Two-component supramolecular hydrogel for controlled drug release

Article abstract of DOI:10.1039/d0cc03962d

A hybrid gel has been developed by combining two supramolecular gelators. Each gelator endows the hybrid gel with its own characteristics. One gelator enables pH-mediated controlled release of the active pharmaceutical ingredient naproxen, while the other new gelator enhances mechanical stability. Self-assembly thus gives multi-functional gels with potential applications.

Full text of DOI:10.1039/d0cc03962d

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Two-component supramolecular hydrogel
for controlled drug release†
Cite this:DOI: 10.1039/d0cc03962d
Anna K. Patterson and David K. Smith
*
Received 5th June 2020,
Accepted 5th August 2020
DOI: 10.1039/d0cc03962d
rsc.li/chemcomm
A hybrid gel has been developed by combining two supramolecular leading to ‘self-sorted’ fibres.9 Alternatively, each gelator may
gelators. Each gelator endows the hybrid gel with its own char- preferentially interact with the other gelator, and fibrils will
acteristics. One gelator enables pH-mediated controlled release of assemble with alternating gelators.10 If the gelators have no
the active pharmaceutical ingredient naproxen, while the other new strong preference for either interaction, then random mixing
gelator enhances mechanical stability. Self-assembly thus gives can occur.11 Finally, one gelator may disrupt the assembly of
multi-functional gels with potential applications.
the other through non-productive interactions, causing loss of
gelation. Self-sorting gelators are desirable because orthogonal
Supramolecular hydrogels, soft materials typically comprising networks can be exploited to control the properties of the gel
ca. 99% water immobilised by a network of entangled fibres, (Fig. 1).12
are of increasing interest in biomedical applications.1 The
In recent years, we have investigated hydrogels based on
fibres self-assemble into a ‘solid-like’ network from a low- the industrially-relevant 1,3:2,4-dibenzylidenesorbitol (DBS)
molecular-weight gelator (LMWG).2 Self-assembled hydrogels framework.13 The parent molecule (DBS) and simple deriva-
have many attractive properties, particularly their responsive tives, have long been exploited for thickening properties, but
nature and generally high biocompatibility.3 There is also the were not previously capable of forming hydrogels as they
potential to program desired functionality into the bulk gel, are too insoluble in water. By modifying the aromatic
through simple molecular-scale modifications of the LMWG.4 ‘wings’ of DBS, we developed hydrogelators.14 One of these is
This has led to increasing interest in their use in biomedical DBS-CONHNH2, with hydrophilic acyl hydrazide groups.15 It
settings, including drug delivery.5 However, supramolecular forms a hydrogel using a simple heat/cool cycle, and is stable
gels often have low mechanical strength, making them difficult across a range of pH values. Hydrogels formed from DBS-
to manipulate.
CONHNH2 have been used for environmental remediation,15
One way of enhancing the mechanical strength of supra- drug delivery,16 and cell culture.17 Controlled release of active
molecular hydrogels is to combine them with polymer gelators, pharmaceutical ingredients (APIs) like naproxen (NPX) can be
which are generally more mechanically robust, albeit less achieved as a result of interactions between the DBS-CONHNH2
responsive to external stimuli.6 Increasingly, hybrid gels com- fibres and the API carboxylic acid. However, the gels are
prising two or more supramolecular gelators are also being mechanically weak and difficult to manipulate. We have pre-
studied, with the hope that the properties of the overall gel can viously combined DBS-CONHNH2 with polymer gelators to
be modified by tuning the ratio of the components, although
applications of this approach remain rare.7 Combining multi-
ple gelators is potentially a simple way of accessing gels with
different properties.8 When two LMWGs are combined, self-
assembly can occur in different ways giving different types
of self-organised material. The individual gelators may pre-
ferentially interact with other gelators of the same type,
Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK.
E-mail: david.smith@york.ac.uk
Fig. 1 The structures of hydrogelators DBS-CONHNH2 and MBS-CO2Me
† Electronic supplementary information (ESI) available: Materials and methods,
investigated in this paper.
characterisation and assay data. See DOI: 10.1039/d0cc03962d
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Scheme 1 The reaction of D-sorbitol and 4-methylcarboxaldehyde to
give MBS-CO2Me and DBS-CO2Me.
