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Mesoporous Nanotubes As Biomaterials Mesoporous Biomater. 2015; 2:33–48 Review Article Open Access Jeffery L. Coffer* Mesoporous nanotubes as biomaterials DOI 10.1515/mesbi-2015-0005 studies (within five years or less, where possible) ofse- Received October 1, 2015; accepted November 23, 2015 lected nanotubes that retain the desired porous dimen- sion in the mesoporous range relevant specifically to ei- Abstract: This review provides an overview of selected re- ther biosensing or therapeutic (e.g. principally drug deliv- cent research efforts that employ the use of mesoporous ery) applications. The discussion presented herein is or- nanotubes in a biomaterial context, e.g. principally as a ganized according to composition, sub-classified within therapeutic or biosensing platform. We focus on the com- each by fabrication, fundamental properties (biocompat- positions of alumina, boron nitride, silica, silicon, tita- ibility/biodegradability), and application. Highlights of a nia, and zinc oxide, along with selected accounts involv- given material’s desirable properties for a particular bio- ing single-walled carbon nanotubes. Where known, atten- relevant application are identified where possible, along tion is directed toward the biodegradability and biocom- with remaining challenges for clinical implementation. patibility of a given nanotube type, its tunability of size and surface chemistry, and relevance of these parameters to its function as a biomaterial. Keywords: nanotube, drug delivery, biosensor, alumina, 2 Alumina Nanotubes boron nitride, silica, silicon, titania We begin with a brief but focused discussion on nan- PACS: 68; 81 otubes of aluminum oxide (alumina, Al2O3). It is appro- priate to begin with this composition, given the fact that nanoporous alumina membranes are used in a widespread 1 Introduction manner as templates for the attempted formation of other nanotube types (titania, silicon) via infiltration, anneal- ing, and etching. While a diverse range of mesoporous morphologies are Interestingly, prior investigations have established the available for investigation, the appealing simplicity of a utility of nanoporous alumina membranes to possess one-dimensional hollow nanotube construct, with associ- improved osteoblast adhesion and proliferation (relative ated high surface area along with interior/exterior curved to amorphous alumina) for orthopedic-relevant applica- interfaces, provides unique opportunities in the observa- tions [3] and also support viability and functionality of tion of new physical properties and chemical reactivity rel- encapsulated beta cells for the ultimate use in immuno- evant to a range of disciplines. While tubular crystals of isolated devices [4]. Nevertheless, in order to legitimately naturally-occurring minerals have been known for some probe size dependent effects, methods must be employed time [1], single walled carbon nanotubes (SWCTs) have gar- to separate membrane assemblies into individual nan- nered a lion’s share of attention in the last decade [2]. The otubes. These are highlighted below. range of commonly-investigated nanotube materials has subsequently been extended to a rather lengthy list, as out- lined in Table 1. Much of the initial focus of investigations of these nan- 2.1 Alumina Nanotube Fabrication otubes has centered on their relevance to energy-related The base alumina nanoporous membranes are prepared areas such as battery technologies and photovoltaics. by anodization in dilute H PO /H SO [3, 4]. Some pro- Given the charge of the journal Mesoporous Biomateri- 3 4 2 4 cedures add a second anodization step under pulsed gal- als, this specific review entails highlighting known recent vanostatic conditions to improve pore structure. Any re- maining aluminum substrate can be removed by wet chemical etching in a mixture of dilute CuCl2 and HCl. *Corresponding Author: Jeffery L. Coffer: Department of Chem- Free-standing alumina nanotubes are obtained by immer- istry, Texas Christian University, Fort Worth, TX 76129, E-mail: sion into the same acid solution followed by ultrasonic [email protected] © 2015 J. L. Coffer, published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. 