Moist Wound Healing with Commonly Available Dressings

Abstract

Significance: A moist wound environment has several benefits that result in faster and better quality of healing. It facilitates autolytic debridement, reduces pain, reduces scarring, activates collagen synthesis, facilitates and promotes keratinocyte migration over the wound surface, and supports the presence and function of nutrients, growth factors, and other soluble mediators in the wound microenvironment.

Recent Advances: Wound dressings can be utilized to create, maintain, and control a moist environment for healing. Moist wound dressings can be divided into films, foams, hydrocolloids, hydrogels, and alginates. We are also including negative pressure wound therapy systems in the moist dressings.

Critical Issues: An optimal wound dressing should provide a moist environment and have an optimal water vapor transmission rate (WVTR) and absorptive capacity. It should also protect the wound against trauma and contamination and be easy to apply, painless to remove, and esthetically acceptable or even pleasing.

Future Directions: Interventions, particularly dressing changes, by medical caregivers are labor intensive and expensive and there should be a continuous effort to reduce their number per week. Smart dressings with integrated microsensors and delivery capabilities that would allow wireless real-time monitoring and treatment of the wound would be very advantageous. This way the state of the wound as well as the wear time of the dressing could be assessed without dressing removal or visit to the wound care center. In addition, an ability to adjust the WVTRs to the exudate level of the wound (or having a large absorptive capacity without changing the WVTR) would be useful. This feature would guarantee an optimal level of hydration of the wound surface throughout the treatment.

Kristo Nuutila, MSc, PhD

Scope and Significance

Wounds heal faster in a moist environment. In addition, the moist environment has other benefits that make the wounds heal with less scarring.^1,2 The purpose of this article is to review the literature on moist wound healing, summarize its benefits, describe currently available moist wound dressing types, and discuss their key characteristics.

Translational Relevance

Dr. George Winter was the first to demonstrate in a porcine model that wounds heal faster when kept moist.³ Subsequently, several other preclinical and clinical studies have compared moist and dry healing and shown that the moist environment accelerates reepithelialization and reduces scar formation.^4–8 Preclinical studies have also been used to test and improve various dressings providing a moist wound environment.^9–11

Clinical Relevance

Besides providing faster wound healing, the moist wound environment has other clinically relevant benefits. It facilitates autolytic debridement, reduces pain, reduces scarring, activates collagen synthesis, facilitates and promotes keratinocyte migration over the wound surface, and supports the presence and function of nutrients, growth factors, and other soluble mediators in the wound microenvironment.^1,2,12 The moist environment is also favorable for various topical treatments as well as for tissue and cell transplantation (Table 1).^13,14 Consequently, moist wound dressings play a vital role in the management of both acute and chronic wounds.¹⁵

Table 1.

A moist wound environment has several benefits that result in faster and better quality of healing

Benefits of the Moist Wound Environment	References
Increases keratinocyte migration and reepithelialization	5,24
Increases collagen synthesis	25
Increases autolytic debridement	26
Decreases necrosis	8,9,29
Decreases pain	32
Decreases inflammation	28
Decreases scarring	7
Allows cell–cell signaling and supports function of soluble mediators such as GFs	23
Allows precise delivery of topical treatments	9,28,29
Provides desirable environment for cell and tissue transplantation	13,14,31

It facilitates autolytic debridement, reduces pain, reduces scarring, activates collagen synthesis, facilitates and promotes keratinocyte migration over the wound surface, and supports the presence and function of nutrients, growth factors, and other soluble mediators in the wound microenvironment. The moist environment is also favorable for various topical treatments as well as for tissue and cell transplantation.

GF, growth factor.

Background

Moist wound healing has been practiced since at least the seventh century (Paulus of Aegina).¹⁶ In 1850, Langenbeck used wet treatment for surgical wounds and eight years later, Passavant reported on wet (immersion in a bath tub) treatment of burns.¹⁷ A similar report was published 3 years later by Hebra.¹⁸ Bunyan used an oilskin dressing and irrigation with a dilute solution of bleach.¹⁹ Winter is usually credited with initiating moist treatment of wounds. His dressing consisted of an impermeable polyethylene membrane, thus really creating a “wet” wound environment with excess water over the wound surface.³ It is also interesting to note that Winter, a professor of biomechanics, was affiliated with the same University as Bunyan who had used moist/wet treatment of hundreds of traumatic wounds with oilskin 20 years earlier. Winter's findings, published in Nature (1962), moved wound care toward moist treatment and the development of more than 100 different moist dressings.^3,15 Regardless of the early evidence, the benefits of the moist wound environment versus dry treatment was a topic of debate for a long time. Many wound care providers were concerned that the warm, moist environment provided by occlusive dressings would favor the growth of bacteria and other pathogens leading to clinical infection and therefore refrained from using moist dressings. However, these concerns were later proven groundless and moist wound treatment became a standard practice in the 1970s and 1980s.^2,20 In this review, we refer to all dressings that hydrate the wound surface (more or less) as moist.