Fig. 2 Left: TEM image of an MBS-CO2Me hydrogel; scale bar 100 nm.
Right: SEM image of an MBS-CO2Me hydrogel; scale bar 1 mm.
enhance rheological performance.18 Here, we wanted to com-
bine DBS-CONHNH2 with a LMWG to produce a new tuneable,
responsive gel.
In the hunt for suitable LMWGs to combine with DBS-
CONHNH2, we decided to develop a new gelator, having noticed
environment (Fig. S10, ESI†). This CD band was temperature-
dependent, proving the chiral environment is generated by self-
assembly. The most significant change in ellipticity was at
45–55 1C, supportive of a gel–sol transition at this temperature.
Simple tube inversion methodology confirmed MBS-CO2Me
hydrogels indeed had Tgel values at ca. 55 1C, significantly lower
than DBS-CONHNH2 (ca. 80 1C), despite the higher concen-
tration (Table S1, ESI†). We reason the greater hydrophilicity of
MBS-CO2Me makes it more temperature-sensitive, i.e., it dis-
solves more easily on heating. When the resulting hot solutions
were left to stand overnight, gels re-formed. However, rather
than being transparent, they were white, with some precipitate,
indicative of greater aggregation and less controlled assembly.
When Tgel values for these re-formed gels were determined,
those at lower concentrations had lower Tgel values, while there
was little impact on the Tgel values at higher concentrations.
These gels at higher loadings also re-formed opaque gels for a
second time. Rheology indicated a slight increase in G0 values.
MBS-CO2Me gels were then tested for their ability to encap-
sulate and release the API naproxen (NPX, Fig. 3). This was
achieved by mixing the API and gelator as solids in advance of
the heat/cool cycle. Hydrogels were still formed (Table S2 and
Fig. S8, ESI†), but when API release was investigated, the MBS-
CO2Me hydrogel broke-down rapidly (within 3 h) once the
supernatant was placed on top of the gel (Fig. 3). This occurred
in the presence of any supernatant, including water and a range
of buffers. This was not observed with DBS-CONHNH2, and was
initially surprising given the MBS-CO2Me gel is more robust.
We reason that the higher solubility of MBS-CO2Me makes
the gel less stable to the presence of excess aqueous medium.
interesting properties of
a by-product from its synthesis
(Scheme 1). DBS-CO2Me is made by condensation of D-sorbitol
and 4-methylcarboxy benzaldehyde, giving mono-, di- and tri-
substituted sorbitol derivatives (MBS, DBS and TBS derivatives).
This reaction can be optimised for either DBS (2 : 1 aldehyde :
sorbitol) or MBS product (1 : 4 aldehyde : sorbitol) (for full method
and characterisation see ESI,† Fig. S1 and S2). We noticed
MBS-CO2Me formed effective gels in water. The ability of simpler
MBS derivatives to form hydrogels is known in the literature.19
DBS-CO2Me does not form hydrogels because it is insufficiently
soluble, but for MBS-CO2Me, the lack of one aromatic ring, and
the presence of two additional OH groups increases hydrophili-
city, making it sufficiently water-soluble. The effect of solubility on
gelation is increasingly understood.20
We investigated the properties of this new hydrogelator. Gela-
tion was achieved via a heat/cool cycle, giving transparent gels
(Fig. S4, ESI†). The minimum gelation concentration (MGC) was
0.75% wt/vol, with lower loadings giving solutions. SEM and TEM
indicated a fibrous network. TEM shows relatively homogeneous
helical fibres, with a diameter of ca. 8 nm (Fig. 2, left). Drying gels
can cause morphological change21 – in an attempt to limit this, we
cryo-dried samples for SEM. The small nanofibres and relative
lack of aggregation are consistent with the observation that gels
formed by MBS-CO2Me in water are transparent. This is similar to
gels formed by DBS-CONHNH2, which also have nanofibres ca.
