6. Modifications after wound regeneration
After the remodeling stage, wound closure istraditionally
complete.However, there are still some problems, such as scar
hyperplasia, loss of hair follicles and sensory disturbances. In the
past, post-healing repair was often overlooked. With the improvement of
medical technology, more attention has been given to aestheticsand
functional recovery. As a new therapeutic material, nanomaterials have
also been developed and applied forthe post-healing repair of wounds.
6.1Scar prevention
Scarsmanifest as excessive fibroblastproliferation, disordered cell
growth and abnormal collagen deposition, which not only affect the
aesthetic appearance but also cause poor local traction and elasticity,
resulting in limb dysfunction;there is also a risk of cancer.
Currently,scars are mostly treated by surgical resection, local
injection and radiation. Nanomaterials havebeen gradually applied in the
treatment of scars. For example, some scholars developed
glucocorticoid-loadedhydrogel particles to realize the long-term release
of glucocorticoids, and the effectson the inhibition of scars has been
confirmed in rabbits(Guo et al., 2018). AuNPs have been used as carriers
of photosensitizersbecause ofthe photothermal effects of AuNPs
and local surface plasmon
resonance (LSPR) to enhance oxidative stress, leading to fibroblast
apoptosis or necrosis(Zhang et al., 2017).However, these methods are
used after the scar has formed, although ideally, scar formation would
be prevented before the wound is healed. Nanomaterials have been
explored in this area. Studies have found that some nanomaterials, such
as carbon nanotubes,can inhibit the excessive proliferation of
fibroblasts by inhibiting the TGF-β-SMAD pathway. Moreover,these
nanomaterials can regulate the expression of various enzymes in the
cellular microenvironment, such as promoting the expression of MMP-1 and
inhibiting collagen and fibronectin levels in the ECM. In addition,
nanomaterials can induce the oriented movement of
fibroblasts(Poormasjedi-Meibod et al., 2016; Weng et al., 2018).All of
these observations indicate that nanomaterials have the potential to
prevent scar formation before epithelialization. However, there is still
a lack of in-depth mechanistic research, such as whether these
nanomaterials can affect other scar pathways, such as Wnt/β-catenin, and
regulate macrophage polarization to inhibit scarring hyperplasia
(Hesketh et al., 2017; Yu et al., 2016)(Figure 2).In addition, most of
the current studies have beenin vitro experiments, and there is a lack
of in vivo confirmation studies, which are difficult to carry out.
Reviewing the process of wound healing, the hyperplasia process of
granulation tissue before epithelialization requires the activation of
the TGF-β-SMAD pathway to promote the proliferation of fibroblasts. This
processalso promotes the deposition of collagen during the remodeling
phase, which is exactly the opposite of the antiscarring mechanism.
Therefore, the application of nanomaterials to prevent scars before
epithelialization may inhibit the active proliferation of granulation
tissue. If we want to prevent scar formation before epithelialization,
we need to consider other aspects. For example, we can implant nanotubes
and other nanomaterials duringthe granulation tissue proliferation stage
or use 3D printing technology to fill wound defects with nanoscaffolds
to guide the directed and moderate growth of fibroblasts. We can also
develop new nanomaterials to reduce the high levels of cytokines such as
IL-6, IL-8, and IL-17 in the scar and adjust the expression of miR-146a,
miR-21, neurotransmitters such as bradykinin, and substance P. Further
experiments are needed as to whether these ideas are
sufficient(Lebonvallet et al., 2018; Lee et al., 2018; Zhang et al.,
2018).
6.2Hair follicle reconstruction
When the defect of the wound reaches the dermis, it will lead to loss of
the hair follicle, so regeneration of the hair follicle is another part
of wound healing. However, in wounds with skin substitute treatment and
scar formation, regeneration of the hair follicles is the technical
bottleneck for the complete restoration of skin structure and function.