It has been shown that the moist environment has several benefits that result in faster and better quality of healing^{1,2,5,7–9,13,14,21} (Table 1). In the first place, the moist wound environment reduces the formation of an eschar saving tissue, time, and energy. Therefore, in a moist environment without an eschar, the wound can start moving toward healing sooner.²² The moist environment is also essential for the function of cells. When a wound occurs, the cells need to be able to communicate with each other by sending and receiving signals to orderly repair the tissue loss. People can communicate in air, but cells communicate by secreting growth factors and other signaling molecules that require a liquid medium for intercellular cross talk to promote healing.²³ In a moist environment, epithelial cells can also migrate and reepithelialize the wound more efficiently than in a dry environment. The moist environment allows keratinocytes to easily and rapidly migrate over the wound surface, whereas in a dry environment, they need to migrate under the dry crust on the wound surface.²⁴ It has also been shown that the moist environment promotes collagen synthesis during wound healing by stimulating collagen-producing fibroblasts.²⁵ The moist environment promotes autolysis of necrotic tissue in the wound by allowing endogenous enzymes to break down dead tissue. Autolytic debridement is a selective process, where only necrotic tissue is dissolved to accelerate healing (Table 1).²⁶

In response to wounding, inflammatory cells are recruited to the injury site. Inflammation is an integral part of wound healing and crucial for the removal of pathogens from the wound. However, an excessive inflammatory process in the wound slows down the shift from the inflammation phase to the proliferation phase and delays the healing process.²⁵ A correlation between the magnitude of early inflammatory response and the size of the scar in the healed wound has been demonstrated.⁷ Many other studies have also shown that healing in a moist environment results in less inflammation than in a dry environment and improves the quality of healing with less scar.²⁸

Several studies have also shown that if a moist wound environment is established soon after injury, it decreases tissue loss and burn wound progression.^8,9,29 Breuing et al.⁸ treated partial-thickness burns in a porcine model in moist and dry environments. Their results showed that early continuous treatment with normal room temperature saline significantly reduced the early progression of the injury and resulted in no or minimal eschar, and only little tissue necrosis was detected. On the contrary, the burns that were left dry formed a thick eschar and a deep layer of necrosis.⁸ Similarly, Grolman et al.²⁹ treated deep partial-thickness porcine burn wounds with an agarose hydrogel. Histological analysis of the burns on day 4 postburn showed that a topical application of agarose hydrogel immediately after the burn reduced the depth of tissue necrosis in comparison to control treatments. The burn depth in agarose hydrogel-treated and gauze-treated burns was 227 and 358 μm, respectively. Thus, it was concluded that immediate establishment of a moist environment and maintenance for 4 days reduced the amount of tissue necrosis by almost 40% (Fig. 1).²⁹ Svensjö et al. did a similar study in full-thickness porcine wounds and found that moist wounds healed significantly faster.⁴

Figure 1.

Several studies have also shown that if a moist wound environment is established soon after injury, it decreases tissue loss and burn wound progression. Grolman et al. ²⁹ treated deep partial-thickness porcine burn wounds with an agarose hydrogel. Histological analysis of the burns on day 4 postburn showed that a topical application of agarose hydrogel immediately after the burn reduced the depth of tissue necrosis in comparison to control treatments. The burn depth in agarose hydrogel-treated and gauze-treated burns was 227 and 358 μm, respectively. Thus, it was concluded that immediate establishment of a moist environment and maintenance for 4 days reduced the amount of tissue necrosis by almost 40%. This picture is modified from Grolman et al.²⁹ ****p < 0.0001.

In addition, the controlled moist wound environment can allow precise delivery of topical treatments such as antimicrobials, analgesics, growth factors, and other bioactive molecules to the wound.^9,28,30 A moist environment is also desirable for cell and tissue transplantation. It has even been shown that the moist environment enables an orientation-independent transplantation of minced or meshed skin grafts.^13,14 In a porcine model, Zuhaili et al. applied split-thickness meshed skin grafts upside down (epidermis facing the wound bed) and demonstrated no difference in graft take or quality of healing, compared to conventional mesh grafting, in a moist wound environment.³¹ Another important benefit is that when wounds are treated using moist dressings, the patients experience less pain. The moist environment reduces the pain caused by the damaged nociceptors in the wound by preventing them from desiccating. In addition, moist wound dressings do not adhere firmly to the wound bed, whereas force is often required to remove dry dressings, causing severe pain.³²

As the cells like the moist environment, so do the microbes. Bacteria and other pathogens need water and nutrients to grow and therefore the moist wound environment provides conditions that support microbial colonization and proliferation. Despite these favorable conditions for microbial growth, it has been reported that wounds treated with moist dressings have had lower infection rates than wounds treated with dry dressings. Lawrence demonstrated that bacterial content of wounds under occlusive moist dressings was less compared with similar wounds treated with dry dressings.³³ Hutchinson and Lawrence showed that treating wounds with an impermeable occlusive dressing reduces infection rates by 50% in comparison to dry treatment.³⁴ Furthermore, Zhao et al. found that eschar resulting of dry wound healing contains a high biofilm bacterial load.³⁵ In addition, it has been suggested that both the humoral and tissue defense mechanisms are more active in a moist environment.³⁶

