versatility of chitosan make it an attractive material 

for a range of biological and biomimetic applications. In addition to the investigations 

mentioned here, chitosan is being considered for use in aerogel-related materials,65 in 

compositions with collagen for piezoelectric properties,82 in water-soluble derivatives for 

improved properties for biological applications,83 and in fabric treatment schemes to 

improve properties and instill antimicrobial capabilities.84

S-layer Proteins 

S-layer proteins are natural proteins originating from the surface layer of bacterial cells 

and archaea. They are crystalline and have the ability to self-assemble into twodimensional arrays. They can be lithographically patterned and have drawn interest 

because, in addition to patterning, they can act as templates for the deposition of 

 

78 Wang, S.-F.; Shen, L.; Zhang, W.-D.; Tong, Y.-J. Preparation and Mechanical Properties of 

Chitosan/Carbon Nanotube Composites. Biomacromol. 2005, 6(6), pp 3067-3072. 79 Zhang, M.; Smith, A.; Gorski, W. Carbon nanotube-Chitosan System for Electrochemical Sensing Based 

on Dehydrogenase Enzymes. Anal.Chem. 2004, 76(17), pp 5045-5050. 80 Luo, X.-L.; Xu, J.-J.; Wang, J.-L.; Chen, H.-Y. Electrochemically Deposited Nanocomposite of Chitosan 

and Carbon Nanotubes for Biosensor Application. Chem. Commun. 2005, pp 2169-2171. 81 Luo, X.-L.; Xu, J.-J.; Wang, J.-L.; Chen, H.-Y. Electrochemically Deposited Nanocomposite of Chitosan 

and Carbon Nanotubes for Biosensor Application. Chem. Commun. 2005, pp 2169-2171. 82 Silva, C.C.; Lima, C.G.; Pinheiro, A.G.; G∴es, J.C.; Figueir∴, S.D.; Sombra, A.S.B. On the 

Piezoelectricity of Collagen-Chitosan Films. Phys. Chem. Chem. Phys. 2001, 3, 4154-4157. 83 Cho, J.; Grant, J.; Piquette-Miller, M.; Allen, C. Synthesis and Physicochemical and Dynamic 

Mechanical Properties of a Water-Soluble Chitosan Derivative as a Biomaterial. Biomacromol. 2006, 7, pp 

2845-2855. 

84 Jeong, Y.J.; Cha, S.Y.; Yu, W.R.; Park, W.H. Changes in the Mechanical Properties of Chitosan-Treated 

Wool Fabric. Tex. Res. J. 2002, 72(1), pp 71-76. 
24

inorganic moieties with electronic and/or optical properties. For these reasons they are 

considered important components for “biomolecular construction kits.”85

S-layer protein lattices are disrupted by chaotropic agents, pH control, or metal chelating 

agents, after which they reassemble upon removal of the disrupting agent. Typically, 

reassembly can take the form of sheets, cylinders or vesicles with two-dimensional lattice 

parameters characteristic of the specific protein, being square, hexagonal or oblique. Slayer thicknesses are on the order of 4 to 10 nanometers and lateral domain measurements 

range from nanometers to many microns, with morphology dependent on environmental 

factors, especially temperature, pH and ionic composition and strength. 

One of the important features of S-layer proteins is their ability to self-assemble into 

these arrays at a variety of interfaces. Upon removal of the disrupting agent, S-layers will 

readily form at liquid-gas or liquid-solid interfaces. These assembled arrays are molecular 

monolayers with well-defined two dimensional crystal symmetries. Formation of S-layers 

at solid-liquid interfaces provides a valuable tool for surface preparation for subsequent 

functionalization, and surfaces that have been reported as substrates for S-layers include 