The application of nanomaterials in medicine has attracted the attention
of scholars, who have begun to note the influence of nanomaterials on
hair follicles. Synthetic composite electrospun membranes can promote
the
recruitment
and proliferation of hair follicle stem cells by releasing zinc
ions(Zhang et al., 2019). The nanofiber scaffold seeded with hair
follicle stem cells can promote the attachment, proliferation and
differentiation of hair follicle stem cells on the scaffold(Hejazian et
al., 2012). These studies indicate that nanomaterials have the potential
to promote hair follicle regeneration, but there are few studies in this
field. Most of the studiesare in vitro experiments with limited types of
nanomaterials. Although an in vivo study
inSprague-Dawley (SD)rats showed
that the number of hair follicles and hair follicle stem cells in the
nanodressinggroupwas higher than that in the control group, further
experiments are needed to explain the mechanism. The reconstruction and
regeneration of hair follicles are difficulties that are faced in wound
recovery(Zhang et al.,
2019).Currently,
surgical hair transplantation is widely used for hair follicle
reconstitution, but the survival rate is limited. This may be related to
factors such as damage to the transplanted hair follicle during the
operation and local inflammation after transplantation. Inspired by the
fact that nanomaterials can promote the proliferation of hair follicle
stem cells and inhibit inflammation, we couldtry to use
anti-inflammatory nanomaterials or composite nanomaterials loaded with
stem cells, PRP and other drugs that promote the survival of hair
follicles as auxiliary treatments to improve the viability and success
rate of hair transplants (Figure 2).
After hair follicle reconstruction,
whether nanomaterials can promote hair growth can also be studied in the
future. It was reported that
poly(glutamic acid) (PGA)NPscan
increase the expression levels of the proteinscyclin D1 and CDK4 and
induce the development of the hair follicle cycle by activating the
Wnt/β-catenin pathway. These proteinsalso increase the expression of
type II keratin and melanin and promote the proliferation of dermal
papilla cells toencourage hair growth(Lee et al.,
2019).However, most current studies
have focusedon nanomaterials as drug carriers for hair loss, and there
is still a lack of research on the effects of the nanomaterials
themselves on hair growth. Because of itsspecial structure, drugs can be
stored within the hair follicle and slowly released for a long time.
Nanomaterials are small in size and more easily enter the hair follicle
for storage. Compared with other drugs for hair loss treatment,
nanomaterials have the advantages of higher efficiency and longer
lasting effects. Further exciting progress is expected in the
exploration ofnanomaterials to promote hair growth.
6.3Skin sensory regulation
There are many cutaneous nerves and peripheral nerves in the skin, and
skin wounds are bound to damage these nerves. Local infection,
inflammation or improper nerve regeneration will cause paresthesia such
as pain, itching, and hypoesthesia after healing, which affect the
quality of life of patients.
Nanomaterials are widely used in the development of topical drug
delivery systems due to their superior drug-carrying properties. To
increase percutaneous drug penetration, prolong the release time and
reduce side effects, some analgesics, such as nonsteroidal
anti-inflammatory drugs, local anesthetics, capsaicin, and antipruritic
drugs, such as glucocorticoids, have been developed as nanoemulsions,
transfersomes, solid lipid
nanoparticles (SLNs) and as other systems for the treatment of pain and
itching after healing.In addition,
topical ZnO NPs have been observed to have anti-itch effects(Aman et
al., 2019; Andreu et al., 2018; Bikkad et al., 2014; Ghiasi et al.,
2019; Nafisi et al., 2018).These formulations are feasible for most
kinds of wounds, such as postoperative wounds, diabetic wounds, and burn
wounds. However, postherpetic neuralgia woundsare a special
circumstance. Neuralgia is severe and continuous and requires more
powerful analgesics. We couldtry to apply an anestheticnanosystem for
local pain relief or to develop delivery nanosystems for neuralgia drugs
such as gabapentin and pregabalin. The actual clinical effects still
need to be confirmed by further studies. In addition, there are few
studies on whether nanomaterials can relieve pain and itching symptoms.
The mechanisms of pain and itching overlap, and both are related to the
sensory transmission of nerve cells mediated by neuropeptides, proteases
and other mediators. Whether certain nanomaterials can achieve pain
relief and antipruritus by inhibiting the secretion of these nerve
mediators or inhibiting nerve signal transduction is a direction that
can be explored in the future.
