# Perspectives in Liquid-Crystal-Aided Nanotechnology and Nanoscience
## 5 Biological Applications: Biosensors and Drug Delivery
### Drug Delivery  
As one of the key features providing biological systems with unique physical and chemical properties, the self-assembly of biological materials, such as amphiphilic lipids, has been the focus of an increasing number of studies. Lytotropic LC droplets with diameters in the range of 100 nm, usually referred to as cubosomes and hexosomes, are potential candidates for constructing novel matrices mimicking biological systems in the formation of new nanoparticle carriers for delivering drugs and nutrition. These self-assembled nanostructures are spontaneously formed by biocompatible amphiphilic lipids, such as phospholipids, monoplycerides, glycolipids and urea-based lipids, mainly in water, and contained in a thin shell of amphiphilic block co-polymer (Figure 19). Figure 18: (**a**) The orientations of 5 CB in the LC cells fabricated with DMOAP-coated glass slides (upper slides) and modified slides (**I–III**; **lower**): (**I**) (3-aminopropyl)trimethoxysilane (APS)/DMOAP-coated glass slide, (**II**) streptavidin alkaline phosphatase (Sv-ALP) modified glass slide, and (**III**) silver-deposited glass slide. (**b**) Optical images under crossed polarizers of LC cells with 5 CB sandwiched between a DMOAP-coated glass slide and a silver-deposited glass slide with complementary target DNA at concentrations of (**I**) 0, (**II**) 0.1, (**III**) 1, (**IV**) 10, (**V**) 100 pM, and (**VI**) two-base mismatched DNA at concentration of 100 pM. Reproduced with permission from [434]. Copyright (2010) John Wiley and Sons. (**c**) Sequential micrographs (**I–III**) and corresponding illustrations (**IV–VI**) of a LC film containing microdroplets before (**I,IV**) and after addition of cationic amphiphile DTAB to the overlying aqueous phase, at 0 (**II,V**) and 60 min (**III,VI**). (**d**) Release of microcargo from LCs by the heat of a human finger or by motile bacteria that generate interfacial shear stresses and interact with microcargo in a self-regulated manner. Reproduced with permission from [441]. Copyright (2018) Springer Nature. Unlike traditional liposomes, which can only load a hydrophilic drug cargo and release it to the target immediately once the lipid membrane is broken, these LC nanoparticles have cores with equal amounts of polar and non-polar regimes, and thus can carry either hydrophilic or hydrophobic cargo, or even both simultaneously. At the same time, the interior drugs are protected from chemical and enzymatic degradation during the passage through the digestive system to the site of release, leading to a sustained release over time [15]. Due to such a slow release, the drugs are inclined to accumulate as much as possible in sickness-related tissues or cells to generate a maximal effect and reduce the negative effects on human bodies such as the toxicity of the drugs, making these LC colloidal nanoparticles particularly interesting in medical treatments [443, 444]. Solid lipid nanoparticles (SLNs) have also attracted great attention as cosmetic and pharmaceutical formulations [445]. Due to their relatively small particle size but large surface area, SLNs possess very strong adhesive characteristics. This can induce film formation on the human skin, which may be helpful to keep the skin moist and even restore the previously damaged protective lipid film on the body through occlusion. Incorporation of active ingredients into a solid lipid matrix can provide protection against hydrolytic degradation, allowing drug release at a controlled rate. This implies either a transient or a continuous drug release, depending on the particle size and polymorphic transitions of the lipid matrix [446]. Continuous release is critical for active ingredients that may irritate the body if the concentration is too high or when a prolonged period of drug transport time is desired, whereas transient release will be helpful to improve the penetration of drugs. In addition to LC phases and solid lipids, a large variety of materials can be fabricated into drug-carrier nanoparticles.