Si with a native oxide layer, silanized silicon wafers, and silicon coated with carbon or 

photoresist, as well as silicon nitride, gallium arsenide, glass, polymers, noble metals, 

mica and graphite. The substrate surfaces are typically hydrophobic, hence the surface 

preparations for silicon substrates, although some proteins have been found to assemble 

on hydrophilic surfaces as well.85 For assembly on solid surfaces, the coherence and 

domain size of the layer depend on the substrate surface as well as the identity and 

environment of the protein. One relevant application of S-layers on solid substrates is the 

stabilization of supported lipid membranes. Lipids on S-layers are both more fluid and 

more stable than lipids supported in other ways. This is significant in applications 

associated with pore-forming proteins, which tend to rupture lipid membranes that are not 

supported on S-layers. Sleytr et al86 demonstrated the enhanced bilayer membranestabilizing effect of S-layer lattices over microfiltration membranes as well as 

reconstitution of functional αHL pores within these S-layer ultrafiltration membranes. 

At the liquid-air interface S-layers assemble as monolayers on the liquid, typically water, 

surface. The morphology of the monolayer is highly dependent on ion content in the 

solution, with a 10 millimolar concentration of bivalent cation being optimum for longrange order. This suggests a salt bridge mechanism for stabilizing coherent layers. 

Patterning is often required for practical applications. S-layers are easily patterned 

lithographically using photomasking and deep ultraviolet radiation. Combining this with 

functionalization allows precise positioning of a range of molecules on the nanometer 

scale. In the case of macromolecules, this can be viewed as preferential deposition on the 

S-layer molecules resulting in patterning of the functional species that mimics the 

 

85 Pum, D.; Neubauer, A.; Gyoervary, E.; S<ra, M.; Sleytr, U.B. S-Layer Proteins as Building Blocks in a 

Biomolecular Construction Kit. Presented at the 7th Forsight Conference on Molecular Nanotechnology and 

accessed online http://www.foresight.org/conference/MNT7/Papers/Pum/index.html. 86 Schuster, B.; Pum, D.; S<ra, M.; Braha, O.; Bayley, H.; Sleytr, U.B. S-layer Ultra-Filtration Membranes: 

A New Support for Stabilizing Functionalized Lipid Membranes. Langmuir 2001, 17, pp 499-503. 
25

underlying pattern of the protein layer. Smaller inorganic molecules the S-layer can act as 

a mask, with precipitation of the inorganic species occurring in the pores of the S-layer to 

produce an array of nanoparticles with well defined symmetry. Batt et al87 determined 

that this biotemplating is dependent on interactions between the growing nanoparticles as 

well as on interactions between the S-layer and the growing particles. They used a 

hexagonally packed intermediate layer of Dinococcus radiodurans, a hexameric structure 

with pores at the centers of the hexamer units and large openings at the vertices 

visualized as the corners of the hexagonal units, used to template ordered arrays of 

citrate-capped gold nanoparticles. Nanoparticle binding at the vertices was found to be 

controlled by ionic strength, with low ionic strength allowing ionic repulsions leading to 

nanoparticle deposition on alternating vertices while higher ionic strength lowered the 

effects of interparticle repulsion and allowed deposition on each vertex. Mark et al88

extended this work to use both Deinococcus radiodurans and Sulfolobus acidocaldarius

S-layers to template citrate-capped gold and thiol-functionalized CdSe/ZnS quantum dots 

to control the patterning of the nanoparticle arrays. 

Batt et al89 have recently demonstrated the ion transport properties of S-layers via 

electrical impedance spectroscopy. Their work indicates that calcium ions are transported 

through the vertices of the lattice, with transport controlled primarily by electrical 

gradients. Thus, while it is possible85 to insert ion channel pores into bilayers and Slayers, using the pore structures inherent in the S-layer may also be a viable route to 

biologically based nanofiltration and sensing. 

