A review and current state of autonomic self-

healing microcapsules-based dental resin

composites

K. Althaqafi, J. Satterthwaite, N. Silikas*

Khaled Abid Althaqafi, a, b Julian Satterthwaite, c Nikolaos Silikas, d*

a Faculty of Dentistry, Collage of Dental Medicine, University of Umm Al Qura, Makkah, Kingdom of Saudi Arabia.

b Division of Dentistry, School of Medical Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom.

c Division of Dentistry, School of Medical Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom.

d Biomaterials Science Research Group, Division of Dentistry, School of Medical Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom.

* Corresponding author at: Biomaterials Science Research Group, Division of Dentistry, School of Medical Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom.

E-mail address: Nikolaos.Silikas@manchester.ac.uk

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Abstract:

Figure 1 Graphical abstract illustrating the mechanism of microcapsules response to a crack in a photo-cured resin composite model

Objectives: This study systematically reviews the literature on self-healing microcapsule

technology and evaluates the biocompatibility of self-healing microcapsules and the efficiency of

crack repair within resin-based dental composites.

Methods: An electronic search was carried out using the following databases: MedLine

(PubMed), Embase, the Cochrane Library and Google Scholar. All titles and abstracts of the

articles and patents found were analysed and selected according to the eligibility criteria. Only

studies published in English were included; the outcomes sought for this review were dental

resin composites with self-healing potential. There were no restrictions on the type of self-

healing system involved in dental resin composites.

Results: The search yielded 10 studies and 2 patents involving self-healing approaches to dental

resin composites. According to the current literature on self-healing dental resin composites,

when a crack or damage occurs to the composite, microcapsules rupture, releasing the healing

agent to repair the crack with a self-healing performance ranging from 25% to 80% of the virgin

fracture toughness.

Significance: Self-healing strategies used with resin composite materials have, to date, been

bioinspired. So far, self-healing microcapsule systems within dental composites include poly

urea-formaldehyde (PUF) or silica microcapsules. The main healing agents used in PUF

microcapsules are DCPD monomer and TEGDMA-DHEPT, with other agents also explored.

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Silica microcapsules use water/polyacid as a healing agent. All self-healing systems have shown

promising results for self-repair and crack inhibition, suggesting a prolonged life of dental

composite restorations. More investigations and mechanical enhancements should be directed

toward self-healing technologies in dental resin composites.

Keywords: self-healing, self-sealing, microcapsules, resin composites, dental composites.

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1 Introduction:Composites in dentistry are widely used as they have the ability to bond to the tooth structure,

repair damaged or decayed teeth to an acceptable aesthetic standard; and have satisfactory

mechanical properties and ease of use as a direct-filling material [1] or as an indirect restoration.

Although they are the most common restorative material of choice in today’s dentistry [2, 3],

composite restorations have been shown to encounter two main downsides: secondary caries and

bulk fracture [4]. The longevity and durability of resin composites are still limited and one of the

primary reasons for failure is fracture, with half of all restorations failing in less than 10 years [5-

7].

The survival rate of composite restorations can be affected by many factors such as patient

compliance (caries risk), operator performance and material properties [3, 4]. Composite

restorations often fail due to the accumulation of micro-cracks resulting from factors such as

masticatory forces and thermal stresses [8]. Development of composite materials aims to

increase fracture resistance and improve the service life of dental restorations [9-12]. Composites

are still brittle and prone to fracture in large cavities with high stress bearing areas [1], hence the

need to inhibit fracture and crack propagation in resin-based restorations is considered

fundamental [13]. The healing potential and repair strategies found in living organisms has

inspired material designers to include self-healing mechanisms to increase longevity of materials

[14]. However, these bioinspired approaches do not necessarily involve mimicry of natural

biological processes which are too complex to replicate [14, 15].

Microencapsulation is defined as “a technology of packaging solids, liquids or gaseous materials

in miniature, sealed capsules that can release their contents at controlled rates under the influence

of specific conditions” [16]. Typically, microencapsulation facilitates the delivery of reactive

components in various applications ranging from cosmetics to advanced coatings and nutrient

retention [17-19]. Promising outcomes in minimalizing enamel demineralisation have been

shown with remineralizing orthodontic cements and pits and fissure sealants incorporating

polyurethane microcapsules with different bioactive water mixtures such as Ca(CO3), NaF and/or

K2HPO4 for fluoride release [20].

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The potential for repairing crack damage and mechanical performance recovery in a polymeric

resin matrix occurs when mechanical damage allows active materials to be released from the

microcapsule and repair the damage [21, 22]. Self-healing mechanisms have been achieved in

bulk thermosetting polymers [21, 23-26], fibre reinforced composites [27-31], dental resin

composites [32-40], adhesives [41], elastomers [42, 43], and coatings [44]. Also, nanocapsules

enabling healing of submicron crack propagation have been accomplished [21], i.e. self-healing

bonding resins [45].

The purpose of this review is to gather and reflect upon the ongoing progression in autonomic

self-healing microcapsules with dental resin composites, and to take the opportunity to categorise

different types of microcapsule healing systems as described in the literature.

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2 Methods:2.1 Electronic databases

The systematic review was conducted according to Preferred Reporting Items for Systematic

Reviews and Meta-Analysis (PRISMA) guidelines [46]; only systematic qualitative synthesis

was implemented at the exclusion of quantitative synthesis (meta-analysis). The literature search

was carried out by two independent reviewers (KA and NS). An electronic search from 1998 to

2018 was performed using the following databases: MedLine (PubMed), Embase, the Cochrane

Library and Google Scholar. The search strategy involved the following keywords: (Self-healing

OR Self-sealing OR Microcapsules) AND Resin composites, (Self-healing OR Self-sealing OR

Microcapsules) AND Dental composites.