10 nm in diameter.15
Rheology was carried out to explore the mechanical proper-
ties of the MBS-CO2Me hydrogel. At a loading of 0.85% wt/vol,
G0 4 G00 in the linear viscoelastic region (LVR), with G0 being ca.
2800 Pa (Fig. S6a, ESI†). This is a significantly stiffer gel than
that formed by DBS-CONHNH2 (G0 = 1200 Pa). The two gels had
similar elasticity, with G0/G00 crossover points of ca. 12%.
Interestingly, MBS-CO2Me was much more robust and easier
to physically manipulate than DBS-CONHNH2, with gel discs
being easily transferred for rheology, with no breakage.
Variable temperature (VT) circular dichroism (CD) spectro-
scopy of MBS-CO2Me had a maximum ellipticity at ca. 280 nm,
showing the aromatic ring of MBS-CO2Me experiences a chiral
Fig. 3 Release of naproxen into pH 7 buffer from MBS-CO2Me hydrogels
showing rapid release driven by erosion of the gel.
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NPX has a maximum absorbance at 329 nm, allowing API internal standard. This allows determination of the gelator
release to be quantified via UV-vis spectroscopy. When incubated assembled in the ‘solid-like’ network, because mobile gelator
1
at 37 1C with pH 7 buffer, around 85% of NPX was released in the ‘liquid-like’ phase is visible in the H NMR, and can be
within 3 h, roughly the point at which the gel broke down quantified by relative integration. At room temperature, neither
completely – we thus propose an ‘erosion’ mode of API release gelator was observed in the spectrum, indicating both are
for MBS-CO2Me.
assembled into ‘solid-like’ networks. As temperature increased,
We combined MBS-CO2Me and DBS-CONHNH2, hoping the signals for MBS-CO2Me became visible first, from around
their different properties would be synergistic. Our goal was a 40 1C with around 90% being free at 65 1C. This reflects the
hybrid gel with mechanical properties enhanced by MBS- higher solubility of MBS-CO2Me and its lower Tgel value (see
CO2Me, but maintaining the controlled release and water above). The signal corresponding to mobile DBS-CONHNH2
stability of DBS-CONHNH2. Initially, gelation was optimised. first appears at around 55 1C, and 90% of the gelator is free
DBS-CONHNH2 and MBS-CO2Me hydrogels have very different at 85 1C, in-line with its higher Tgel value. A small amount of the
MGCs (0.20% and 0.75% wt/vol respectively). Varying concen- DBS-CONHNH2 disassembles at ca. 55 1C before MBS-CO2Me
trations were explored, with the gelators mixed as solids. The disassembly is complete. It is therefore possible that self-
total MGC for the hybrid gel was 0.3% wt/vol (DBS-CONHNH2 at sorting in this system is not perfect, but nonetheless, the
0.2% wt/vol, and MBS-CO2Me at 0.1% wt/vol). Hybrid hydrogels networks do still largely disassemble in a sequential way and
formed even with both gelators below their individual MGCs thus with a good degree of self-sorting.
(DBSCONHNH2 at 0.16% wt/vol and MBS-CO2Me at 0.2% wt/vol).