Molecular Imprints 

Molecular imprinting is the practice of synthesizing an abiotic material which is 

morphologically equivalent to a biological functional element whose function is 

connected to morphology. In simplistic terms, an example might be a polymer that has 

been molded around an antigen so that, on removal of the antigen, the polymer takes on 

the antibody morphology and can act as an artificial antigen capture agent. The selectivity 

of this approach was demonstrated by Valdes, Thompson, et al in a study in which they 

used molecularly imprinted polymers (MIPs) to selectively capture closely related 

conotoxins.90 In practical terms, using the analyte as the imprint template, although direct 

and effective, is not ideal as it introduces analyte to the sensor production process and 

resulting MIPs may be self-contaminating, a condition referred to as template bleeding. 

 

87 Bergkvist, M.; Mark, S.S.; Yang, X.; Angert, E.R., Batt, C.A. Bionanofabrication of Ordered 

Nanoparticle Arrays: Effect of Particle Properties and Adsorption Conditions. J.Phys.Chem.B. 2004, 108, 

pp 8241-8248. 

88 Mark, S.; Bergkvist, M.; Yang, X.; Teixeira, L.M.; Bhatnagar, P.; Angert, E.R.; Batt, C.A. 

Bionanofabrication of Metallic and Semiconductor Nanoparticle Arrays using S-Layer Protein Lattices 

with Different Lateral Spacings and Geometries. Langmuir 2006, 22, pp 3763-3774. 

89 Sotiropoulou; S.; Mark, S.S.; Angst, E.R.; Batt, C.A. Nanoporous S-Layer Protein lattices: A Biological 

Ion Gate with Calcium Selectivity. J.Phys.Chme.C. 2007, 111, pp 13232-13237. 

90 Iqbal, S.S., Lulka, M.F., Chambers, J.P., Thompson, R.G., Valdes, J.J. Materials Science & Engineering 

C-Biomimetic And Supramolecular Systems, 2000, 7, pp77-81. 
26

In chromatography, MIPs have received a great deal of interest for their ability to 

selectively separate specific molecules from solution and they have proven quite useful in 

this capacity.91 In the field of drug delivery, MIPs have been considered favorably for 

controlled and sustained release, enantio-selective loading and release and improved 

loading.92 In the biological arena, MIPs are of interest for a number of applications. 

Bolisay et al93 have been working on molecularly imprinted polymers for the recognition, 

differentiation and, ultimately, removal of viruses. In contrast with typical MIP synthesis 

strategies that involve monomers in organic solutions, they use polyamine cross-linked in 

aqueous solutions. Their poly(allylamine hydrochloride) hydrogels imprinted against 

baculovirus appear to be effective at removing baculovirus from solution, a result that is 

very significant for its prospects in removal of viruses for a broad range of requirements. 

Another important discovery in this work is the relative insensitivity of the hydrogel MIP 

to swelling effects, long considered a problem in monomer-derived MIPs but apparently 

not significant with the polymer synthesis approach. Chen et al94 and Waigmann et al95

have reported surface imprinted polymers used to detect low concentrations of virus, 

while Bolisay et al96 work with three dimensional polymer gels for collection of large 

amounts of virus. In this recent work they have demonstrated the shape selectivity of 

MIPs imprinted for virus binding. 

Several approaches have been considered to overcome the issue of template bleeding. 

Sellergren97 and Takeuchi98 developed combinatorial methods for synthesizing and 

screening MIPs for specific targets while Piletsky et al99 use modeling for the same 

purpose. While these approaches address the problem from the aspect of avoiding the 

target analyte in MIP synthesis, Chang et al100 approach the problem from the template 

release aspect. They used a thermally reversible bond for imprinting silica spheres while 

introducing functional groups into the cavity vacated by the template. 

Molecularly imprinted polymers have been investigated as sensors in a range of schemes. 

They have been used on electrodes for electrochemical sensing,101, 102 on quartz crystal 

 

91 Widstrand, C.; Yilmaz, E.; Boyd, B.; Rees, A. the Column [Online] 2006, Feb., pp 20-24.

92 Hilt, J.Z.; Byrne, M.E. Configurational Biomimesis in Drug Delivery: Molecular Imprinting of 

Biologically Significant Molecules. Adv. Drug Deliv. Rev. 2004, 56(11), pp 286-315. 

