therapeutic drugs. Some therapeutic drugs (nucleic acid, etc.) are unstable under
physiological condition and could not be internalized by cell membrane. It is
necessary to deliver these drugs to achieve therapeutic efficacy. Bioimaging is
another important biomedical application to visualize structure of cells, tissues, or
organs. Self-assembly could be used as facile strategy to construct biomaterials with
multiple components and functions to achieve desired applications.
To render biomedical applications possible, it is often necessary to introduce
stimuli responsiveness to self-assembly design. Stimuli are generally divided into
two categories: (1) physiological stimuli, such as pH, reducing environment, and
reactive oxygen species (ROSs), and (2) external stimuli, including photo-irradiation
and temperature [7]. Physiological stimuli are existing in physiological or pathological tissues. External stimuli are locally applied to target tissues. Either stimuli types
are used to trigger responsiveness of self-assembly systems. For bioimaging applications, self-assembly systems are designed as probes to detect physiological or
pathological environment with certain stimuli. For drug delivery purposes, physiological or external stimuli are used to trigger the release of encapsulated cargo.
Macrocyclic hosts are important building blocks for the construction of stimuliresponsive self-assembly systems. By using host-guest interaction as supramolecular
“cross-linker,” it is possible to construct complex self-assembly systems with
variable morphology, which is crucial to achieve biomedical applications. Stimuli
may trigger the morphology conversion of self-assembly systems and release of
encapsulated cargo. Importantly, host-guest interaction is dynamic and reversible.
Macrocyclic host-based self-assembly systems may respond to biomarkers and
demonstrate stimuli responsiveness based on guest displacement.
Different types of macrocyclic hosts are used to construct stimuli-responsive selfassembly systems, including cyclodextrin (CD), cucurbituril (CB), calixarene (CA),
pillararene (PA), and crown ether. CD is generally considered to be biocompatible
compound for biomedical purpose. CD is also low cost and commercially available.
As a consequence, CD is a useful macrocyclic host for drug delivery and bioimaging
purposes, which may even be used for translational medicine. CB is known to be a
high-affinity and selective host for suitable guests in water [8]. CB is a powerful tool
to construct self-assembly systems in water. In recent years, PA has emerged as a
new type of macrocyclic host to construct functional materials [9]. The above types
of macrocyclic hosts are used to assemble nanoparticles, microparticles, and
hydrogels. The resulting biomaterials are used for drug delivery and bioimaging
applications. With the combination of stimuli responsiveness and host-guest interaction, sophisticated biomedical applications could be achieved.
23.2 Macrocyclic Host Molecules
Among various types of macrocyclic host molecules, CDs, CBs, CAs, and PAs are
the most widely used in biomedical applications. As shown in Fig. 1, macrocyclic
hosts are composed of different building blocks, which render them variable architectures. All the four types of macrocyclic hosts are composed of homologues with
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physiological condition and could not be internalized by cell membrane. It is
necessary to deliver these drugs to achieve therapeutic efficacy. Bioimaging is
another important biomedical application to visualize structure of cells, tissues, or
organs. Self-assembly could be used as facile strategy to construct biomaterials with
multiple components and functions to achieve desired applications.
To render biomedical applications possible, it is often necessary to introduce
stimuli responsiveness to self-assembly design. Stimuli are generally divided into
two categories: (1) physiological stimuli, such as pH, reducing environment, and
reactive oxygen species (ROSs), and (2) external stimuli, including photo-irradiation
and temperature [7]. Physiological stimuli are existing in physiological or pathological tissues. External stimuli are locally applied to target tissues. Either stimuli types
are used to trigger responsiveness of self-assembly systems. For bioimaging applications, self-assembly systems are designed as probes to detect physiological or
pathological environment with certain stimuli. For drug delivery purposes, physiological or external stimuli are used to trigger the release of encapsulated cargo.
Macrocyclic hosts are important building blocks for the construction of stimuliresponsive self-assembly systems. By using host-guest interaction as supramolecular
“cross-linker,” it is possible to construct complex self-assembly systems with
variable morphology, which is crucial to achieve biomedical applications. Stimuli
may trigger the morphology conversion of self-assembly systems and release of
encapsulated cargo. Importantly, host-guest interaction is dynamic and reversible.
Macrocyclic host-based self-assembly systems may respond to biomarkers and
demonstrate stimuli responsiveness based on guest displacement.
Different types of macrocyclic hosts are used to construct stimuli-responsive selfassembly systems, including cyclodextrin (CD), cucurbituril (CB), calixarene (CA),
pillararene (PA), and crown ether. CD is generally considered to be biocompatible
compound for biomedical purpose. CD is also low cost and commercially available.
As a consequence, CD is a useful macrocyclic host for drug delivery and bioimaging
purposes, which may even be used for translational medicine. CB is known to be a
high-affinity and selective host for suitable guests in water [8]. CB is a powerful tool
to construct self-assembly systems in water. In recent years, PA has emerged as a
new type of macrocyclic host to construct functional materials [9]. The above types
of macrocyclic hosts are used to assemble nanoparticles, microparticles, and
hydrogels. The resulting biomaterials are used for drug delivery and bioimaging
applications. With the combination of stimuli responsiveness and host-guest interaction, sophisticated biomedical applications could be achieved.
23.2 Macrocyclic Host Molecules
Among various types of macrocyclic host molecules, CDs, CBs, CAs, and PAs are
the most widely used in biomedical applications. As shown in Fig. 1, macrocyclic
hosts are composed of different building blocks, which render them variable architectures. All the four types of macrocyclic hosts are composed of homologues with
604
W. Mao and D. Ma