2.2 Screening and study selection

All titles and abstracts of the articles and patents found were independently analysed and selected

by two reviewers according to the eligibility criteria (Figure 2). Only studies published in English

were included; the outcomes sought for this review were dental resin composites with self-

healing potential. There were no restrictions on the type of self-healing system involved in dental

resin composites. Hand-searching of reference lists by the articles was carried out to identify

missing literature. After duplicate removal, a full text assessment was undertaken against the

inclusion and exclusion criteria.

Figure 2 Selection criteria

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- Case control studies, case reports, case series, expert opinion.- Studies in which self-healing microcapsules were not incorporated in dental composites.- Studies not conducted in English language.

Exclusion

- Studies with self-healing microcapsules within dental resin composites.- Patents related to self-healing microcapsules within dental resin composites.

Inclusion

Eligibility criteria

3 Results:3.1 Study selection process

The initial search retrieved 128 articles. After removing duplicates, 127 studies were screened on

the basis of title and abstract. 116 articles were excluded as they did not satisfy the selection

criteria, leaving 11 articles for full text assessment against the eligibility criteria. One Chinese-

language paper was excluded (non-English literature). 10 articles were included for qualitative

synthesis, but no articles were included for quantitative synthesis (meta-analysis). The patent

search revealed two patents for self-healing microcapsules with dental composites. A summary

of the selection process (Figure 3).

3.2 Study characteristics

Table 1 summarises the included articles about self-healing dental composites. Table 2

summarises the patents. All articles and patents were published between 2010 and 2018. The

majority of self-healing systems in dental composites used a microcapsule healing approach; the

differences were in the shell materials and the healing agents involved. Poly urea-formaldehyde

(PUF) capsular shells were the most common, followed by silica microcapsules. In PUF

microcapsules, the healing agents used were DCPD monomer, TEGDMA-DHEPT, with other

agents also explored. The healing agent used in silica microcapsules was water/polyacid. Other

reinforcing inorganic fillers varied among the selected studies.

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Figure 3 Search flowchart as described in the PRISMA guidelines

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12 records included in this study (qualitative synthesis)

Patent search revealed 2 patents included in

qualitative synthesis

Included

Eligibility

Screening

Identification

Citation search, no

extra papers

Consensus meeting, 1 study was excluded - published in Chinese (non-English literature)

10 studies included in qualitative synthesis

Initial screening (titles & abstracts), consensus meeting held, 11 records screened

116 records excluded on the basis of titles & abstracts assessment

0 studies included in quantitative synthesis (meta-analysis)

11 full-text articles assessed for eligibility

127 records after duplicates removal

128 records retrieved from all databases

Table 1 Included studies in the review - Self-healing dental composites (SHDC)

Authors Self-healing system Self-healing concept & repair mechanism

Wertzberger et al., 2010 [32]

PUF microcapsules Bioinspired SHDC, using (DCPD monomer) microcapsules and Grubb’s catalyst. Some experimental models required external intervention to initiate healing. Average healing performance 57% recovery rate.

Then et al., 2011 [36]

Melamine-modified UF microcapsules

Bioinspired SHDC, using melamine UF microcapsules (DCPD monomer) with no catalyst in the composite model. No healing capability was studied, only mechanical properties and microcapsules performance evaluated.

Wu et al., 2015 [33],Wu et al., 2016 [34],Wu et al., 2016 [35]

PUF microcapsules Bioinspired SHDC, using (TEGDMA-DHEPT amine) microcapsules and benzoyl peroxide (BPO) catalyst in the composite mixture. Average healing performance 65-81% recovery rate. They developed a triple action dental composite with antibacterial, remineralizable and self-healing potential.

Huyang et al., 2016 [37]

Silanized silica microcapsules

Bioinspired SHDC, glass ionomer cement (GIC) repair technology made of contemporary dental materials plus silica microcapsules (aqueous solution of polyacrylic acid) and a healing powder (strontium fluoroaluminosilicate particles) in the composite mixture. Average healing performance up to 25% recovery rate.

Sharma et al., 2017 [38]

Silanized silica microcapsules

Bioinspired SHDC, GIC repair technology made of contemporary dental materials plus silica microcapsules (aqueous solution of polyacrylic acid) and a healing powder (strontium fluoroaluminosilicate particles) in the composite mixture. No healing capability was studied, only static and dynamic mechanical responses evaluated.

Kafagy, 2017 [39] PUF microcapsules Bioinspired SHDC, using microcapsules of TMPET, UDMA, and Bis-GMA or a mixture of these monomers as healing agents and (MBDMA) amine. A catalyst mixture consisted of benzoyl peroxide (BPO) and phenyl acetate solvent (PA) in another microcapsules. Average healing performance around 40% recovery rate.

Chen et al., 2017 [47]

PUF microcapsules Bioinspired SHDC, using (TEGDMA-DHEPT amine) microcapsules and benzoyl peroxide (BPO) catalyst in the composite mixture. They reported the formulation design and synthesis of a protein-resistant dental composite composed of 2-methacryloyloxyethyl phosphorylcholine (MPC) that also can self-repair damage. Average healing performance 57-71% recovery rate.

Yahyazadehfar et al., 2018 [40]

Silanized silica microcapsules

Bioinspired SHDC, GIC repair technology made of contemporary dental materials plus silica microcapsules (aqueous solution of polyacrylic acid) and a healing powder (strontium fluoroaluminosilicate particles) in the composite mixture. Testing durability in terms of SHDC resistance to fracture and healing capacity of damage under monotonic and cyclic loading.