93 Bolisay, L.D.V.; March, J.F.; Bentley, W.E.; Kofinas, P. Separation of Baculoviruses Using Molecularly 

Imprinted Polymer Hydrogels. In Mat. Res. Soc. Symp. Proc. Vol 787, G3.1.1-G.3.1.5. 

94 Tai, D.F.; Lin, C.Y.; Wu, T.Z.; Chen, L.K. Recognition of Dengue Virus protein Using EpitopeMediated Molecularly Imprinted Film. Anal. Chem. 2005, 77(16), pp 5140-5143. 

95 Dickert, F.L.; Hayden, O.; Bindeus, R.; Mann, K.J.; Blaas, D.; Waigmann, E. Bioimprinted QCM 

Sensors for Virus Detection – Screening of Plant Sap. Anal. Bioanal. Chem. 2004, 378(8), pp 1929-34. 96 Bolisay, L.D.; Culver, J.N.; Kofinas, P. Molecularly Imprinted Polymers for Tobacco Mosaic Virus 

Recognition. Biomaterials 2006, 27, pp 4165-4168. 97 Lanza, F.; Sellergren, B. Method for Synthesis and Screening of Large Groups of Molecularly Imprinted 

Polymers. Anal. Chem. 1999, 71, pp 2092-2096. 

98 Takeuchi, T.; Fukuma, D.; Matsui, J. Anal. Chem. 1999, 71, pp 285-290. 99 Piletsky, S.A.; Pilestska, E.V.; Day, C.J.; Freebairn, K.W.; Legge, C.; Turner, A.P.F. Analyst 2001, 126, 

pp 1826-1830. 

100 Ki, C.D.; Oh, C.; Oh, S.-G.; Change, J.Y. The Use of a Thermally Reversible Bond for Molecular 

Imprinting of Silica Spheres. J. Am. Chem. Soc. 2002, 124, pp 14838-14839. 101 Shoji, R.; Takeuchi, T.; Kubo, I. Atrazine Sensor Based on Molecularly Imprinted Polymer-Modified 

Gold Electrode. Anal. Chem. 2003, 75(18), pp 4882-4886. 
27

microbalances for gravimetric sensing103 including whole cell applications,104 and in 

arrays for colorimetric identification.105

In addition to bulk and thin film morphologies and porous monoliths, there is a growing 

body of work applying imprinting techniques to nanoparticles. For example, Mayes et 

al106 have demonstrated the binding of propanolol by shell-imprints on core-shell 

nanoparticles and report the effect of solvent on binding affinity, while Zhu et al107 have 

devised composites of multiwalled carbon nanotubes with MIPs to join the electronic 

properties of nanotubes with the recognition capabilities of the MIPs. 

Nanotubes 

No discussion of emerging materials is 

complete without touching on nanotubes. 

Although nanotubes are not relevant to the 

current discussion as stand-alone substrates 

for abiotic sensing applications they are 

extremely interesting as composite elements in 

the substrates, and have in fact been 

mentioned previously for cases where the 

composites are already under investigation. 

By far the most commonly studied nanotubes 

are carbon nanotubes, which come in both the 

single walled (SWCNT) and multiwalled 

(MWCNT) varieties. SMCNTs can be envisioned as single planar sheets of graphite, 

graphene, wrapped into tubes with diameters on the order of a nanometer and lengths up 

to the micron range. They have excellent electrical properties and can be modified to 

make both p- and n- field effect transistors and therefore can be used to construct 

nanoscale logic gates.108 Generally the electrical properties of SWCNTs are dictated by 

the wrapping symmetry of the tube, which in turn dictates the available electron paths. 