Hypoesthesia after wound healing is difficult. Generally, physical
therapy is used to improve symptoms, but the therapeutic effect is poor.
In recent years, electronic skin represented by carbon-based
nanomaterials has emerged in the field of body surface monitoring.
Electronic skincan collect information of temperature, pain sensation,
and even taste sensation from the skin and convert it into electronic
signals for transmission(Qiao et al., 2018; Zhao et al., 2018b).Then,the
application of this kind of equipment to patients with hypoesthesia
after a large area of wound healing could be imagined. When the
patientsare exposed to a harsh environment of heat, chemical damage and
electrical stimulation, they cannot respond in time due to hypoesthesia.
The application of electronic skin can transmit danger signals on a more
appropriate timescale and protect the body from danger. Unfortunately,
the current technology just convertsthese signals into digital signals,
which cannot be directly transmitted to human nerves and fed back to the
brain. If this difficulty is overcome, holistic functional wound healing
wouldbe further realized(Ma et al., 2019).Moreover, before electronic
skin is applied to the body, it is necessary to consider the harmonious
symbiosis between the electronic skin and the surrounding normal nerves,
muscles, lymphatics and glands, as well as ensure the precise
transmission of nerve instructions. These may be the next directions for
researchers (Figure 2).
7. The toxicity of nanomaterials in wound healing
Nanomaterials have an excellent ability to promote wound healing, and
there is still great potential for their application and development in
the future. However, it should be noted that the wound surface is not
protected by intact skin. The nanomaterials directly contact the tissue
inside the wound, and the biological safety of the products must be
considered before application.
The most reported transdermal toxicities of nanomaterials are skin
irritation and allergies(Ema et al., 2013; Palmer et al.,
2019).Theseside effectshave individual differences and may be
unavoidable. There havealso been frequent reports ofnanomaterials
causingoxidative stress, autophagy and apoptosis in keratinocytes and
fibroblasts(Wang et al., 2018). These toxicities depend on the particle
size, shape, surface charge and concentration of the nanomaterial.
Therefore, these factors should be adjusted to reduce toxicity to skin
cells when developing new nanomaterials used in wounds(Hashempour et
al., 2019).In addition, it has been reported that nanomaterials cause
DNA damage and decrease gene methylation, suggesting the potential of
cell canceration(Ali et al., 2016; Sooklert et al., 2019).However, there
is still no direct evidence to prove that nanomaterials can cause
malignant transformations and inherited gene mutations in skin cells.
Thisdoes not mean that nanomaterials are safe. It is not known whether
the long-term percutaneous exposure and deposition of nanomaterials in
the skin will cause severe consequences, which will need to be confirmed
by long-term exposure experiments in the future.
Nanomaterials will come in contact with blood cells in ruptured blood
vessels in wounds and enter the blood circulation. This phenomenonbrings
two consequences, one of which is hemolysis. Some metal nanomaterials,
such as AgNPs and ZnO NPs, have been found to cause hemolysis. To solve
this problem, we can adjust the physical and chemical properties of the
materials or wrap biologically active substances such as phospholipids
and polysaccharides onto the surface of the nanomaterials(Bakshi,
2017).Another consequence is that the nanomaterials will spread
throughout the body into various organs after entering the blood,
causing multisystem effects. Compared with the original concentration of
the nanomaterials, the concentration of nanomaterials after entering the
blood circulation is greatly reduced, and they can be partly excreted
through urine and feces. Weight loss and even death have been observed
in animal experiments, but in practical use, there is no conclusive
evidence regarding whether nanomaterials will cause organ failure and/or
tumors, whether exposure during pregnancy will affect offspring and what
the safe concentrationis(Hadrup et al., 2018).
In general, nanomaterial toxicity studies in wound healing havemainly
concentrated on local acute adverse reactions. There are relatively few
studies on systemic and chronic toxicity. On the other hand, most of the
studied nanomaterials are metal nanomaterials, carbon-based
nanomaterials and nanotubes, whereas nanofibers, films and other novel
nanomaterials are still rarely studied(Teixeira et al.,
2020).Future
toxicity studies urgently need to solve these problems.
