Biomedical Engineering Reference
In-Depth Information
10.3.2.1
pH-Controlled Macromolecule Encapsulation
It has been found that hollow PAH/PSS capsules can open at lower pH values for macromolecule
permeation and close at higher pH values for encapsulation [30,120]. The loading process is simple
and only requires a few steps of washing. First, proteins and polyelectrolyte capsules are coincu-
bated at a lower pH value at which shells are open for easy penetration of macromolecules. Second,
after being mixed for a certain time, the mixture's pH is adjusted to a higher value for closing the
capsule. Finally, the capsules are washed a few times, usually through centrifugation at this higher
pH environment for removing free proteins.
CLSM is an excellent tool for monitoring capsules “open” and “close” when they are in micron
size. Observing the loading procedure of smaller capsules (
200 nm) requires other tools such
as AFM or TEM. The opening state of PAH/PSS capsules for a model drug FITC-dextran (MW:
75,000) was observed for pH values up to 6, and most capsules were closed from pH 8 upwards [30].
The surface structure of polyelectrolyte capsules is affected by pH values. Holes up to hundreds
of nanometers could be formed in capsule wall when pH was lower than 6.5, which obviously
facilitated protein (most in the range of 5-50 nm) penetration into shells [120]. In addition, it was
suggested that the opening of the capsule wall presumably occurs as a cooperative process and
appears like defect formation [30]. Possibly, changes of the polyelectrolyte charge upon pH varia-
tion are able to induce pore formation or loosen the polyelectrolyte network, thus enabling macro-
molecules to penetrate [30,131]. Further studies are necessary to fully understand the mechanism of
pH-induced permeability changes.
In addition to the capsule interior cavity, its membrane also serves as an important reservoir
for protein encapsulation. For example, α-chymotrypsin concentration into shells (inside capsule
<
+
capsule wall) was in the range of 40-50 mg/mL, which was much higher than the incubation protein
concentration of 3.75 mg/mL and even above the solubility limit of α-chymotrypsin (
10 mg/mL)
[120]. So the concentration gradient is not the only driving force for protein penetration into shells.
After loading, proteins are distributed into two different locations: one is the capsule membrane and
the other is inside capsules. The capsule membrane contains 90% loaded proteins because of the
high-affi nity to proteins and higher capacity and leads to the overall high-protein loading.
A method introduced by Lowry et al. [132] can be used to determine the protein concentra-
tion inside shells [120-122,124]. The principle behind the Lowry method of determining protein
concentrations lies in the reactivity of the peptide nitrogen[s] with the copper [II] ions under alkaline
conditions (pH 10
∼
10.5) and the subsequent reduction of the Folin-Ciocalteay phosphomolybdic-
phosphotungstic acid to heteropolymolybdenum blue by the copper-catalyzed oxidation of aromatic
acids [133]. The Lowry method is sensitive with the protein detection limit as low as 0.005 mg/mL
[134]. To avoid any turbidity caused by microcapsules, ultrasonication and purifi cation are neces-
sary before optical measurements.
∼
10.3.2.2
Switch On/Off Capsule Opening through External Magnetic Field
It is of special interest that drug carriers can be triggered to open and release contents upon appli-
cation of a harmless external magnetic fi eld. This enables one to control a burst release of drugs
inside microcapsules at the site of choice when necessary. It has been demonstrated that magnetic
nanoparticles and drug can be embedded inside alginate or liposome matrixes, and controlled drug
release was achieved when using a low-frequency external magnetic fi eld [135,136]. To apply this
methodology to polyelectrolyte capsules, it has to meet two conditions: (1) incorporation of mag-
netic nanoparticles into the shell wall and (2) movement of magnetic nanoparticles under external
magnetic fi eld.
In an earlier attempt, the Lvov group incorporated magnetite nanoparticles into polyelectrolyte
shell wall for enhanced bioreaction under external magnetic fi eld [137]. Later, they embedded ferro-
magnetic cobalt nanoparticles (
3 nm) (Co@Au nanoparticles) into (PSS/PAH)
n
polyelectrolyte cap-
sules for capsule wall permeability control and loading of macromolecules [119]. Dextran was used
∼