It was hoped the stiffer MBS-CO2Me would reinforce the
Hydrogels with a range of LMWG concentrations were pre- more interactive, but mechanically weak, DBS-CONHNH2, giv-
pared, and their thermal properties investigated. Given that ing a gel that was easier to manipulate. Hybrid hydrogels with
MBS-CO2Me hydrogels have lower Tgel values (ca. 55 1C) than varying loadings of MBS-CO2Me (‘high’: 0.80% wt/vol, ‘low’:
DBS-CONHNH2 hydrogels (ca. 80 1C), it was predicted that 0.10% wt/vol) and DBS-CONHNH2 (‘high’: 0.28% wt/vol, ‘low’:
increasing the proportion of MBS-CO2Me in the hybrid gel 0.24% wt/vol) were tested by rheology (Table 1 and Fig. S6, S7,
may lower Tgel, providing access to a range of Tgel values. This ESI†). Pleasingly, with higher proportions of MBS-CO2Me, the
was indeed the case, with Tgel values from 27 1C to 4100 1C hybrid gel was stiffer than either DBS-CONHNH2 or MBS-
being obtained (Tables S3 and S4, ESI†). In general, a greater CO2Me alone (G0 = 9600 Pa). At higher MBS-CO2Me concentra-
proportion of MBS-CO2Me gave a lower Tgel, and a greater tions, increased loading of DBS-CONHNH2 leads to a slightly
proportion of DBS-CONHNH2 gave a higher Tgel. Total gelator weaker gel, perhaps as a result of the more extensive weaker
loading was also important – more gelator gave higher Tgel values. DBS-CONHNH2 network being more dominant. However, at the
SEM and TEM indicated nanofibres of ca. 10 nm diameter lowest concentrations of MBS-CO2Me, where the concentration
(Fig. S4 and S5, ESI†). It was not possible to differentiate of MBS-CO2Me here is below the MGC, the gels are slightly
nanofibres for the individual gelators, as they are similar in weaker than DBS-CONHNH2 alone, suggesting a small amount
morphology, but it was clear self-assembly in the mixed system of MBS-CO2Me network can disrupt the DBS-CONHNH2 net-
was not disrupted, nor did different types of assembly arise as a work. Therefore, there is scope to tune the rheological proper-
result of mixing.
ties of the hybrid hydrogel, to give properties suitable for the
1
VT H NMR studies investigated gelator immobilisation in desired application.
the hybrid gel, and understand how the LMWGs respond to an
Given DBS-CONHNH2 can achieve pH-mediated naproxen
increase in temperature (Fig. 4). Hybrid gels (0.80% wt/vol release,18 we wanted to determine if this ability was retained by
MBS-CO2Me and 0.22% wt/vol DBS-CONHNH2) were prepared the hybrid gel, and thus investigated encapsulation and release of
in an NMR tube using D2O : H2O (50 : 50, D2O alone leads to NPX. Stable gels were still formed in the presence of NPX, and the
reduced solubility of MBS-CO2Me), with DMSO (2 ml) as an addition did not have a significant impact on the rheological
properties of the hydrogels – there was perhaps a slight stiffening
reflected by a small increase in G0 (Fig. S8 and S9, ESI†). We
studied the interaction of the hybrid gels with NPX by 1H NMR. A
known mass of each gelator and NPX, were dissolved in D2O:H2O
Table 1 G0 and G00 values, from the LVER, for MBS-CO2Me (high – 0.80%
wt/vol, low – 0.10% wt/vol) and DBS-CONHNH2 (‘high’ – 0.28% wt/vol,
‘low’ – 0.24% wt/vol) gels
Gelator
G0/Pa
G00/Pa
DBS-CONHNH2
MBS-CO2Me
High MBS, low DBS
High MBS, high DBS
Low MBS, low DBS
Low MBS, high DBS
1200
2800
9600
6300
300
60
550
700
900
25
Fig. 4 The breakdown of the hybrid hydrogel on increasing temperature,
with free gelator monitored by 1H NMR spectroscopy. The loading of MBS-
CO2Me is 0.80% wt/vol, and of DBS-CONHNH2 is 0.22% wt/vol.
500
30
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well as investigating such gels in wider biomedical applications
like tissue engineering or wound healing.
Conflicts of interest
There are no conflicts to declare.
Notes and references
1 (a) B. Hu, C. Owh, P. L. Chee, W. R. Leow, X. Liu, Y.-L. Wu, P. Guo,
X. J. Loh and X. Chen, Chem. Soc. Rev., 2018, 47, 6917–6929;
(b) L. Saunders and P. X. Ma, Macromol. Biosci., 2019, 19, 1800313.
2 (a) R. G. Weiss, J. Am. Chem. Soc., 2014, 136, 7519–7530;
(b) E. R. Draper and D. J. Adams, Chem, 2017, 3, 390–410.
Fig. 5 Release of NPX, from hybrid hydrogels based on DBS-CONHNH2
(0.20% wt/vol) and MBS-CO2Me (0.70% wt/vol) at varying pH values.