Discussion

Optimizing the wound microenvironment is essential for healing. Several factors such as temperature, pressure (positive or negative), hydration, gases (oxygen supply and CO₂), pH, microbial content, and antimicrobial treatment affect the healing outcome. Particularly, the moist wound environment has shown to provide the best conditions for faster and better wound healing by promoting tissue regeneration and mitigating infection, scarring, and pain. In wound care, wound dressings can be utilized to create, maintain, and control the desirable moist environment for healing (Fig. 2).³⁷ An optimal wound dressing should protect the wound, provide a moist environment, and maintain an appropriate wound temperature that promotes tissue regeneration, and be easy to apply and remove.¹⁵ Currently, based on the wound type, there are hundreds of wound dressings available on the market. Traditional dry wound dressings such as gauze (including wet to dry gauze) and bandages are dry. Until the 1960s, it was strongly believed that wounds healed better if kept dry. Now that multiple preclinical and clinical studies have extensively demonstrated the benefits of moist healing, often dry dressings have been replaced by modern dressings, providing a moist environment.³⁸ In a porcine model of full-thickness wounds, Vogt et al. compared gauze to a moist dressing (hydrocolloid) and demonstrated that the wounds treated with a moist dressing resulted in less necrosis and faster and better quality of healing.⁵ In a clinical trial of split-thickness skin graft (STSG), donor site dry dressings were compared to dressing providing moist wound environment and the results showed improved healing rates in wounds treated with the moist dressings. Therefore, dry dressings are nowadays only used as secondary dressings or to cover clean and dry wounds with mild exudate levels, and moist dressings have become the standard of care.³⁹ Before modern moist dressings, allografts and xenografts were commonly used as biological dressings to provide, in addition to other benefits, a moist wound environment.⁴⁰ Moist wound dressings can be divided into films, foams, hydrocolloid dressings, hydrogels, and alginate dressings. In this review, we are also including negative pressure wound therapy (NPWT) systems as moist dressings (Fig. 3). In addition, it should be mentioned that there are film-, foam-, hydrogel-, and hydrocolloid-based oxygen dressings (such as OxyBand Wound Dressing^TM and OxySpur^®) that provide a moist wound environment and simultaneously supply topical oxygen therapy from the dressing material to the wound.^41–43

Figure 2.

Optimizing the wound microenvironment is essential for healing. Several factors such as temperature, pressure (positive or negative), hydration, gases (oxygen supply and CO₂), pH, microbial content, and antimicrobial treatment affect the healing outcome. Wound dressings can be utilized to create, maintain, and control a moist environment for healing. In addition, NPWT is commonly used to accelerate the healing process. The WVTR of a wound dressing is significant because it directly regulates and adjusts the moisture level of the wound. The lower the WVTR of the dressing, the smaller the water loss, and the higher the WVTR of the dressing, the bigger the water loss. Another very important property of a wound dressing is its capacity to absorb fluid, which should match the amount of fluid the wound is producing to avoid accumulation of exudate or dehydration. NPWT, negative pressure wound therapy; WVTR, water vapor transmission rate.

Figure 3.

Moist wound dressings can be divided into films, foams, hydrocolloid dressings, hydrogels, alginate dressings, and NPWT dressings. Examples of different moist dressing types that are on the market.

Moist dressings

Film dressings are semipermeable and transparent polymer membranes (most commonly made of polyurethane) that are coated with an adhesive. They maintain a moist environment by having reduced permeability to water. Semipermeable film dressings are commonly used to cover minor burns, STSG donor site wounds, postoperative wounds, and a variety of minor injuries, including abrasions and lacerations. There are multiple semipermeable films on the market that mainly differ in relationship to their water vapor transmission rate (WVTR).^15,44–46 Tegaderm™ and Opsite™ are among the most commonly used film dressings.

Foam dressings are polymers, most commonly made of polyurethane, polyethylene, or silicone. The foams are designed to maintain a moist environment and absorb exudate from the wound. They usually consist of a hydrophobic permeable outer layer and a hydrophilic layer that is in direct contact with the wound bed. Typically, the foam dressings also contain a permeable polyurethane film with an adhesive that has a high WVTR and that is placed on the top of the foam and attaches the dressing to the peri wound skin. They are the most useful in moderate- to high-exuding wounds.^15,44–46 Common foam dressings include Tegaderm Foam, Lyofoam^TM, Allevyn^TM, and Mepilex^TM.

NPWT dressings are also moist dressings. NPWT dressings are most commonly made of foam or gauze and covered with a semipermeable membrane. A tube that connects the space under the membrane to a suction pump is attached to the dressing. Negative pressure creates an environment that has a high humidity under the membrane. NPWT is used to promote and accelerate healing in all kinds of acute and chronic wounds.⁴⁷ Common NPWT dressings include V.A.C^® Therapy Dressings (foam), Prevena^TM (foam), and Pico^TM (gauze).

Hydrocolloids are wound dressings comprising cross-linked matrix gelatin, pectin, and carboxymethyl cellulose sheets, powders, or pastes that together with adhesives usually are attached to a semipermeable film or foam. They maintain the moist environment by forming a gel with the wound fluid and maintaining their permeability. They are a good option for shallow and low-exuding wounds.^15,44–46 Common hydrocolloid dressings include DuoDERM^®, CombiDERM^TM, and GranuGel^TM.

Hydrogels are three-dimensionally cross-linked polymer networks composed of hydrophilic polymers (such as agarose, alginate, carboxymethyl cellulose, or collagen) with high water content. They are biocompatible, as they are structurally similar to the extracellular matrix. Hydrogels maintain the moist wound environment by delivering water molecules to the wound. They are used for many different types of wounds, including leg ulcers and pressure sores. Hydrogel dressings are available as amorphous (with no shape), impregnated into a secondary dressing such as gauze or foam and as sheets.^15,44–46 Commonly used hydrogels include IntraSite^⋄ Gel, Purilon^® Gel, and AquaSite^® Hydrogel Sheet.

Alginate wound dressings are nonwoven and nonadhesive fibers that are produced from calcium and sodium salts of alginic acid. Alginic acid is composed of mannuronic and galuronic acid residues and their ratio affects the chemical and physical properties of the alginate. A high concentration of mannuronic acid promotes gelling and high galuronic acid concentration promotes fiber integrity. When in contact with wound exudates, alginates partly dissolve and form a hydrophilic gel that establishes a moist wound environment. The alginate dressings with high mannuronic acid content gel rapidly and are therefore called rapid gelling fiber dressings.⁴⁸ Alginates are suitable for highly exuding wounds.^15,44–46 Examples of alginate wound dressings include Algisite^⋄ and Kaltostat^®.