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Table 2 Patent data related to self-healing dental composites

Patent Inventor Country Title Year Self-healing system Claims

US9763858B2 Stephen Gross and Mark Latta [48]

United States

Self-healing dental restorative formulations and related methods

2014 Polyoxymethyleneurea (PMU) microcapsules, DCPD healing agent

Self-healing dental composites

US9931281B2 Jirun Sun [49]

United States

Multi-functional self-healing dental composites, methods of synthesis and methods of use

2018 Silanized silica microcapsules, GIC repair technology

Self-healing, antibacterial, reminerizable dental composites

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4 Discussion4.1 Self-healing strategies

Self-healing approaches are either bioinspired or biomimetic mechanisms. Nature’s ability to

self-heal has inspired engineers and chemists who aim to restore the mechanical properties of a

material by suggesting different healing approaches [14]. The bioinspired approach is considered

achievable following observation of healing in natural systems, e.g. coagulation mechanism [14,

15]. Biomimetic self-healing mechanisms are still in their infancy, with mimicry of tissue

bruising, blood clotting and tailoring of healing networks all being studied. [14].

An example of a bioinspired self-healing concept is the fusion of broken surfaces by reversible

cross-linking polymers. Under controlled load fracture propagation and heat application of up to

150°C, a healing efficiency of 57% of the original fracture toughness has been achieved in a

thermally reversible polymeric material [50]. Other examples include healing in polymeric

material by adding a second solid-state polymer phase [51] and a two-phase solid-state repairable

polymer [52]. These systems, although offering self-healing abilities, may be considered

impractical as they require external intervention, such as a heat source, to activate the healing

[14].

Another approach involves self-healing nanoparticles dispersed in polymer films to be released

at a crack site (resembling blood clotting). An computer simulation has been used to model the

self-healing efficiency, which could potentially reach 75-100% recovery of mechanical

properties of the composites [53]. This system is considered relevant to biomedical engineering,

optical communication and display technologies [14]. A later work, involving multilayer

composites for use in microelectronic and bio-engineering applications, employed fluorescent

nanoparticles with ligands (bonding molecules) to control the movement of the nanoparticles to

the crack site through a microelectronic film layer [54].

The third area of study involves self-healing hollow fibres and microcapsules. Microcapsules of

dicyclopentadiene (DCPD) monomer in poly urea-formaldehyde (PUF) shells are dispersed

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within a polymer host and rupture when subjected to a load that causes crack propagation. The

healing agents (DCPC) will draw along the fissure line where it encounters a chemical catalyst

(usually ruthenium based ‘Grubbs’) incorporated in the polymer matrix. This initiates

polymerization and healing occurs [22, 28, 31, 32, 55]. (Figure 4) However, DCPD is no longer in

use in dental materials. This is perhaps due to biocompatibility complications, probable toxicity

of DCPD and Grubb’s catalyst, in addition to the high cost of the material [24, 56].

Figure 4 Typical method of microcapsules approach (Left), SEM image illustrating ruptured microcapsule (Right). Reproduced with permission from Nature [22]

Self-healing dental composites (SHDC) with PUF microcapsules of triethylene glycol

dimethacrylate (TEGDMA) monomer and N, N-dihydroxyethyl-p-toluidine (DHEPT) amine

accelerator (both as a healing agent) with a benzoyl peroxide (BPO) catalyst incorporated into

the composite mixture have been developed. Self-healing properties and fracture toughness (K IC)

recovery of approximately 65% after composite fracture have been observed [34].

NIR spectroscopy has shown that TEGDMA-DHEPT microcapsules crushed with BPO initiator

powder by manual spatulation were polymerized, and after 24h in an FT-IR spectroscopy the

degree of conversion was around 67.2% [34]. This is in agreement with typical dimethacrylate

degree of conversion values which typically range from 55-75% [57-59]. Thermal analysis

reveals that PUF shells of TEGDMA-DHEPT microcapsules could be chemically stable at

150°C, which is considered a good thermal stability for dental applications [34]. TEGDMA-

DHEPT microcapsules within resin composites show no toxicity during human fibroblast

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cytotoxicity testing in vitro, hence incorporation of microcapsules into resins does not

significantly compromise cell viability [34].

It has also been shown that different resin monomers can be encapsulated in PUF microcapsules

such as TMPET, UDMA, and Bis-GMA or a combination of these monomers and amine

activator 4,4’-methylene-bis (N, N-dimethylaniline) (MBDMA). The catalyst BPO and phenyl

acetate solution have also been encapsulated and dispersed within dental resin composites. This

demonstrated a successful self-healing capability for specimens in water at 37°C, with a healing

performance of around 40% recovery of the virgin fracture toughness [39].

Different types of healing agents within the core materials of microcapsules have been studied,

including three different solvents (chlorobenzene, phenylacetate and ethyl phenylacetate), plus

two reactive epoxy resins (diglycidyl ether of bisphenol-F (DGEBF Epon 862 resin) and

diglycidyl ether of bisphenol-A (DGEBA Epon 828 resin)). The thermal stability of this type of

microcapsule ranged from 150-180°C, due to differences the in boiling temperatures of the

encapsulated solvents [21]. Nanocapsules without a catalyst in the resin have been developed,

containing polyurethane (PU) microcapsules of TEGDMA which were then added to dental

adhesives to improve the bond strength to dentine tissues. However, these particular

nanocapsules have not demonstrated a healing potential since they have no catalyst for

polymerization [45].

