Stem cell and tissue engineering approaches in pressure ulcer treatment

Abstract

Context:

Pressure ulcers or injuries, arise from ischemic damage to soft tissues induced by unrelieved pressure over a bony prominence. They are usually difficult to treat with standard medical therapy and often they recur. In the search for better treatment options, promising alternative forms of treatment are today emerging. Within the field of regenerative medicine, ongoing research on advanced therapies seeks to develop medicinal products based on gene therapy, somatic cell therapy, tissue-engineering and combinations of these.

Objective:

The main objective is to perform an overview of experimental and clinical developments in somatic cell therapy and tissue engineering targeting the treatment of pressure injuries.

Methods:

Searching terms as “PRESSURE ULCER”, “STEM CELL THERAPY”, “TISSUE ENGINEERING” or “WOUND HEALING” were used in combination or alone, including publications refered to basic and clinical research and focusing on articles showing results obtained in a clinical context. A total of 80 references are cited, including 23 references published in the 3 last years.

Results:

The results suggest that this form of treatment could be an interesting option in patients with difficult-to-treat ulcers as spinal cord injury patients.

Conclusion:

This field of regenerative medicine is very broad and further research is warranted.

Introduction

Pressure ulcers (PU), or pressure injuries as recently named,¹ are defined as localized damage to the skin and/or underlying tissue (epithelial, dermal and subcutaneous tissues, such as fat or muscle). They usually appear over a bony prominence or at the site of medical devices (or others), and are the consequence of severe and/or prolonged pressure, sometimes involving shear and friction forces. The pathophysiological explanation for how a PU arises is that sustained external pressure inhibits blood flow, and this leads to local tissue ischemia and eventually to necrosis. An ischemic episode is usually followed by reperfusion of affected tissue (once external pressure ceases). This can also provoke damage through the generation of reactive oxygen species and the onset of a pro-inflammatory cytokine cascade impacting the extracellular matrix.² About 70% of PUs in adults appear over the bony prominences of the sacrum, ischial tuberosity or greater trochanter, while 15% to 25% affect the lower extremities, typically the heel or lateral malleolus.³

While several staging guidelines are available for pressure injuries, or ulcers, six- stage classification systems have been widely accepted internationally. Thus, both the last version of the International Classification of Diseases (ICD-11, published in 2018 by the WHO), and the 2019 NPIAP/EPUAP/PPPIA International Guideline, differentiate four stages: I (or 1), intact skin with a localized area of non-blanchable erythema; II (or 2), partial thickness loss of skin with exposed dermis; III (or 3), full thickness skin loss with possibility of subcutaneous fat visibility; and IV (or 4), full thickness skin and tissue loss with exposure of tissues (fascia, muscle, tendon, ligament, cartilage or bone) in the ulcer (Fig. 1). Two further categories complete the six stages: unstageable, full thickness skin loss with depth of the ulcer completely obscured by slough; and “suspected” deep tissue ulcer, localized area of soft tissue damage which is anticipated to evolve into a deep PU.

Figure 1.

Schematic representation of pressure ulcer (PU) staging: Stage I (or 1), intact skin with a localized area of non-blanchable erythema; II (or 2), partial thickness loss of skin with exposed dermis; III (or 3), full thickness skin loss with possibility of subcutaneous fat visibility; IV (or 4), full thickness skin and tissue loss with exposure of tissues (fascia, muscle, tendon, ligament, cartilage or bone) in the ulcer; unstageable, full thickness skin loss with depth of the ulcer completely obscured by slough; and “suspected” deep tissue ulcer, localized area of soft tissue damage which is anticipated to evolve into a deep PU.

Pressure injury management is one of the most challenging clinical problems in hospitals. The risk of developing PU is increased by limited mobility and/or tactile sensation, frequently associated with poor nutrition and subsequent loss of muscle and body mass. This determines a high prevalence and burden of PU in older adults and patients in intensive and long-term care settings.⁴ Individuals with spinal cord injury (SCI) are a subset of patients who are especially prone to PU development due to factors including motor impairment, sensory loss; skin changes and prolonged hospital stay.⁵ In effect, PU are the most common secondary complication observed in SCI patients, with an overall pooled prevalence of 32.36%, although this figure could be much higher in middle to high income countries.⁶ The described impacts of PU are reduced quality of life due to physical, social and psychological factors,⁷ high economic costs for the individual and health care system arising from a need for recurrent hospitalizations, longer hospital stays and multiple surgeries, along with potentially devastating complications and significantly increased morbidity and mortality.⁸ Indeed, in second place only to arterial ulcers, PU have been described as the costliest chronic wounds⁹ and these costs increase with their severity due to longer healing times and a higher incidence of complications.¹⁰

