Lipid-coated hydrogel shapes as components of electrical circuits and mechanical devices

SUBJECT AREAS: MATERIALS SCIENCE MATERIALS CHEMISTRY SYNTHETIC BIOLOGY Lipid-coated hydrogel shapes as components of electrical circuits and mechanical devices K. Tanuj Sapra & Hagan Bayley BIOPHYSICS Department of Chemistry, University of Oxford, Oxford OX1 3TA, UK. Received 4 April 2012 Accepted 30 October 2012 Published 14 November 2012 Correspondence and requests for materials should be addressed to K.T.S. (tanuj.sapra@ chem.ox.ac.uk) Recently, two-dimensional networks of aqueous droplets separated by lipid bilayers, with engineered protein pores as functional elements, were used to construct millimeter-sized devices such as a light sensor, a battery, and half- and full-wave rectifiers. Here, for the first time, we show that hydrogel shapes, coated with lipid monolayers, can be used as building blocks for such networks, yielding scalable electrical circuits and mechanical devices. Examples include a mechanical switch, a rotor driven by a magnetic field and painted circuits, analogous to printed circuit boards, made with centimeter-length agarose wires. Bottom-up fabrication with lipid-coated hydrogel shapes is therefore a useful step towards the synthetic biology of functional devices including minimal tissues. T issues and organs are organized cellular assemblages capable of internal and external communication by means of chemical, electrical and mechanical signals1,2. An important undertaking of synthetic biologists is the realization of minimal cells (protocells)3 and minimal tissues (prototissues)4. Like their natural counterparts, synthetic minimal tissues should be compartmented, and able to communicate and support emergent properties. Communication might be through chemical (e.g., the movement of ions or small molecules) or physical (e.g., the detection of force or electrical potentials) means. Up to now, lipid vesicles have been the system of choice for the development of artificial cells5–7. However, the small size of these compartments limits their manipulation, including the ability to measure ionic currents through the bilayer envelopes. Systems based on droplet interface bilayers (DIB) can be more readily controlled8. A DIB is formed when two lipid monolayer-coated aqueous droplets in an oil are brought together. Several such droplets can be assembled to form a network, and when membrane proteins are included in the DIBs, functional systems are produced8. DIBs including those formed on hydrogels have been used to study the fundamental properties of membrane proteins9–14, and aqueous DIB networks have been used to construct devices12, including a light sensor8, a battery8, and half- and full-wave rectifiers15. Recently, droplet networks that function in aqueous media have been devised16. In a synthetic biology context, these networks can be regarded as minimal tissues4. However, aqueous droplet networks are delicate. Robust 3D systems that incorporate engineered membrane proteins for inter-compartment communication are required. Hydrogels are ideal scaffolds to realize such systems; they conduct ions, they can be molded, and many are biocompatible. Here, we present a simple approach in which shaped lipid-coated hydrogel objects are used to assemble networks both with and without bilayers between them. Both proved useful in devices. The hydrogel bilayer networks employ electrical and chemical signals for communication, and are capable of performing electrical and mechanical tasks. The modular hydrogel system expands the scope of DIB networks by enabling the construction of devices with enhanced structural complexity and functionality (Fig. 1), which has not been achieved thus far with aqueous droplets. Results Lipid monolayers self-assemble on various shaped objects made from a hydrogel and immersed in a lipid/oil mixture (Fig. 1a). As described in detail below, depending on the lipid concentration in the oil and the force applied to the objects, we have been able to (i) form bilayers between two millimeter-sized hydrogel shapes (with 1–5 mg mL21 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) in hexadecane) by controlling the distance between the objects with a micromanipulator (Fig. 1b); (ii) form stable patterned assemblies with bilayers between more than two hydrogel shapes without the need for precision control with a micromanipulator (10 mg SCIENTIFIC REPORTS | 2 : 848 | DOI: 10.1038/srep00848 1 www.nature.com/scientificreports Figure 1 | Lipid-coated hydrogel networks. (a) Hydrogel objects with various shapes immersed in a lipid/oil solution become coated with a lipid monolayer. (b) When two lipid-coated shapes are brought together with a micromanipulator, a bilayer can be formed at the interface. (c) At high lipid concentrations, a stable bilayer network is formed. (d) When the shapes are pressed against each other, the bilayers rupture at the hydrogel interfaces, and a network coated with a single external lipid monolayer is obtained. The positions of the hydrogel shapes in both bilayer and no-bilayer networks can be rearranged to change the network topology. (e) Networks can be formed with and without bilayers between specific hydrogel objects. In such a network, a new bilayer can be created between two shapes (which did not originally have a bilayer) by pulling them apart and forming the bilayer by bringing them back together. Bilayer networks can be used to form functional electrical and mechanical devices. (f) For example, aHL pores can be used to carry an ionic current between two hydrogel shapes separated by an interface bilayer. (g) A hydrogel rotor is an example of a mechanical device. mL21 DPhPC) (Fig. 1c); (iii) self-assemble hydrogel shapes in arbitrary patterns without bilayers between the shapes (up to 5 mg mL21 DPhPC) and later rearrange the assemblies in patterns of specific design (Fig. 1d); and (iv) build hydrogel networks with a bilayer between some shapes and none between the others (5–10 mg mL21 DPhPC) (Fig. 1e). A high lipid concentration (.5 mg mL21 DPhPC) is required to stabilize a bilayer network. However, even at high lipid concentrations, bilayers between hydrogel objects can be ruptured and expelled by applying a force to the objects that results in pressure normal to the bilayer surface. SCIENTIFIC REPORTS | 2 : 848 | DOI: 10.1038/srep00848 Bilayer formation between hydrogel shapes. A lipid monolayer is formed on a hydrogel surface upon immersion in a lipid / oil mixture. Previously, an individual bilayer has been formed at the interface of an aqueous droplet and a flat hydrogel surface11,14,17. We supposed that two hydrogel shapes, encased in lipid monolayers, brought close to each other with a micromanipulator, would form a bilayer at the interface (Fig. 1b), which could be functionalized by the incorporation of transmembrane pores (Fig. 1f). To test this, initially, hydrogel spheres were brought into contact with other hydrogel shapes (Fig. 2a–d, Supplementary Fig. S1). When two 2 www.nature.co (...truncated)