3 X. Du, J. Zhou, J. Shi and B. Xu, Chem. Rev., 2015, 115, 13165–13307.
4 Molecular Gels: Structure and Dynamics, ed. R. G. Weiss, Royal Society
of Chemistry, Cambridge, 2018.
5 (a) K. J. Skilling, F. Citossi, T. D. Bradshaw, M. Ashford, B. Kellam
and M. Marlow, Soft Matter, 2014, 10, 237–256; (b) A. Vashist,
A. Kaushik, K. Alexis, R. D. Jayant, V. Sagar, A. Vashist and
M. Nair, Curr. Pharm. Des., 2017, 23, 3595–3602; (c) J. Mayr,
(50 :50), and the hydrogel formed in an NMR tube. An internal
standard (DMSO) was added to allow quantification of any
‘mobile’ NPX. These studies indicated that, like DBS-CONHNH2
gels, over 90% of the NPX was not visible in the NMR spectrum,
and is therefore likely bound to the gel network as a result of non-
covalent interactions between the acid on NPX and the hydrazide.
The interactive properties of DBS-CONHNH2 thus appear to
translate into the mechanically-robust hybrid gel.
The release of NPX from the hybrid gels, into buffers with
different pH values, was then investigated by monitoring the
UV-vis absorbance at 329 nm. Pleasingly, the supramolecular
hybrid gel showed pH-dependent release (Fig. 5). The NPX
release profile at pH 7 is similar to that observed previously
for DBS-CONHNH2,16 with rapid initial release, and total
release of 85–95%. In contrast, release at pH 5.5 is slower and
much less favoured, reaching only ca. 35% in the first 3 h, and a
maximum of just 55%. This is a result of the interactions
´
´
´
C. Saldıas and D. Dıaz Dıaz, Chem. Soc. Rev., 2018, 47, 1484–1515.
6 D. J. Cornwell and D. K. Smith, Mater. Horiz., 2015, 2, 279–293.
7 (a) L. E. Buerkle and S. J. Rowan, Chem. Soc. Rev., 2012, 41, 6089–6102;
(b) J. Raeburn and D. J. Adams, Chem. Commun., 2015, 51, 5170–5180.
8 E. R. Draper and D. J. Adams, Chem. Soc. Rev., 2018, 47, 3395–3405.
9 (a) J. R. Moffat and D. K. Smith, Chem. Commun., 2009, 316–318;
(b) E. R. Draper, E. G. B. Eden, T. O. McDonald and D. J. Adams, Nat.
´
Chem., 2015, 7, 848–852; (c) N. Singh, K. Zhang, C. A. Angulo-Pachon,
E. Mendes, J. H. van Esch and B. Escuder, Chem. Sci., 2016, 7, 5568–5572;
(d) S. Onogi, H. Shigemitsu, T. Yoshii, T. Tanida, M. Ikeda, R. Kubota and
I. Hamachi, Nat. Chem., 2016, 8, 743–752; (e) E. R. Draper, B. Dietrich and
D. J. Adams, Chem. Commun., 2017, 53, 1864–1867; ( f ) R. Kubota, S. Liu,
H. Shigemitsu, K. Nakamura, W. Tanaka, M. Ikeda and I. Hamachi,
Bioconjugate Chem., 2018, 29, 2058–2067; (g) C. C. Piras and D. K. Smith,
Chem. – Eur. J., 2019, 25, 11318–11326; (h) B. O. Okesola, Y. Wu,
B. Derkus, S. Gani, D. Wu, D. Knani, D. K. Smith, D. J. Adams and
A. Mata, Chem. Mater., 2019, 31, 7883–7897.
between DBS-CONHNH2 fibres and NPX, which occur when 10 (a) M. Tena-Solsona, S. Alonso-de Castro, J. F. Miravet and
NPX is protonated. The pKa of NPX is 4.15,22 and at pH 5.5 a
B. Escuder, J. Mater. Chem. B, 2014, 2, 6192–6197; (b) G.-F. Liu,
W. Ji, W.-L. Wang and C.-L. Feng, ACS Appl. Mater. Interfaces, 2015,
7, 301–307.