A promising strategy in SHDC studied by a number of researchers involves the introduction of a

healing powder (strontium fluoroaluminosilicate particles) into the composite along with a

healing agent (aqueous solution of polyacrylic acids) encapsulated in silanized silica

microcapsules. This concept has a unique mechanism of action as it forms reparative glass

ionomer cement (GIC) within the crack when microcapsules are ruptured [37, 38, 40]. (Figure 5)

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Figure 5 Self-healing mechanism in SHDC (A) a crack propagates, and water inflow, (B) microcapsule ruptures and healing liquid outflows, (C) reacts with healing powder and produce GIC with an ionic crosslinking network. Reproduced with permission from Materials and Design [37]

Self-healing microcapsules can be easily dispersed within a resin matrix. However, there are

some disadvantages related to the use microcapsules, such as the necessity of incorporating a

catalyst within the resin matrix to initiate polymerization, and the need for microcapsules to

rupture when a load is applied. This rupturing is highly dependent on shell thickness and the

surface morphology (roughness) of microcapsules that facilitates retention to the resin matrix

[14]. Good catalyst distribution can offer uniform healing, however limited amount of healing

agents within microcapsules, and formation of voids when microcapsules empty are downsides

of microcapsule self-healing systems [14]. Other problems arise in fibre-reinforced resin

composites; the size of microcapsules (typically 10-100 μm) can disrupt fibre architecture [14].

Self-healing hollow fibres that act similarly to blood vessels in a natural system have been

explored in various engineering materials. A recent work studied the ability to form a ‘bruise’

within a self-healing hollow fibre resin composite, described as ‘bleeding composite’. The author

of this project designed a visual damage enhancement method which illustrates the ‘bleeding

action’ of an UV fluorescent dye (resembling healing liquid) flow to a crack site and restore

mechanical properties of the material [29, 30]. (Figure 6) The downsides are the fact that self-

healing hollow fibres are of large diameter compared to reinforcement fibres, and the necessity

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for the fibres to contain low-viscosity healing agents to promote recovery and damage infusion

[14].

Figure 6 Visual damage enhancement in composite with hollow glass fibres showing a bleeding action of UV fluorescent dye. Reproduced with permission from Composites Part A: Applied Science and Manufacturing [30].

4.2 Methods of encapsulation

The procedure of encapsulation has been outlined by many scholars since 1980s; a basic review

of in situ encapsulation technique has been reported [60-63]. Microencapsulation can be

achieved by in situ polymerization of a urea-formaldehyde (UF) in an oil-in-water (O/W)

emulsion. This was documented by Brown et al., 2003 [55] then adopted by other researchers

with a few procedural modifications.

Shell-forming materials for microcapsules involve urea, formaldehyde, ammonium chloride and

resorcinol. All combine together to form a solid capsular shell of poly urea-formaldehyde (PUF).

A copolymer such as ethylene-maleic anhydride (EMA) is required (typically 2.5% EMA in

aqueous solution) to act as a surfactant which forms an O/W emulsion, wherein oil is the healing

liquid [22, 28, 31, 32, 34, 55]. Ammonium chloride is used to catalyse the reaction of urea with

formaldehyde to form PUF shells at the oil-water interface, and the use of resorcinol in the

reaction is to enhance shell rigidity [55]. Another way to improve the shell strength is to replace

part of the urea with melamine, resulting in a melamine-modified urea-formaldehyde polymer

shell, which is known to have a higher bond strength and enhanced microcapsules properties,

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particularly adhesion to dental resin composite matrix upon microcapsule dispersion [36].

Different shell materials have also been used: silica microcapsules created by a silica

condensation method [37, 38, 40] and polyurethane (PU) nanocapsules by in situ polymerization

[45].

During synthesis of PUF microcapsules, measured amounts of distilled water and surfactant plus

shell-forming materials are all mixed in a flask with continuous stirring. The mixture pH is

usually adjusted to reach 3 to 3.5 by drop-wise addition using pH regulators such as sodium

hydroxide (NaOH) and/or hydrochloric acid (HCL) [21, 34, 55].

Different healing agents (core material) encapsulated within PUF shells have been reported, with

varying materials (monomers) and/or solvents used according to current literature. Noticeably,

most of the studies of neat epoxy resin involved DCPD monomer in PUF microcapsules [22, 28,

31, 32, 55]. Other studies involving dental resin-based composites described the use of

TEGDMA monomer with 1 wt% DHEPT amine as a healing agent in PUF microcapsules [33-

35, 45]. One study involves encapsulating Bis-GMA, UDMA, and TMPET or a mixture of these

monomers with 0.5 wt% MBDMA amine as a healing agent in PUF microcapsules [39]. In

addition, catalyst PUF microcapsules were also synthesized which contained 90.1 wt% BPO

catalyst and 9.9 wt% phenyl acetate mixture solution [39]. An alternative approach has been

described that involves incorporating an aqueous solution of polyacrylic acid in silica

microcapsules [37, 38, 40].

Typically, the encapsulation procedure is performed with an external heat source to optimize in

situ polymerization reactions. The mixed solution should be agitated throughout the entire

encapsulation procedure by magnetic stirring [33, 34], mechanical stirring [21, 55, 64] and/or

sonication [21, 45, 64]. The agitation speed plays an important role in capsular diameter: the

higher the speed of agitation the smaller the diameter of the microcapsules [21, 55, 65-68].

Nanocapsules usually need a higher speed of agitation using mechanical stirring with a

sonication horn used for a short duration (usually less than 5 minutes) [21, 45, 64]. It has been

found that microcapsule diameter, along with shell surface roughness, has a significant effect on

capsular rupture and healing performance within self-healing composites [22, 23].

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The encapsulation procedure, at a target temperature of 55°C, continues with agitation for 4

hours. A suspension of microcapsules will be formed. Once cooled to ambient temperature,

filtration and air-drying should follow [21, 33, 34, 45, 55, 64]. The formed suspension of

microcapsules can be separated under vacuum filtration with continuous rinsing with deionized

water [21, 33, 34, 55]. Some studies have used solvents in filtration such as acetone [34], ethanol

[37] or methanol [69] for rinsing, and others have included a drying agent such as anhydrous

magnesium sulphate [64]. The purpose of rinsing with these solvents is to eliminate excess EMA

surfactant from the microcapsule suspension [64]. However, the use of a solvent is controversial

as it may cause capsular surface changes and damage the PUF shells. The harvested

microcapsules require air-drying for 24-48 hours [21, 34, 55]. This will result in a free-flowing

white powder of microcapsules. Other methods have been used to aid separation of

microcapsules in order to obtain a free-flowing powder. Methods include centrifugation and

sedimentation with continuous rinsing with deionized water, removing the water after each cycle

then spray-drying [31].