Controlling these properties is at the heart of SWCNT applications in nanoelectronics. 

 102 Kitada, T.; Kitamura, K.; Konishi, T.; Takegami, S.; Okuno, T.; Ishikawa, M.; Wakabayashi, M.; 

Nishikawa, K.; Muramatsu, Y. Potentiometric Immunosenso Using Artificial Antibody based on 

Molecularly Imprinted Polymers. Anal. Chem. 2004, 72(22), pp 6802-6807. 103 Das, K.; Penelle, J.; Rotello, V.M. Selective Picomolar Detection of Hexachlorobenzene in Water Using 

a Quartz Crystal Microbalance Coated with a Molecularly Imprinted Polymer Thin Film. Langmuir 2003, 

19(9), pp 3921-3925. 104 Dickert, F.L.; Hayden, O. Anal. Chem. 2002, 74, pp 1302-1306 105 Buryak, A.; Severin, K. A. Chemosensor Array for the Colorimetric Identification of 20 Natural Amino 

Acids. J. Am. Chem. Soc. 2005, 127(11), pp 3700-3701. 106 Perez-Moral, N.; Mayes, A.G. Noncovalent Imprinting in the Shell of Core-Shell Nanoparticles. 

Langmuir 2004, 20, pp 3775-3779. 107 Kan, X.; Zhao, Y.; Geng, Z.; Wang, Z. Zhu, J.-J. Composites of Multiwalled Carbon nanotubes and 

Molecularly Imprinted Polymers for Dopamine Recognition. J. Phys. Chem. C. 2008, 112, pp 4849-4854. 108 Martel, R.; Derycke, V.; LaVoie, C.; Appenzeller, J.; Chan, K.K.; Tersoff, J.; Avouris, Ph. Ambipolar 

Electrical Transport in Semiconducting Single-Wall Carbon Nanotubes. Phys. Rev. Lett. 2001, 87(25), pp 

256805. 

Figure 3.4. Conceptual Drawing of a 

Single-Walled Carbon Nanotube 
28

The drawbacks of SWCNTs are the synthetic expense and susceptibility to chemicals. A 

special case of the multiwalled nanotube, the double walled nanotube, is significantly less 

susceptible to chemicals but remains difficult and expensive to manufacture. 

Multiple layers of graphene wrapped into a tube constitute a MWCNT. In comparison 

with SWCNTs, multiwalled tubes sacrifice electrical properties in favor of stability and 

ease of manufacture. MWCNTs are larger than SWCNTs, with outer diameters ranging 

from >1 to 100 nanometers and variable inner diameters. In many applications where the 

electrical properties of SWCNTs are not required, these less expensive relatives are 

appropriate. 

Composites of multiwalled carbon nanotubes in chitosan have already been mentioned78

and these composites have been cast79 and electrochemically deposited81 on electrodes 

for biosensing applications. SWCNTs have likewise been used in chitosan composites on 

glassy carbon electrodes for galactose sensing.81 

Another potential value of carbon nanotubes is their ability to cross cell membranes. 

They can be readily functionalized and used to ferry functionality into a cell.109 This 

property can be employed to move functional elements across abiotic membranes. 

Additionally it is possible to incorporate nanotubes into membranes, with the nanotubes 

acting as pores. In microfluidics it has been found that the hydrophobicity and smooth 

inner surface of carbon nanotubes accelerates the motion of fluids through nanotubes.110

This concept has been used in aligning MWCNTs in polymer membranes used to 

demonstrate molecular transport through the tubes.111 Functionalization of the inner 

surfaces of the CNTs is also possible. Similar to the enhanced flow through CNTs, the 

concentric layers of MWCNTs move freely across each other, allowing telescoping 

action and free rotation which has been demonstrated in molecular machines. 