There is no one dressing that fits all wound types. Moreover, as the healing of a wound progresses and the amount of exudate decreases, one single dressing may not be optimal for the different phases of healing. When choosing the best dressing for the wound, it is important to evaluate whether the wound is low or high exuding, condition of the peri wound area, location, wound size, and depth, as well as the risk/presence of infection.^15,36–38 The following pages will discuss and compare the different functional characteristics of the moist dressings such as WVTR, absorptive capacity, wear time, and antimicrobial properties (Table 2).

Table 2.

Moist wound dressing types, their indications, water vapor transmission rates, absorptive capacities, wear times, and examples brands

Dressing	Indication	WVTR	Absorptive Capacity	Wear Time	Examples
Film	Superficial low-exuding wounds	Moderate	None	Several days	TegadermTM, OpsiteTM
Foam	Moderate- to high-exuding wounds	Moderate	Moderate to high	A few days	AllevynTM, MepilexTM
NPWT	All kinds of acute and chronic wounds	Moderate	Moderate to high	Several days	PrevenaTM, PicoTM
Hydrogel	Full- and partial-thickness wounds	High	None to low	A few days	IntraSiteà, AquaSite®
Hydrocolloid	Shallow and low-exuding wounds	Low	Moderate	Several days	DuoDERMTM, CombiDERMTM
Alginate	Highly exuding wounds	High	Moderate to high	A few days	Algisiteà M, Kaltostat®

NPWT, negative pressure wound therapy; WVTR, water vapor transmission rate.

Water vapor transmission rate

One very important physical characteristic of a moist dressing is its ability to transmit water vapor between the wound and the external environment. WVTR, also known as moisture vapor transmission rate, is the steady state at which water vapor permeates the dressing at specified conditions of temperature and relative humidity. There are two methods to measure WVTR: the desiccant method and the water method. In the desiccant method, the dressing is sealed to the top of a dish that is filled with desiccant. The dish is placed in an environment with high relative humidity and after a predetermined testing time, it is measured how much moisture passes through the dressing and been absorbed by the desiccant. In the water method, a dish is filled with water and like in the desiccant method, the dressing is sealed to the top of the dish. The dish is placed in an environment with low relative humidity and then weighed to measure the amount of water vapor that has transferred from the dish during the test period. These different procedures lead to different results and therefore the standard test method of WVTR for materials addresses both testing procedures. Not all publications that report WVTR are clear about what method they have used, or even if they have followed the standard methods or some modified version, therefore, it is difficult to compare the WVTR values between different studies, unless they have used the same standardized method. Values are usually expressed in metric units as g/m²/24 h. WVTR from the intact skin is normally between 200 and 300 g/m²/24 h, whereas the water vapor loss from an open wound can be more than 10 times higher.^49,50 Wu et al.⁵¹ studied the WVTRs of different wounds using a probe evaporimeter and concluded that the water vapor loss mainly depends on the wound depth. They measured WVTRs for superficial, deep-partial thickness, and full-thickness burns, the rates being 427, 1,480, and 1,953 g/m²/24 h, respectively. In addition, the WVTR of a chronic ulcer (1,877 g/m²/24 h) was found to be at the same level as full-thickness burns. Hence, the deeper the wound, the bigger the water loss.⁵¹

An important function of the wound dressings is to contain the existing moisture in the wound environment and control evaporation of the moisture from the wound. Therefore, the WVTR of a wound dressing is significant because it directly regulates the level of hydration of the wound. The lower the WVTR of the dressing, the smaller the water loss, and the higher the WVTR, the bigger the water loss. As depicted in the Fig. 2, WVTR goes both directions. Generally, the discussion is the transfer of water mass away from the wound, but in some dressings, the environment can transfer water mass to the wound environment. To find a dressing with an optimal WVTR for the wound is important because too high WVTR can result in desiccation of the wound and too low WVTR may cause accumulation of wound fluid. Also, some wounds require moisture to be removed; other wounds require that all in situ moisture be maintained inside the wound dressing, while some wounds require the addition of moisture. In their study, Queen et al. suggested that a WVTR rate of 2,000–2,500 g/m²/24 h would provide adequate level of moisture without the risk of desiccation.⁵² In acute wounds, the amount of exudate from the wound is higher during the first 24–48 h, than at later times.⁵² A dressing that has a WVTR accommodating the early high level of exudate may desiccate the wound later. Conversely, a dressing with a lower WVTR appropriate for later healing may show accumulation of fluid under the dressing, requiring removal or dressing change.⁵²

Today, the semipermeable film dressings are usually composed of polyurethane that is impermeable to liquids and bacteria, but allow transmission of gases (water vapor, oxygen, and carbon dioxide) between the wound and the external environment. The WVTR of the films can be modified by adjusting the polymer and the film structure. It has been reported that WVTRs of >3,000 g/m²/24 h can be achieved. Xu et al. performed in vitro and in vivo experiments to determine an ideal WVTR for a semipermeable film dressing.⁵³ They used polyurethane membranes with graded WVTRs and concluded, using the water method, that the membrane with WVTR of 2,030 g/m²/24 h was the best in terms of maintaining an optimal moist environment for cells both in culture and in a murine model of full-thickness wound healing.⁵³ Thomas studied the WVTR characteristics of Tegaderm and Opsite film dressings and measured the WVTR both from the inside out and from the outside in.⁵⁴ The WVTR from the wound to the external environment in Tegaderm and Opsite was 794 and 839 g/m²/24 h, respectively, and from the external environment to the wound was 846 g/m²/24 h (Tegaderm) and 862 g/m²/24 h (Opsite), thus demonstrating very similar characteristics.⁵⁴