Encapsulation procedures vary from one study to another; differences can be found in shell-

forming materials, core-forming materials, pH of the mixture, speed of stirring, time and

temperature of the process, and filtration. However, the final product of microcapsules should

have similar properties. Unfortunately, the filtration procedure is rarely elaborated upon in the

literature; and should receive more attention in order to standardise the quality of production.

Further attention is also needed regarding storage condition recommendations to limit healing

agent leakage from the microcapsules.

4.2.1 Microcapsules silanization

Self-healing microcapsules can be treated with silane coupling agent, which facilitates a strong

surface binding within the composite methacrylate resinous matrix, aids microcapsule rupture

when composites are subjected to damage stimuli (fracture) [37], and improves the mechanical

properties of the self-healing composite [40].

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Based on SEM analysis, silica microcapsules (polyacrylic acid healing liquid) treated with a

methacrylate silane (MA-silane) 3-methacryloxypropyltrimethoxysilane [37, 38] demonstrate a

rupture rate of approximately 75% of microcapsules at the fracture surface of the composite,

compared to only 15% of unsilanized microcapsules [37]. Another study compared two saline

coupling agents: MA-silane linked by covalent bonds, and bishydroxybutyl tetramethylsilane

(OH-silane) connected by a H-bond forming hydroxyl silane to the resin matrix. The study found

MA-silane self-healing dental composites had nearly five times more microcapsules rupturing

compared to OH-silane [40].

The effect on interfacial interactions of the addition of 3-aminopropyltriethoxysilane coupling

agent (KH550) to PUF microcapsules has been explored [70]. The X-ray photoelectron spectra

(XPS) analysis of the interfacial action, chemical bond and hydrogen bond revealed three types

of chemical bonding that strongly binds to PUF microcapsules surface. The SEM revealed a thin

layer formed on the microcapsule surface. The interfacial adhesion performance of the silanized

microcapsules to the epoxy composite improved drastically in comparison to the unsilanized

microcapsules [70] (Figure 7).

Figure 7 SEM fractured surface of self-healing microcapsules in epoxy resin (a) before salinization and (b) after salinization with KH550 silane. Reproduced with permission from Applied Surface Science [70].

4.3 Microcapsules characterization4.3.1 Microcapsules size & surface analysis

Optical microscopy has been used to identify microcapsule diameter with the aid of image

analysis software [33, 34]. Another way of measuring capsular diameter is by a laser diffraction

particle size analyser (Mastersizer instrument) [71]. SEM imaging aids in observing

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microcapsule surface texture morphology and shell thickness. Stirring speed plays an important

factor in determining capsular diameter; PUF microcapsules average a diameter of

approximately 100 µm, ranging from 10 to 300 µm, achieved by a 400 rpm agitation rate [21,

34]. A range of 10-1000 µm diameter PUF microcapsules can be obtained by an agitation rate of

200-2000 rpm [55]. Silica microcapsules average a diameter of 30 µm, achieved by an agitation

rate of 400 rpm [37]. For submicron sized microcapsules, the diameter reaches as small as 300

nm through sonication and costabilization technique [64] (by adding an ultrahydrophobe e.g.

hexadecane or octane to the core material to increase the hydrophobicity and decrease the

Ostwald ripening) [72].

SEM investigation of PUF microcapsule surface morphology has shown numerous PUF

nanoparticles on the smooth exterior shell surface that offer a rougher surface texture. This

facilitates resin matrix retention upon microcapsule dispersion and aids breakage upon cured-

resin fracture [21, 34, 55]. However, the interior of the capsular shell shows a smooth and thin

wall surface [21, 55]. (Figure 8) Shell thickness of PUF microcapsules ranges between 160 to

230 nm [21, 34, 55], whereas in silica microcapsules it ranges from 4-8 µm [37].

Figure 8 SEM images show (A) PUF microcapsules (TEGDMA-DHEPT), (B) a higher magnification of the smooth shell surface with PUF nanoparticles. Reproduced with permission from Dentals Materials [34]. (C) PUF microcapsules (DCPD) shell thickness of 170 nm, rough exterior shell wall morphology with smooth and thin interior surface. Reproduced with permission from Journal of Microencapsulation [55].

4.3.2 Microcapsule content

Elemental analysis (CHN) has been used to determine the amount of encapsulated healing liquid.

Newly-made PUF microcapsules of DCPD healing agent after drying contain 83-92 wt% DCPD

and 6-12 wt% urea-formaldehyde shells. After 30 days the average DCPD content falls by 2.3 wt

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% [55]. It has been found that dispersion of microcapsules in a polymer resin matrix limits and

reduces further leakage of DCPD monomer from capsular shells [55]. Another way of measuring

the capsule content, microencapsulation efficiency and encapsulation yield is via extraction

methods [73-75].

4.4 Preparation and mechanical evaluation of self-healing dental resin composite (SHDC)

Table 3 summaries self-healing dental composite content and their mechanical properties.