Although carbon nanotubes are by far the most heavily studied nanotubes, they are by no 

means the only players in the game. Tenne et al112 have made layered tubes of sulfides 

and selenides of tungsten and molybdenum some of which are used commercially as 

solid lubricants. Yang et al113 recognized the suitability of nanowires as templates for the 

growth of nonlayered nanotubes of gallium nitride and silica which have subsequently 

been used to demonstrate regulation of ionic transport. Arrays of titania nanotubes from 

 

109 Pantarotto, D.; Briand, J.-P.; Prato, M.; Bianco, A. Translocation of Bioactive peptides Across Cell 

Membranes by Carbon Nanotubes. Chem. Commun. 2004, pp 16-17. 110 Holt, J.K.; Park, H.G.; Wang, Y.; Stadermann, M.; Artyukhin, A.B.; Grigoropoulos, C.P.; Noy, A.; 

Bakajn, O. Fast Mass Transport through Sub-2-Nanometer Carbon Nanotubes. Science 2006, 312, pp 1034-

1037. 111 Scholl, D.S.; Johnson, J.K. Making High-Flux Membranes with Carbon nanotubes. Science 2006, 312, 

pp 1003-1004. 

112 Rapoport, L.; Fleischer, N.; Tenne, R. Applications of WS2 (MoS2) Inorganic Nanotubes and FullereneLike Nanoparticles for Solid Lubrication and for Structural nanocomposites. J. Mater. Chem. 2005, 15, pp 

1782-1788. 

113 Fan, R.; Yue, M.; Karnik, R.; Majumdar, A.; Yang, P. Polarity Switching and Transient Responses in 

Single Nanotube Nanofluidic Transistors. Phys. Rev. Lett. 2005, 95, 086607. 
29

anodization of titanium electrodes by Grimes et al114 are more sensitive hydrogen 

detectors than any other technology investigated, perhaps as the result of the nanotubes 

and inter-tube spaces providing a well defined straight path at the molecular level. 

A broad array of inorganic nanotubes have been synthesized and studied, both singly and 

in arrays, and a full review of these is not appropriate here. The salient point is that 

nanotube applications need not be limited by the fairly narrow array of sizes and 

properties available from carbon allotropes. A much broader and more controllable array 

of properties and morphologies is available by considering materials other than carbon. 

In the context of designing adaptive abiotic materials and systems, SWCNTs may be of 

interest as embedded electrically active subunits but not as an essential part of the 

substrate or matrix material. MWCNTs on the other hand may be useful in the substrate 

as structural composite reinforcements or as synthetic pores or conduits embedded into 

the matrix. 

 

114 Varghese, O.K.; Mor, G.K.; Grimes, C.A.; Paulose, M.; Mukherjeeb, N. A Titania Nanotube-Array 

Room-Temperature Sensor for Selective Detection of hydrogen at low Concentrations. J. Nanosci. 

Nanotech. 2004, 4, pp733-737. 
30

Conclusions and Recommendations 

The purpose of this review was to survey the field of biologically inspired materials and 

to develop a conceptual path from natural, biotic materials to completely synthetic, 

abiotic materials which can be assembled into systems with unique properties found in 

living systems but which could not be replicated via traditional synthetic chemical 

technologies. The ability to design and manufacture abiotic components which can be 

assembled into adaptive systems will have applications ubiquitous to the defense 

industry. Examples include sensors, adaptive camouflage, antibiotic and antiviral agents, 

compact personal power, coatings, new green manufacturing processes, the list is endless. 

As a specific example, the ANTS platform concept is a new paradigm in the area of CBD 

and is the result of a convergence of a number of rapidly developing technical areas, 

including synthetic biology, nanotechnology, materials science and information 

technology. This “transdisciplinary” approach is inimicable to current DOD research 

practices but is quickly becoming the norm in the commercial and academic sectors. Just 

as the current administration has placed an emphasis on radically altering acquisition 

policy and practice, so too must the DOD research community embrace radical change 

and develop both a philosophy and processes which facilitate transdisciplinary research 

and development.