8.Outlook and future challenges
8.1 Limitations of the existing
research
In summary, it has been demonstrated that nanomaterials have a positive
effect duringeach process of wound healing. But most current studies
have beenconducted in vitro, and the in vivo experiments need to be
improved. The animals used for studies are rats, mice, rabbits and pigs.
Due to the differences in animal cost, size and availability, rats and
mice are most commonly used. However, the skin morphologiesand wound
healing processes of these rodents are different from those of humans.
In comparison, the skin of pigs is the most similar to that of humans.
Because of the high cost and cumbersome operation of experiments with
large animals, pigs are not widely used in wound healing
research(Abazari et al., 2020).In addition, many wounds lack a
standardized and controllable modeling method. For example, wound
biofilms, diabetic wounds and wounds of immune skin diseases are usually
imitated woundphenomena, and it is difficult to reflect the mechanism of
human skin wounds.
Drugs and methods for wound treatment are changing rapidly, and
nanomaterials are being applied to wounds in various formulations.
However, the mechanism by which nanomaterials promote wound healing
hasstill only been superficially studied. For example,macrophage
polarization and the TGF-β1/SMAD signaling pathway are the most commonly
reported pathways in the inflammatoryand proliferation phases,
respectively. The related mechanism by which nanomaterials promote wound
repair after reconstruction is rarely reported. There are many types of
nanomaterials with different characteristics, and the mechanism of wound
healing promotion should also be multifaceted. Deeper and more
innovative mechanisms need to be explored in the future, which would
behelpful to improve treatment methods and avoid unnecessary side
effects(Dukhinova et al., 2019; Janjic et al., 2017).
8.2 Future challenges
Because nanomaterials have superior drug-carrying properties, an
increasing number of new drugs have beenloaded onto nanomaterials, such
as stem cells and PRP. For the treatment of immune skin diseases, the
emergence of a variety of biological agents has gradually replaced
traditional drugs. If these monoclonal antibodies can be loaded onto
nanomaterials, it may lead to an improvement in the absorption rate and
efficacy of the drug.
Although the existing research has confirmedthat nanomaterials can play
a positive role in all phases of the promotion of wound healing, the
same materials cannot be beneficial throughoutthe whole process, and
different types of materials may be required at different stages. Since
it is impossible to visually judge the dividing point of each stage, it
is important to develop a real-time indicator of the wound condition. In
recent years,self-powered implantable electronic skins based on ZnO
nanowires modified by enzymes (urease and uricase) have been developed
for transcutaneous detection of human health, including blood pressure,
temperature, humidity, electrolyte metabolites, etc(Asif et al., 2015;
Ma et al., 2019).Through this technology, electronic skin that monitors
the pH value, humidity, inflammatory factors and signaling pathway
proteins can be developed in the future. Then, the therapist can
accurately control the treatment of wounds and select appropriate
nanomaterials according to the real-time situation.
With the development of medicine, the treatment of wounds not only
pursues the filling of defects but also requires comprehensive
functional and aesthetic recovery. Nanotechnology is rapidly developing
and is considered to be able to solve a number ofproblems in various
situations. Currently, scholars have successfully applied nanomaterials
to promote wound healing and prevent scar formation. Nanotechnology
still has great potential in the regeneration of hair follicles, the
regulation of paresthesia and the improvement of abnormal pigmentation.
Especially with the emergence of electronic skin in recent years, the
combination of nanotechnology and electronic technology has provided new
ideas for the recovery of paresthesia after wound repair and brought an
intelligent concept for wound healing.
9.Conclusion
In recent years, an increasing number of nanomaterials have been used to
treat wounds. This work reviewed the possible ways that nanomaterials
can facilitate wound healing and their related mechanisms, which
improves our understanding of the role of nanomaterials in wound
healing. We pointed out that most of the current studies have focused on
promoting hemostasis, antiinfection, immunoregulation and proliferation,
but there is a lack of research on the in-depth mechanisms and
post-wound modifications. Additionally, we proposed some methods and new
thoughts for subsequent studies, especially for functional and aesthetic
problems after wound healing. We hope our work will provide inspiration
for more exciting progress in the future.