4.4.1 PUF microcapsules (TEGDMA-DHEPT) or (other mixture) in dental composites

SHDC vary among the published studies; differences have been reported in resin matrix

monomers, inorganic fillers, photo-initiators and chemical catalysts. One example of SHDC has

been composed of: Bis-GMA-TEGDMA (1:1), 1 wt% phenyl bis(2,4,6- trimethylbenzoyl)

phosphine oxide (BAPO) photo-initiator, 0.5 wt% BPO chemical catalyst with PUF

microcapsules (TEGDMA-DHEPT). The capsules comprised different percentages 0%, 5%,

10%, 15%, and 20% of the composite [34]. When the microcapsules rupture, the BPO catalyst

reacts with the DHEPT amine included in the microcapsules to initiate the self-repairing

mechanism. A preliminary test confirmed that 0.5 wt% of BPO was sufficient to activate free

radical polymerization without deterioration of the mechanical properties of the resin composite.

However, deterioration of mechanical properties was reported at 20 wt% TEGDMA-DHEPT

microcapsules in dental composites [34].

The flexural strength of SHDC with 5-15 wt% TEGDMA-DHEPT microcapsules was not

significantly different, ranging from 50 to 60 MPa (p>0.1). The elastic modulus was reported to

be between 1.8 GPa (0 wt% microcapsules) and 1.5 GPa (5-15 wt% microcapsules), whereas 20

wt% of microcapsule resulted in significantly lower figures of approximately 30 MPa flexural

strength and 1 GPa elastic modulus (p<0.05) [34]. TEGDMA-DHEPT microcapsules improves

virgin fracture toughness (KIC) of resin composites, reaching 40% higher at 15 wt%

microcapsules compared to controls with no microcapsules (p<0.05). Meanwhile, healed facture

toughness (KIC-healed) significantly improved from no healing at 0 wt% microcapsules to maximum

20

healing at 10-15 wt% microcapsules in composites (p<0.05). Self-healing performance ranged

from 64-68% recovery of KIC with 10-15 wt% microcapsules in composites [34] (Figure 9).

Figure 9 SEM images of the fractured planes from the sample including 15 wt% microcapsules. (Left) The initial virgin fracture of the resin sample revealing the step tail structure, illustrated in the inset in (A). (Right) The healed and re-fractured surface of the resin sample demonstrating the presence of released and polymerized healing agent films. Reproduced with permission from Dentals Materials [34]

Another composite formula with self-healing, antibacterial, and remineralization potential

incorporated: Bis-GMA-TEGDMA (1:1), 1 wt% BAPO, 0.5 wt% BPO, 10 wt%

dimethylaminohexadecyl methacrylate (DMAHDM) antibacterial monomer, 20 wt% nano-

amorphous calcium phosphate (NACP) remineralizing agent, 35 wt% glass fillers (silanated

barium boroaluminosilicate [1.45 µm particle mean size]), with TEGDMA-DHEPT

microcapsules (0, 2.5, 5, 7.5, 10 wt%) [33]. Composites containing up 7.5 wt% microcapsules

showed no significant difference (p>0.1) in the flexural strength and elastic modulus values, 85-

100 MPa and 5.5-6 GPa, respectively. However, flexural strength and elastic modulus

significantly reduced to 60 MPa and 4 GPa with 10 wt% microcapsules within composites

compared the rest of the groups (p<0.05) [33]. Healing performance showed no significant

difference (p>0.1) with TEGDMA-DHEPT microcapsules of up to 10 wt% in composites (65-

81% recovery of KIC) [33].

A dual-function composite which has a promising potential to prevent secondary caries and

reduces restoration fracture incorporated: Bis-GMA-TEGDMA (1:1), 0.2 wt% camphorquinone

(CQ) photo-initiator, 0.8 wt% ethyl-4-dimethylaminobenzoate (EDMAB), and glass fillers

silanated barium boroaluminosilicate [47]. A protein-repelling agent was synthesized, 2-

methacryloyloxyethyl phosphorylcholine (MPC) according to a reported method [76], which

21

played an important role in decreasing protein adsorption and preventing formation of a

conditioning layer that might otherwise enable bacterial anchorage to the restoration surface [47,

77]. MPC powder at 7.5 wt% and TEGDMA-DHEPT microcapsules at 10 wt% were mixed in

composite; this seemed to be the optimum formula for acceptable mechanical properties. The

flexural strength and elastic modulus of the self-healing antibacterial composites were not

statistically significant from the controls without microcapsules [47]. KIC of composites at 10 wt

% microcapsules with or without MPC was 36% higher than controls, however, the inclusion of

MPC in composites did not compromise KIC-healed (p<0.05). The healing performance reported for

this double-action composite was 57-71% recovery of KIC [47].

One study showed the effect of water aging for 6 months on self-healing dental composites [35].

The composite model consisted of: Bis-GMA-TEGDMA (1:1), 1 wt% BAPO photo-initiator, 0.5

wt% BPO chemical catalyst, 70 wt% of silanated barium boroaluminosilicate glass fillers, and

TEGDMA-DHEPT microcapsules at (0, 2.5, 5, 7.5, 10%). Involvement of up to 7.5 % of

microcapsules in composites showed effective self-healing, with insignificant effect on the

mechanical properties [35]. Promising results reported of self-healing efficiency in water reached

up to 77 % recovery of the virgin KIC. Interestingly, the self-healing performance in composites

after 6 month water-aging did not decrease significantly compared to day one, in addition to

composites healed in water was not decreased compared to that healed in air [35].

A different SHDC introduced a self–healing system with two types of microcapsule. The first

PUF microcapsules contained the healing agent: Bis-GMA, UDMA and TMPET or a mixture of

the three, in addition to amine accelerator 0.5 wt% MBDMA. The second PUF microcapsules

consisted of: 90.1 wt% BPO catalyst and 9.9 wt% phenyl acetate mixture solution. This resulted

in a formulation for the SHDC of: Bis-GMA-UDMA-TMPTMA (19.01 wt%:19.01 wt%:1.9 wt

%), 0.08 wt% camphorquinone mixed with inorganic fillers (barium glass) from 40 wt% to 60 wt

%. Next, 5 wt% to 15wt% of the resin microcapsules (healing agent) and 1 wt% to 5 wt% of the

catalyst microcapsules, a sum of 20 wt% of a mixture of these microcapsules added to the

inorganic phase [39]. The healing performance of the self-healing specimens reached up to 40%

recovery of KIC.

