Hydrogels have widely been used as scaffolds due to their high water content, biocompatibility, and biodegradability resembling the extracellular matrix for cells in the human body. Scaffolds, cells, and soluble factors are essential elements to fabricate biological tissues and organs. However, commonly used hydrogels as scaffold materials often have low mechanical properties and electrical conductivity, which limit their performance to fabricate functional tissues of electroactive cells, such as neural, skeletal muscle, or cardiac cells. One simple way to increase mechanical properties and electrical conductivity of hydrogels is the use of carbon-based nanomaterials (e.g., carbon nanotubes (CNTs) and graphene) as supplementary materials in hydrogels. Such carbon-based nanomaterials exhibit unique mechanical properties and electrical conductivity making them excellent candidates to provide hybrid hydrogels having high strength and conductivity. Of course, one can increase mechanical and electrical properties of hydrogels by adding the concentration of carbon-based nanomaterials in hybrid hydrogels; however, this method has adverse effects on porosity, degradability, or biological response of hydrogels. Therefore, there is still a requirement to precisely tune electrical and mechanical properties of hybrid hydrogels-carbon based nanomaterials.
Recently, a combined team of researchers at Tohoku University and Harvard Medical School came up with the idea of using electric fields to manipulate CNTs inside hydrogels [1]. They used dielectrophoresis (DEP) method in their work that provides a simple and rapid technique to arrange CNTs in hydrogels. DEP is a technique by which a particle in a medium is polarized in the presence of an electric field. Polarized particles can be attracted to high electric field regions (positive DEP) or can be distracted to low electric field regions (negative DEP). The CNTs were dielectrophoretically patterned inside gelatin methacrylate (GelMA) hydrogel (Figure 1) that is an inexpensive and semi-natural hydrogel and can be crosslinked by applying UV light. The latter property is crucial as to preserve dielectrophoretically aligned CNTs after switching off the applied current. The CNTs alignment using the DEP method was rapid and took less than one minute. Hybrid GelMA-aligned CNTs hydrogels showed a few orders of magnitude higher electrical conductivity compared with that of hybrid GelMA-randomly dispersed CNTs and pristine GelMA hydrogels (Figure 1). In addition, mechanical properties of hybrid GelMA-aligned CNTs hydrogels was improved compared with hybrid GelMA-randomly dispersed CNTs and pristine GelMA hydrogels. Interestingly, skeletal muscle cells cultured on hybrid GelMA-aligned CNTs hydrogels demonstrated high maturation and contractility upon applying electrical stimulation compared with muscle myofibers obtained on hybrid GelMA-randomly dispersed CNTs and pristine GelMA hydrogels.
DEP method provides this opportunity to selectively pattern CNTs within hydrogels and therefore to tune their electrical and mechanical properties. Hybrid hydrogel-dielectrophoretically aligned CNTs materials with tunable electrical and mechanical properties are highly desirable in real-time monitoring of cellular activities, in developing hybrid three-dimensional materials in bioelectronics, and as scaffolds to engineer biological tissues of electroactive cells, such as muscle or neural cells. In the perspective of tissue engineering, such engineered tissues can be used as implantable tissues in human body or can have other applications, such as in bio-robotics or as tissue models in drug screening and discovery. Electrical stimulation of such tissues is an efficient tool to control metabolism and physiological activity of tissues and in this regard hybrid hydrogel-dielectrophoretically aligned CNTs materials can significantly enhance the effect of electrical stimulation on fabricated tissues. It is hoped that DEP method could further employ to control electrical and mechanical properties of hydrogels to provide novel functional biomaterials for various tissue engineering and applications.
Figure 1. DEP was used to align CNTs within GelMA hydrogels in a facile and rapid manner. Aligned GelMA-CNTs hydrogels showed higher electrical properties compared with pristine and randomly distributed CNTs in GelMA hydrogels. The muscle cells cultured on these materials demonstrated higher maturation compared with cells cultured on pristine and randomly distributed CNTs in GelMA hydrogels (Reproduced with the permission from Reference [1]. Copyright 2013 Wiley-VCH.).
Author
Dr. Samad Ahadian is a research associate at WPI-Advanced Institute for Materials Research, Tohoku University, Japan.
Contact
ahadian@wpi-aimr.tohoku.ac.jp; samad_ahadian@yahoo.com.
Reference
[1] S. Ahadian et al., Advanced Materials, DOI: 10.1002/adma.201301300 (in press).