The foams usually consist of a hydrophobic outer layer that allows gas exchange and a hydrophilic layer that is in direct contact with the wound bed. Zehrer et al.⁵⁵ studied the WVTRs of Tegaderm, Allevyn, Mepilex, Biatain, Optifoam^®, and Versiva^® foams in vitro under wet (water method) and dry conditions (desiccant method). The study concluded that significant differences exist between the different foams in terms of WVTR. In dry conditions, the WVTRs of the foams varied from 80 to 1,620 g/m²/24 h (Tegaderm = 980; Allevyn = 1,620; Mepilex = 1,360; Biatain = 80; Optifoam = 570; and Versiva = 150), and in wet conditions, the WVTR was significantly higher in all foams varying from 830 to 12,750 g/m²/24 h (Tegaderm = 12,750; Allevyn = 11,340; Mepilex = 11,040; Biatain = 1,730; Optifoam = 1,540; and Versiva = 830).⁴⁶ White et al. also studied the WVTRs of different foams. They compared 22 foams to each other and similarly reported substantial variation in their VWTRs.⁵⁶

NPWT dressing that consists of a foam or gauze and a semipermeable film typically have high WVTRs like foam dressings. Also, importantly, negative pressure that creates a vacuum on the wound surface increases the WVTR of the foam dressing by sucking out exudate. Hudson et al.⁵⁷ studied the properties of Pico NPWT gauze dressing. The dressing is composed of four layers, which delivers negative pressure and removes wound exudate primarily through evaporative loss. They demonstrated that 80% of the exudate evaporated through the dressing and its upper layer that has a high WVTR.⁵⁶

Hydrocolloids are dressings with low WVTRs (around 500 g/m²/24 h) in comparison to films and foams. Once the polymers such as sodium carboxymethylcellulose, gelatin, or pectin get in contact with the wound fluid, they absorb water and form a gel. This makes them waterproof and practically impermeable to gases.⁵⁷ Hasatsri et al.⁵⁸ compared the WVTRs of different dressing and showed that hydrocolloids had the lowest WVTRs.⁵⁹ Wu et al.⁶⁰ studied the effect of DuoDERM hydrocolloid dressing on the WVTRs of full-thickness burns, deep-partial thickness burns, and chronic leg ulcers. Their study demonstrated that after placing the DuoDERM (WVTR around 500 g/m²/24 h, water method) on the wounds, their water vapor loss was reduced by 70–80%.⁵⁹

Hydrogels have high WVTR. Wu et al. studied the water vapor loss characteristics of some commercial hydrogel dressings using the water method and found their WVTRs to be more than 9,000 g/m²/24 h.⁶⁰ Balakrishnan et al.⁶¹ formulated a gelatin/alginate hydrogel and studied water vapor transmission loss from the hydrogel over time. They showed that water vapor loss from the wound through the hydrogel increased linearly over time. They also studied evaporative water vapor loss from the hydrogel and demonstrated that the water loss increased linearly for 2 days and was 30–40% after 24 h, and then increased to about 80% over 4 days. After that, no more water was lost and the hydrogel retained ∼15% of its water. In clinical practice, hydrogels are covered with a secondary dressing such as a semipermeable film that lowers the total WVTR.⁶¹

Alginate dressings are usually highly permeable. Yang and Hong⁶² studied water vapor loss characteristics of a dressing made of calcium alginate fibers (Algisite; Smith and Nephew) and demonstrated its WVTR to be 1,267 g/m²/24 h. In most cases, the alginate is combined with a secondary dressing that reduces the water vapor loss.⁶² It can be summarized that, in general, hydrogels and alginates have high, films and foams have moderate, and hydrocolloids low WVTRs (Table 2).

Absorptive capacity

Another very important property of a wound dressing is its capacity to absorb fluid. A dressing saturated with fluid usually requires to be changed. Therefore, it is important to find a dressing whose absorptive capacity matches the amount of fluid the wound is producing to avoid either accumulation of excess exudate or dehydration of the wound. Usually, wound dressing manufacturers inform the absorptive capacity of their dressings in metric units as grams or milliliters of fluid they have been shown to hold in a laboratory test. In addition, volume/area/time to absorb exudate nomenclature has been used.^{15,44–46,63}

Films are nonabsorbent dressings and thus excess wound exudate will accumulate underneath the dressing. This will maintain a moist environment, but if the exudate production is high, the film will not be able to hold all the fluid and will eventually start forming “blisters” or leaking, necessitating a dressing chance. Therefore, films are excellent for superficial, low-exuding wounds such as partial-thickness burns.^{15,44–46,63} In general, hydrogels are also nonabsorbent dressings and meant for low-exuding wounds. However, some hydrogels have an ability to absorb fluid because the polymers are only partially hydrated at the time of application. In addition, it has been shown that hydrogels lose about 40% of their water content within 24 h of application. Consequently, this water loss enables hydrogels to take up exudate from the wound.^61,64 Hydrocolloids can absorb small to moderate volumes of exudate. The polymers of hydrocolloid dressings absorb fluid forming a gel, but they cannot be used for wounds that generate high levels of exudate.⁶⁵