22

4.4.2 PUF microcapsules (DCPD) in dental composites

PUF microcapsules (DCPC healing agent) used in dental resin composite [32] are similar to a

self-healing system in neat epoxy resin [22]. However, DCPD monomer is no longer used in

dental applications [24, 56]. A study involved the use of DCPD microcapsules in dental

composites consisting of: Bis-GMA-UDMA-TEGDMA (1:1:1), 0.5 wt% CQ, 0.5 wt% EDMAB,

2 wt% Grubb’s catalyst, 55 wt% glass fillers silanated barium borosilicate (0.7 µm particle mean

size), and 5 wt% of DCPD microcapsules (average capsular diameter 50 µm) [32]. The intention

was to compare 5 wt% of DCPD microcapsules in composite to the controls (one with no

microcapsules nor Grubb’s catalyst, 5 wt% microcapsules only, and 2 wt% Grubb’s catalyst

only). This found a 57% recovery of virgin fracture toughness in the self-healed composite

specimen. No healing efficiency was reported for all three control groups [32].

Melamine has also been used for the shell materials [36]. Melamine-modified urea-formaldehyde

microcapsules (DCPD healing agent) were synthesised with the purpose of improving the

adhesion of microcapsules to the composite matrix and facilitating breakage upon crack

intrusion. The mechanical properties were not greatly affected by the incorporation of melamine-

modified microcapsules. The composites consisted of Bis-GMA-TEGDMA (7:3), 0.7 wt% CQ,

and 2.3 wt% EDMAB. The flexural strength of composites at 6 wt% microcapsules was (64.6 ±

23.5 MPa) compared to the control with no microcapsules (106.3 ± 19.8 MPa) and to 3 wt%

microcapsules (105.1 ± 24.8 MPa). Vickers micro-hardness (VH) values confirmed these results.

3 wt% to 6 wt% microcapsules in composites (24.1-25.8 HV and 21.7-28.8 HV, respectively)

were hardly affected in comparison to the control (30.7 ± 1.6 HV). As a result, melamine-

modified DCPD microcapsules (0.5-5% of the urea replaced by melamine) did not significantly

impact the mechanical properties of SHDC [36].

4.4.3 Silica microcapsules (water/polyacid) in dental composites

A number of studies have explored the use of dental composites with silica microcapsules filled

with an aqueous solution of polyacid (water/polyacid) [37, 38, 40], with Bis-GMA-HEMA (1:1)

with 0.5 wt% CQ and 0.5 wt% EDMAB, 70 wt% strontium fluoroaluminasilicate glass, and (0%,

2.5%, 5%, 10%) silanized silica microcapsules [37]. Increasing silica microcapsules loading (≥

23

10 wt%) in composite significantly reduced the elastic moduli of the composite to less than 10

GPa (p<0.05) [37].

The healing performance of silanized silica microcapsules (water/polyacid) reaches up to 25%

recovery of virgin fracture toughness at 5 wt% to 10 wt% microcapsules within composites.

However, 10 wt% microcapsules seemed to have a negative impact on the mechanical properties

of the composite. The best overall performance for SHDC with silanized silica microcapsules

was found with 5 wt% microcapsules [37]. Similar findings have shown that 5 wt% of MA-

silanized silica microcapsules in composites was the optimum formula for healing performance

(25% recovery rate of KIC) and fracture toughness [40]. However, less than 5 wt% silanized

silica microcapsules in composites resulted in no healing. Whilst maximum healing was reported

at 25 wt% silanized silica microcapsules in composites, the fracture toughness and fatigue crack

growth were dramatically affected [40].

Another composite formula consisted of Bis-GMA-TEGDMA (50 wt%:48 wt%) with 0.4 wt%

CQ and 1.6 wt% EDMAB, 6 wt% amorphous calcium phosphate (ACP), and 50% strontium

fluoroaluminasilicate glass. Then, silanized silica microcapsules were added at (0, 3, 6, 9 wt%)

to the mix as a substitution from the base resin wt%. The static behaviour of silica microcapsule

dental composites revealed that composites with 3 wt% microcapsules showed an increase in

both surface hardness and flexural strength by 38% (58 HV) and 6% (55 MPa) respectively,

while it decreased the compressive strength by 35% (133 MPa). The flexural strength and

surface hardness improved with increasing the silica microcapsules up to 9 wt% in composites,

which indicates that more inorganic filler content strengthened the resin matrix [38]. The

dynamic behaviour of SHDC at 0 wt% to 9 wt% microcapsule loading in composites showed a

decrease in storage modulus, loss modulus and glass transition temperatures [38].

4.5 The limitation of microcapsule-based system in dental field

The limitation of microcapsules can be found at different stages from production to mixing into

the resin host. If the microcapsules are small (sub-micron sized), they will not have enough core

materials to be functional particles, whereas if the microcapsules are too large, they will

24

deteriorate the mechanical properties of the host resin material. After synthesis, the

microcapsules leak the healing agents when stored at room temperature, however, cool

temperature storage may help to reduce the core materials leakage, also dispersion of the

microcapsules in resin would limit the leaking phenomena [55]. During preparation of SHDC,

the microcapsule’s shell must have enough strength to withstand handling and mixing with resin

composites, and to break upon resin composite fracture [34]. According to the published studies,

dispersion of microcapsules should not exceed 10 wt% in composites, to prevent mechanical

properties deterioration. Treatment of microcapsules by silane coupling agent would facilitate a

strong surface binding within the composite methacrylate resinous matrix, aimed at microcapsule

rupturing by fracture of the composites [37], and improves the mechanical properties of the self-

healing composite [40].