greater proportion of NPX will be protonated than at higher pH
values, and thus able to interact with gel nanofibres, limiting 11 (a) D. M. Ryan, T. M. Doran and B. L. Nilsson, Chem. Commun., 2011,
˜
47, 475–477; (b) H. A. M. Ardona, E. R. Draper, F. Citossi,
release. This controlled release is very different to the erosion
release observed when using MBS-CO2Me alone, and is attrac-
M. Wallace, L. C. Serpell, D. J. Adams and J. D. Tovar, J. Am. Chem.
Soc., 2017, 25, 8685–8692.
tive for oral delivery. It has the potential to reduce uptake in the 12 C. Colquhoun, E. R. Draper, E. G. B. Eden, B. N. Cattoz, K. L. Morris,
L. Chen, T. O. McDonald, A. E. Terry, P. C. Griffiths, L. C. Serpell and
acidic stomach, facilitating drug protection, and may assist
D. J. Adams, Nanoscale, 2014, 6, 13719–13725.
controlled release in the small intestine. This is important for
13 B. O. Okesola, V. M. P. Vieira, D. J. Cornwell, N. K. Whitelaw and
NSAIDs such as NPX, which have known side effects in the
upper gastrointestinal tract23 Changing LMWG concentrations
had little effect on the release process (Fig. S11 and S12, ESI†).
In summary, a two-component supramolecular hydrogel,
formed from two gelators (DBS-CONHNH2 and novel MBS-
CO2M), has been used to encapsulate and release NPX. NMR
studies indicate that the gel has sequentially assembled net-
works. Each gelator has its own unique characteristics pro-
grammed into the performance of the gel. Specifically, the
hybrid gel retains the interactive nature of DBS-CONHNH2,
facilitating controlled drug release, while having improved
mechanical strength as a result of MBS-CO2Me. NPX release
is pH-mediated, lower pH leading to less release, with potential
for controlled release in the small intestine. Further work will
focus on tuning multi-component hydrogels to give different
D. K. Smith, Soft Matter, 2015, 11, 4768–4787.
14 D. K. Smith, Chem. Commun., 2018, 54, 4743–4760.
15 B. O. Okesola and D. K. Smith, Chem. Commun., 2013, 49, 11164–11166.
16 E. J. Howe, B. O. Okesola and D. K. Smith, Chem. Commun., 2015, 51,
7451–7454.
17 V. M. P. Vieira, A. C. Lima, M. de Jong and D. K. Smith, Chem. – Eur.
J., 2018, 24, 15112–15118.
18 (a) P. R. A. Chivers and D. K. Smith, Chem. Sci., 2017, 8, 7218–7227;
(b) C. C. Piras, P. Slavik and D. K. Smith, Angew. Chem., Int. Ed.,
2020, 59, 853–859; (c) P. R. A. Chivers, J. A. Kelly, M. J. S. Hill and
D. K. Smith, React. Chem. Eng., 2020, 5, 1112–1117.
19 (a) J. Song, H. Sun, S. Sun and R. Feng, Trans. Tianjin Univ., 2013, 19,
319–325; (b) K. Fan, H. Kong, X. Yang and J. Song, RSC Adv., 2016, 6,
80934–80938; (c) G. C. Dizon, G. Atkinson, S. P. Argent, L. T. Santu
and D. B. Amabilino, Soft Matter, 2020, 16, 4640–4654.
20 Y. Lan, M. G. Corradini, R. G. Weiss, S. R. Raghavan and M. A. Rogers,
Chem. Soc. Rev., 2015, 44, 6035–6058.
21 D. J. Adams, Gels, 2018, 4, 32–37.
22 F. Barbato, M. I. La Rotonda and F. Quaglia, J. Pharm. Sci., 1997, 86,
225–229.
release profiles, enabling other drug delivery applications, as 23 N. Bhala and J. Emberson, et al., Lancet, 2013, 382, 769–779.
Chem. Commun.
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