If the wound is generating moderate to high levels of exudates, the wound dressing needs to have a high absorptive capacity. Absorbent wound dressings include foams and alginates. Foams have a high WVTR and high absorptive capacity, and are therefore best suited for moderate- to high-exuding wounds. Alginates are very absorbent and can absorb up to 20 times their weight of fluid, making them suitable for highly exuding wounds.^{15,44–46,63} Salmerón-González et al.⁶⁶ studied and compared the absorptive capacity of alginates, foams, and hydrocolloids. In their study, 20 mL of saline was passed through each dressing (16 cm²) during 10 min at a pace of 120 mL/h, and absorptive capacity and final size of the dressing were measured. The results showed that the foam dressings (Biatain and Mepilex) were able to absorb 12.5 and 12.3 mL, respectively. The alginate dressings (Biatain and Kendall) and Comfeel hydrocolloid could take up 2.8, 2.2, and 0 mL, respectively. The results also demonstrated that the foam dressings increased up to 30% in size after absorbing the saline, whereas alginates and hydrocolloid kept their size, suggesting that in situations when the foams do not have room to swell (e.g., when placed under a compressive dressing), they would not be able to reach their absorptive capacity.⁶⁶

Typically, NPWT systems contain an exudate collection canister that can be emptied when needed. Therefore, high absorptive capacity is usually not needed in the NPWT dressings. However, there are NPWT systems on the market that no longer employ a canister, but allow any exudate produced by the wound to stay in or evaporate through a dressing that has a high WVTR. Malmsjö et al.⁶⁷ studied the fluid handling of Pico NPWT dressing that does not contain an exudate collection canister. In an in vitro model, 81.5 mL of simulated wound exudate was delivered through the dressing using negative pressure over 72 h (1.1 mL/cm²/24 h). Their results demonstrated that 14 mL of exudate remained in the absorbent layer of Pico and the rest had evaporated due to the high WVTR of the upper layer of the dressing.⁶⁷

Wear time

Wear time of a dressing can be difficult to predict and it mainly depends on the state of the wound. For example, any sign of wound infection usually results in a dressing change. However, the characteristics of the dressings also make a difference. Some dressings need to be changed daily, whereas some others can be left on the wound for several days.^{15,44–46,68} The most important factors that influence the dressing's wear time are its absorptive capacity and integrity in terms of its structure or attachment to the wound. When the dressing has reached its absorptive capacity, and cannot take up any more exudate, it should be replaced. Another common reason for dressing change is a leakage or detachment of the dressing caused by an adhesive failure at one or more edges. (If the label of a dressing recommends a certain duration of application, this should be followed).^{15,44–46,69}

Film dressings that are nonabsorbent and used for shallow wounds generating low volumes of exudate may be left in place for several days unless the clinical condition of the wound requires a change. As an example, the commonly used Tegaderm film is recommended being changed routinely at 7 days. Transparency of the films also makes them practical because the wounds can be monitored without removing the dressing. Hydrogels that are also meant for the management of low-exuding wounds can be left in place for several days. Since hydrogels lose their water and dry over time, they might need to be reapplied more often.^15,44–46 Changing interval for a commonly used Purilon hydrogel is every 1–3 days according to the manufacturer's protocol. There are also hydrogels such as IODOSORB^⋄ that change their color once it is time for a dressing change.⁷⁰ Hydrogels require a secondary dressing such as films and therefore the wear time of the secondary dressing and especially its WVTR also impact the gels' wear time. Foam dressings that are recommended for moderate- to high-exuding wounds are commonly left in place for 2–3 days. One disadvantage of the foams is that unlike the film dressings, they are opaque, which prevents monitoring of the wound without dressing removal. In addition, a secondary dressing is needed with the foam. Zehrer et al. studied wear times of multiple foam dressings in an in vivo artificial wound model, simulating a highly exudating wound. Their results demonstrated that the median wear time before dressing failure were for Tegaderm foam 7.0, Mepilex foam 2.5, Allevyn foam 1.0, Versiva foam 1.2, Biatain foam 3.2, and Optifoam 3.0 days, respectively.⁵⁵ NPWT dressings that are used to treat various acute and chronic of wounds to accelerate healing are commonly recommended to be changed every 2–3 days. Gauze-based Pico and foam-based Prevena can stay even longer and, according to their manufacturers' recommendations, can be left in place for up to 7 days.^57,67 Hydrocolloids are a good option for shallow, low-exuding wounds and their wear time is usually several days. As an example, the manufacturer of DuoDERM hydrocolloid dressing instructs that it can be worn continuously for up to 7 days unless it is uncomfortable, there is leakage, or there are clinical signs of infection. No secondary dressing is needed with hydrocolloids. Like foams, hydrocolloids are opaque, limiting monitoring of the wound.¹⁵ Solfest et al.⁷¹ measured the wear time of DuoDERM hydrocolloid dressing using in vivo artificial wound fluid model. The dressing was applied to the lower backs of 18 healthy volunteers and adjusted to simulate a moderately draining wound. Interestingly, their results showed a mean wear time of only 1 day for the hydrocolloid dressing.⁷¹ In a clinical study, Brown-Etris et al.⁷² used DuoDERM to cover pressure ulcers and studied its wear time. A total of 72 patients took part in the study and a mean wear time of 4.7 days was found for the hydrocolloid dressing.⁷² Alginate dressings are highly absorbent and proposed for all wound types with high exudates. Like hydrogels and foams, the alginates also require a secondary dressing and they are recommended to be changed every 5–7 days.^15,44–46 According to its manufacturer's instructions, the commonly used Algisite^⋄ M dressing can remain in the wound for 7 days. However, in a clinical study of exuding venous leg ulcers, Schulze et al. found that the alginates could be worn for an average of 3.1 days before dressing failure.⁷³

Antimicrobial properties

Infection control is a crucial part of management of both acute and chronic wounds. Wound infections compromise the healing process and can progress to sepsis and cause pain and discomfort to the patients. Wound infection is also a reason for premature dressing change. To treat and prevent wound infection, several antimicrobial agents have been incorporated into wound dressings to reduce the number of pathogens on the wound surface and within the dressing. The most commonly used antimicrobial agents include silver, iodine, and honey and the dressing manufacturers commonly have silver/iodine/honey versions of their films, foams, hydrogels, hydrocolloids, and alginates on the market. These dressings provide continuous release of an antimicrobial agent when applied on the wound, while also creating and maintaining the moist environment for healing.^74–76 There is an obvious need for a study that would compare the in vitro capacity of topical antimicrobial dressings to kill common bacteria found in wounds. It could probably be done with a technique similar to the testing of minimum inhibitory concentration of antibiotics and would provide valuable information on the antimicrobial properties of these different agents.