During the photo-activation of SHDC, the light curing unit generate and radiate heat, which

might negatively affect the reactivity of the chemical catalyst (BPO) involved in the composite.

For example, the decomposition of benzoyl peroxide (BPO) catalyst is ranging from 55-98 °C

[78], the half-life of BPO is one hour at 92 °C and one minute at 103 °C neither of which is very

toxic [79]. In self-healing performance evaluation, there is a great controversy of the reliability

and repeatability of methods used to measure healing capability in composites, yet, accurate

methodologies must evolve to overcome the limitation of the current methods in the literature

e.g. recovery rate of the virgin fracture toughness (TDCB) and flexural strength (SEVEN). Other

disadvantages of the incorporated microcapsules system in composites can be seen in the

polishability of the material; resultant voids in the surface of composite and also after

microcapsules rupture in cured composites which might weaken the bulk structure.

Biocompatibility concerns also exist, due to the risk of uncured monomer release (core) and

unreacted free formaldehyde (PUF shells) from microcapsules in the oral cavity and the probable

cytotoxicity.

25

Table 3 Summary of SHDC compositions and mechanical properties

Author Type of Microcapsules (MC)

SHDC contents Flexural strength (MPa)

Elastic modulus

(GPa)

Microhardness (VH)

Healing performance

(%)Wu et al., 2016 [34]

PUF-microcapsules TEGDMA-DHEPT

Bis-GMA:TEGDMA (1:1), 1% BAPO, 0.5% BPO, (0%, 5%, 10%, 15%, 20%) MC

0-15% MC:50-60 MPa20% MC:30 MPa

0-15% MC:1.5-1.8 GPa20% MC:1 GPa

N/A 64-68%

Wu et al., 2015 [33],

PUF-microcapsules TEGDMA-DHEPT

Bis-GMA:TEGDMA (1:1), 1% BAPO, 0.5% BPO, 10% DMAHDM, 20% NACP, 35% silanated BBS, (0%, 2.5%, 5%, 7.5%, 10%) MC

0-7.5% MC:85-100 MPa10% MC:60 MPa

0-7.5% MC:5.5-6 GPa10% MC:4 GPa

N/A 65-81%

Chen et al., 2017 [47]

PUF-microcapsules TEGDMA-DHEPT

Bis-GMA:TEGDMA (1:1), 0.2% CQ, 0.8% EDMAB, BBS, 7.5% MPC (antibacterial agent), 10% MC

0% MC:60 MPa10% MC:55 MPa10% MC +7.5% MPC: 57 MPa

0% MC:1.7 GPa10% MC:1.5 GPa10% MC +7.5% MPC: 1.5 GPa

N/A 57-71%

Kafagy, 2017 [39]

PUF-microcapsules (Bis-GMA,UDMA,TMPTMA)+MBDMAPUF-microcapsules BPO+PA (catalyst)

Bis-GMA:UDMA:TMPTMA, 0.08% CQ,40% to 60% BBS,5% to 15% of resin MC1% to 5% of catalyst MC

N/A N/A N/A 40%

Wertzberger et al., 2010 [32]

PUF-microcapsules DCPD

Bis-GMA:UDMA:TEGDMA (1:1:1), 0.5% CQ, 0.5% EDMAB, 2% Grubb’s catalyst, 55% BBS, 5% MC

N/A N/A N/A 57%

Then et al., 2011 [36]

Melamine-modified UF-microcapsules DCPD

Bis-GMA:TEGDMA (7:3), 0.7 wt% CQ, 2.3 wt% EDMAB, (0%, 3%, 6%) MC

0% MC:10619 MPa3% MC:10524 MPa6% MC:6423 MPa

N/A 0% MC:30 VH3% MC:25 VH6% MC:21-28 VH

N/A

Huyang et al., 2016 [37]

Silanized Silica microcapsules (water/polyacid)

Bis-GMA:HEMA (1:1), 0.5% CQ, 0.5% EDMAB, 70 % strontium fluoroaluminasilicate glass, (0%, 2.5%, 5%, 10%) MC

N/A 0% MC:12 GPa2.5-5% MC:11 GPa10% MC:7.5 GPa

N/A 25%

Sharma et al., 2017 [38]

Silanized Silica microcapsules (water/polyacid)

Bis-GMA:TEGDMA, 0.4% CQ, 1.6% EDMAB, 6% ACP, 50% strontium fluoroaluminasilicate glass, (0%, 3%, 6%, 9%) MC

0% MC:52 MPa3% MC:55 MPa6% MC:62 MPa9% MC:67 MPa

N/A 0% MC:42 HV3% MC:58 HV6% MC:66 HV9% MC:78 HV

N/A

26

5 Conclusion

In the last couple of decades, improvements have been made to dental resin composites including

manufacturing and structural design modifications. However, composite cracking and fracture

has limited the long-term success rate of the material. Nature has inspired new technologies such

as self-healing and repair mechanisms that can improve the survival rate of the material.

All self-healing systems have shown promising results for self-repair and crack inhibition,

suggesting a prolonged life for dental composite restorations. Overall healing performance

reported in dental composites ranges from 25% to 80% recovery rate after fracture. The self-

healing systems used were PUF microcapsules of DCPD and TEGDMA-DHEPT or silica

microcapsules of water/polyacid healing agent.

More investigations should be directed toward the limitations of microcapsules within resin

composites i.e. resultant voids after microcapsules rupture on mechanical properties and surface

polishability of the composites, the biocompatibility of the released microcapsule’s components

to the oral cavity. Cytotoxicity testing should be taken into consideration towards self-healing

dental materials before any in vivo studies involvement.

27

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