34 Ë J. L. Coffer Table 1: Known mesoporous nanotubes investigated for biomaterial applications Nanotube Type Focus (+) Advantages / (−) disadvantages References Al2O3 Drug delivery (+) Ease of preparation / (−) brittleness [3–6] BN Drug delivery, tissue (+/−) Chemical inertness (both a [7–22] engineering, radiation positive & negative effect, depending therapies on application) / (−) length control SiO2 Drug delivery, tissue (+) Diverse range of preparative routes / [23–36] engineering, biosensing (−) Agglomeration (depending on surface chemistry) Si Drug delivery, tissue (+) Bioresorbability; semiconductor / [37–46] engineering (−) multistep fabrication & low yield SWCNT Drug delivery, tissue (+) Ease of large scale preparation / [47–75] engineering, biosensing (−) toxicity in the absence of surface modification TiO2 Drug delivery, tissue (+) Large scale fabrication and ease of [76–86] engineering, biosensing osteointegration (in orthopedics) ZnO Drug delivery, biosensing (+) Cost, ease of fabrication / [87–92] (−) challenges with drug loading ica and titania, for example. Alumina is typically consid- ered as a bioinert material; nevertheless, its ability to be carefully constructed in nanotube form under controlled fabrication conditions does make it an excellent candidate for an evaluation of the toxicity of high aspect ratio nano- materials in general. A detailed study by Wang and co- workers evaluated alumina nanotubes with aspect ratios ranging from 7.8 to 63.3, and multiple cytotoxicity assays (beyond simple cell viability and morphology) were con- ducted with RAW 264.7 mouse macrophage cells and MDA- MB 231-TXSA human breast cancer cells [5]. Not surpris- ingly, the resultant toxicity patterns were cell-type depen- dent and strongly related with nanotube dose, length of time, and very importantly, nanotube aspect ratio. Long ratio nanotubes triggered enhanced cell death, morpho- Figure 1: Alumina nanotubes: (a) generation of discrete alumina NTs logical changes, tumor necrosis factor α (TNF-α) release, from alumina nanoporous membranes [6]; (b) TEM image of iso- etc. than short nanotubes. The toxic aspect ratio ‘window’ lated alumina NTs with widths on the order of 100 nm and lengths of these nanotubes was determined to be 7.8, reported to be ~700 nm [6]; (c) TEM image of alumina NTs internalized by RAW relatively shorter than that of other high aspect ratio nano- 264.7 macrophage cells [6]. materials [5]. treatment [5]. Typical inner diameters for the nanotubes prepared by this route are on the order of 30 nm, falling 2.3 Alumina nanotubes - therapeutic within the mesoporous regime [5]. relevance Drug Delivery 2.2 Alumina Nanotube Biocompatibility Given the above results, other research groups have eval- Mesoporous alumina nanotubes have not enjoyed the uated the ability of a non-toxic alumina nanotube mate- widespread investigation as witnessed for the case of sil- rial to host and release a tumor necrosis factor-relevant due to the induction of apoptosis (as monitored by changes in Caspase-3 activity). Importantly, and encouragingly, these high loading capacities facilitated cancer cell death in relatively short times. 6 Given cost considerations and ease of fabrication, these results for this relatively under-explored composition of nanotubes will likely stimulate further work as a consequence. III. Boron Nitride Nanotubes In nanotube form, boron nitride forms a unique contrast to its isoelectronic analog, carbon (CNTs). Its chemical inertness, specifically resistance to oxidation, along with mechanical strength and intrinsic radiation adsorption properties, suggests novel 7 utility in selected biomedical applicationsMesoporous. nanotubes as biomaterials Ë 35 apoptosis-inducing ligand, Apo2L/TRAIL [6]. Experiments with these nanotubes using a combination of transmission (a) electron microscopy (TEM) and fluorescence microscopy demonstrated significant uptake of alumina nanotubes by the same RAW 264.7 mouse macrophage cells and MDA-MB 231-TXSA human breast cancer cells noted earlier. These alumina nanotubes could load more than 100 micrograms of the Apo2L/TRAIL ligand per mg of nanotube, and in studies with MDA-MB 231-TXSA human breast cancer cells, (b) (c) an associated significant reduction in cell viability is ob- served due to the induction of apoptosis (as monitored by changes in Caspase-3 activity). Importantly, and encourag- ingly, these high loading capacities facilitated cancer cell death in relatively short times [6]. Given cost considerations and ease of fabrication, Figure 2: Boron Nitride (BN) nanotubes: (a) schematic illustra- tion of glycinetemplated biopolymer coating process for these these results for this relatively under-exploredIII. A. composi-Boron Nitride Nanotube Fabrication nanoubes [15]; (b) Internalization of functionalized BN NTs by a tion of nanotubes will likely stimulate further work as a mesenchymal stem cell [17]; (c) Plasma-assisted functionalization of Typical fabrication techniques for boron nitride (BN NTs) nanotubes have been consequence. BN NTs with Au nanoparticles [16]. inspired by methods established for the growth of CNTs, mainly via arc-discharge and can provide meso-scale inner widths in the range of 20– 3 Boron Nitride Nanotubes 40 nm [15]. Figure 2. Boron Nitride (BN) nanotubes: (a) schematic illustration
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