Silver is considered a broad-spectrum antimicrobial that can be used as a topical treatment in the management of superficially infected wounds. However, there is no agreement in the literature on its effectivity in infection control. Struik et al.⁷⁷ performed a randomized controlled trial on the incidence of surgical site infections (SSIs) after application of a silver hydrocolloid dressing (Aquacel Ag) after breast cancer surgery. Their results concluded that the silver dressing did not significantly reduce the occurrence of SSIs compared to a standard gauze dressing.⁷⁷ Kalemikerakis et al.⁷⁸ compared foam dressings with silver to foam dressings without silver in the care of malodorous malignant fungating wounds. Their results demonstrated that the wounds treated with silver-containing foams showed a significant reduction of the odor compared to foam dressings without silver. They concluded that silver dressings can reduce odor.⁷⁸ Carter et al.⁷⁹ studied the antimicrobial efficacy of silver-impregnated dressings for the healing of leg wounds and ulcers by performing a systematic review and meta-analysis of published randomized clinical trials. Their analysis concluded that silver-impregnated dressings improve the short-term healing of wounds and ulcers. Recently, silver-impregnated dressings that allow slow release of silver ions have been designed, which enables prolonged application.⁷⁹

Iodine, a chemical that is used extensively as a powerful disinfectant, is also considered a broad-spectrum antimicrobial, and like silver, it can be incorporated into dressings. Although there is evidence that supports the efficacy of iodine as an antimicrobial agent, it has been used cautiously in wound healing due to perceived issues with toxicity, systemic absorption, sensitivity reactions, and delayed healing. Therefore, silver-impregnated dressings are much more common.⁸⁰ Dumville et al. compared the effects of hydrocolloid wound dressings with silver, iodine, or hydrocolloid without an antimicrobial component on the healing of foot ulcers in people with diabetes. Their analysis demonstrated that there was no statistically significant difference in healing between the dressings.⁸¹

Honey's antimicrobial properties have been known for thousands of years and it has been used in wound treatment ever since. Honey is a biologic wound dressing that independently creates a moist wound environment. It has high osmolarity due to its high sugar content. When applied on the wound, the osmotic effect draws water out of the wound, creating a moist wound environment. Typically, honey is incorporated into dressings such as foams, hydrocolloids, hydrogels, and alginates. Multiple studies have demonstrated that when applied topically, honey clears wound infection. It has been shown to inhibit the growth of over 50 species of bacteria without developing microbial resistance.^82,83 In their clinical study with 69 patients, Lund-Nilsen et al.⁸⁴ compared influence of honey- and silver-impregnated dressings on wound healing. Their results showed both dressings improved wound healing, but no difference was found between the two regimens.⁸⁴ In addition to silver-, iodine-, and honey-based products, benzalkonium chloride (BlastX™ Antimicrobial Wound Gel)-, polyhexamethylene biguanide (Prontosan^® Wound Gel X)-, poloxamer 188 (PluroGel^®)-, octenidine (Octenilin^®)-, and hypochlorous acid (Anasept^®)-based antimicrobial hydrogels are available.

NPWT is also used to treat infected wounds. It has been suggested that NPWT systems remove pathogens from the treated wounds and therefore reduce the risk of infection.⁸⁵ Nuutila et al.¹¹ compared the effect of NPWT and silver sulfadiazine ointment on bacterial removal in artificially infected porcine wounds. The results showed that although no statistical difference between the groups was observed, there was a strong trend showing that NPWT dressing was more effective at reducing bioburden in the wound than the silver sulfadiazine ointment.¹¹ Similarly, Wang et al.⁸⁶ demonstrated NPWT reduced bacterial counts, virulence factors, and environmental DNA in a Pseudomonas aeruginosa wound infection model in rabbits. In addition, there are NPWT dressings on the market that have been coated with silver to add an antimicrobial feature to the therapy.⁸⁶

Future Directions

Existing dressings provide limited information about the status of the wound; thus, repeated examinations of the wound by medical professionals are required, which add to health care costs. This problem is more severe in a remote setting where medical professionals are less likely to be available. There are already a few smart dressings on the market and many more in development, which monitor and collect information about the status of the wound through integrated oxygen, CO₂, pH, moisture, and temperature sensors. Pang et al.⁸⁷ developed a smart dressing that is composed of a silicone layer with integrated sensors capable of monitoring wound temperature to diagnose infection. The lower layer of the dressing is an antibacterial hydrogel providing on-demand release of antibiotics to treat infection.⁸⁷ Qiao et al. fabricated a smart hydrogel that contains pH-responsive fluorescent nanoprobes to detect bacterial infection.⁸⁸ Similarly, Tamayol et al. engineered a hydrogel that allows wireless real-time monitoring of the wound pH levels.⁸⁹

Smart dressings with integrated micro sensors allow wireless real time monitoring of the wound that would be very advantageous for wound care providers and potentially very cost effective for the health care system.⁸⁹ This way the state of the wound as well as the wear time of the dressing could be assessed without dressing removal or visit to the wound care center. In addition, an ability to adjust their WVTRs based on the exudate level of the wound would be useful. This way an optimal level of moist treatment would be maintained throughout the treatment.

Gas flow wound therapy is another emerging moist wound dressing. It implements a wound dressing technology that applies a gentle flow of gas into the wound environment. The gas flow is capable of providing moisture to or removing moisture from the wound by adjusting the relative humidity of the gas flow. Other parameters such as oxygen, pH, temperature, and pharmaceuticals can also be provided in the gas flow. Case studies result indicate significant improvements in the time to heal, but formal clinical trials are needed to determine the efficacy of this gas flow wound therapy moist wound treatment approach.⁹⁰

The authors have developed a wound dressing that they call Platform Wound Device (PWD). It consists of an almost impermeable polyurethane membrane (WVTR = 122 g/m²/24 h and oxygen transmission rate = 1,200 mL/m² · 24 h). It has an adhesive base and a bellows structure (allowing containment of exudate) that form a self-contained space over the wound. The PWD is designed to be applied as soon as possible after injury and to be used throughout the treatment period, providing a moist environment to facilitate healing. It can be used either alone as a primary dressing or as a secondary dressing, for example, for a hydrogel. The PWD has a port that allows easy administration of liquids and hydrogels to the wound. The interior surface of the dressing contains pyramid-like structures protruding toward the wound and creating a three-dimensional space above the wound surface to facilitate distribution of topically administered liquid- and gel-based therapeutics. The PWD has been validated in multiple preclinical and in a few clinical studies.^6,9–11,91 In addition, it can be used as an NPWT device without foam or gauze.⁹² Current development includes manufacturing of dressings that can enclose an entire limb and integration of pH, oxygen, and temperature microsensors allowing wireless, real-time monitoring of the wound.

Summary

Wounds heal faster in a moist environment. The moist wound environment facilitates autolytic debridement, reduces pain, reduces scarring, activates collagen synthesis, facilitates and promotes keratinocyte migration over the wound surface, and supports the presence and function of nutrients, growth factors, and other soluble mediators in the wound microenvironment. It has also been shown that immediate wound treatment with moist dressings decreases tissue loss and mitigates wound progression, especially in burns. Many studies have demonstrated that healing in a moist environment results in less inflammation than in a dry environment and thus also improves the quality of healing (less scarring). In addition, the moist environment provides a good platform for cell and tissue transplantation, as well as for topical application of drugs and active soluble molecules such as growth factors (Table 1).^1–12

Wound dressings can be utilized to create, maintain, and control the desirable moist environment for healing. One of the most important physical properties of a moist dressing is its ability to transmit water vapor between the wound and the external environment. To find an optimal WVTR for the dressing is important because too high VWTR can result in desiccation of the wound and too low WVTR may cause accumulation of wound fluid. Another very important property of a wound dressing is its capacity to absorb fluid. A typical reason for a dressing change is that the dressing's ability to handle exudate has been reached or exceeded. Therefore, it is desirable to find a dressing whose absorptive capacity matches the amount of fluid the wound is producing to avoid accumulation of exudate or dehydration. In addition, together with the dressing's integrity, the absorptive capacity impacts the dressing's wear time. Antimicrobial properties are important characteristics for a dressing, especially when treating an infected wound. Moist wound dressing can be divided into semipermeable film and foam dressing, hydrocolloid dressings, hydrogels, alginates, and NPWT dressings (Table 2).^{15,44–46,75}

Take-Home Messages

• It has been shown that moist environment has several benefits that result in faster and better quality of healing.

• The moist environment facilitates autolytic debridement, reduces pain, reduces scarring, activates collagen synthesis, facilitates and promotes keratinocyte migration over the wound surface, and supports the viability and function of nutrients, growth factors, and other soluble mediators in the wound microenvironment.

• Wound dressings can be utilized to create, maintain, and control the optimal moist environment for healing.

• Moist wound dressings can be divided into films, foams, hydrocolloid dressings, hydrogels, alginate dressings, and NPWT dressings.

• The WVTR of a wound dressing is significant because it directly regulates and adjusts the moisture level of the wound. The lower the WVTR of the dressing, the smaller the water loss, and the higher the WVTR of the dressing, the bigger the water loss.

• Another very important property of a wound dressing is its capacity to absorb fluid, which should match the amount of fluid the wound is producing to avoid accumulation of exudate or dehydration.

• The most important factors that influence the dressing's wear time are its absorptive capacity and integrity in terms of its structure or attachment to the wound.

Abbreviations and Acronyms

GF

growth factor

NPWT

negative pressure wound therapy

PWD

platform wound device

SSI

surgical site infection

STSG

split-thickness skin graft

WVTR

water vapor transmission rate

Author Disclosure and Ghostwriting

Both authors are employed by Applied Tissue Technologies, LLC, that manufactures the Platform Wound Device dressing, which is depicted in the discussion of this review. No ghostwriters were involved in the writing of this article.

About the Authors

Kristo Nuutila, MSc, PhD, is a burn and wound healing researcher. He currently works as the Director of Research at Applied Tissue Technologies. In addition, he holds a position at Brigham and Women's Hospital of Harvard Medical School and is an adjunct professor of pharmacology at the University of Helsinki, Finland. Elof Eriksson, MD, PhD, is the Joseph E. Murray Professor Emeritus of Plastic Surgery at Harvard Medical School. He was the chief of plastic and reconstructive surgery at Brigham and Women's hospital in Boston from 1986 to 2016. He has been conducting NIH- and DoD-funded wound healing research for over 25 years. Currently he works as the Chief Medical Officer of Applied Tissue Technologies.