It is widely accepted that costs associated with PU treatment, especially for the more severe cases, are substantially higher than those related to their prevention.¹¹ Pressure ulcer prevention comprises different strategies that aim to reduce the magnitude and duration of mechanical load (repositioning, mobility promotion, support surfaces or off-loading, among others) or enhance tissue tolerance (preventive skin care, nutrition or electrical stimulation).¹² The treatment of a PU, is dependent on PU stage: while stage I and II injuries can be treated by applying occlusive or non-occlusive dressings as well as maintaining a clean wound environment to avoid contamination, stage III and IV lesions, in which necrotic tissue is present, require debridement, dressings and topical agents, as well as additional therapies such as negative-pressure wound therapy, electromagnetic therapy, phototherapy, therapeutic ultrasound, growth factors or even reconstructive surgery.¹³ Surgery is usually indicated after failure of conservative measures, and usually commences with surgical debridement (involving excision of the fibrotic capsule or bursa) followed by the selected surgical method (primary wound closure, skin graft, flaps or tissue expansion) based on patient and PU characteristics.¹⁴ However, while serious complications are rare, post-operative complications are relatively common. In the case of SCI patients, surgery and anesthesia are especially challenging, as there is a risk of both spinal cord dysregulation¹⁵ and postoperative infection.¹⁶

Advanced therapies represent an emerging research field of medicinal products based on gene therapy, somatic cell therapy, tissue-engineering (scaffolds with or without cells) and their combinations. These therapies, offer healing options to manage several diseases for which there are limited or no treatments. In this context, several approaches have been developed in the last few years to improve the treatment of PU. In this review, we focus on recent advances in somatic cell therapy and tissue engineering, alone or combined, designed for this purpose (summarized in Table 1).

Table 1.

Cells and scaffolds used in pressure ulcer (PU) treatment.

Cell	Scaffold	Recipient	PU stage	References
ASC	–	Mouse	III	Strong, AL et al.19
ASC	–	Mouse	N.I.	Bukowska J et al.20 Bukowska J et al.21 Xiao S et al.22
WJ-MSCs	–	Goat	Full-thickness Wound (III–IV)	Azari O et al.23
BM-MNCs	–	Human	III–IV	Sarasua J et al.25
–	Collagen	Human	N.I.	Ichioka S et al.36
–	Chitosan	Mouse	N.I.	Park CJ et al. 42
–	Chitosan	Human	II	Campani V et al.49
–	Chitosan	Human	N.I.	Xiaohui M et al.50
–	Hyaluronic Acid	Human	Prevention	Beniamino P et al.56
–	Hyaluronic Acid + PRGF	Human	II–III	Ramos-Torrecilla J et al.59
–	Silk	Mouse	N.I.	Seo SR et al. 63
–	Acellular dermal matrix	Human	IV	Jeon M et al.66
–	Acellular dermal scaffold	Human	N.I.	Strauss NH et al.67
Native cells	Placental membrane allograft	Human	IV	Golla D et al.68
Native Cells	Placental membrane allograft	Human	N.I.	Suzuki et al.69
Native Cells	Amniotic membrane	Human	II–III	Dehgani M et al.70
–	Amnion/Chorion membrane	Human	N.I.	Garoufalis M et al.71
–	Amnion/Chorion membrane	Human	II–III	Berhane CC et al.72
–	PLLA + gelatin + PRF	Human	III–IV	Sun H et al.74
BM-MSCs	Albumin	Rabbit	Full-thickness wound (III–IV)	Feldman DS et al.75
Fibroblasts + Keratinocytes	NaCMC + Collagen	In vitro	–	Bektas CK et al.76
WJ-MSCs	PVA	Dog	N.I.	Ribeiro J et al.77
Fibroblasts	Collagen	Human	N.I.	Kuroyanagi Y et al.78
Native cells	Dermal micrografts ± Collagen	Human	N.I.	De Francesco F et al.79
BM-MSCs	Fibrin	Mouse	Full-thickness wound (III–IV)	Falanga et al.80

Abbreviations: ASC, Adipose Tissue-Derived Stem Cells; WJ-MSCs, Wharton’s Jelly Mesenchymal Stem Cells; BM-MNCs, Bone Marrow-Mononuclear Cells; BM-MSCs, Bone Marrow Mesenchymal Stem Cells; PLLA, Poly(L-lactide) acid; PRF, Platelet Rich Fibrinogen; NaCMC, Sodium Carboxymethyl Cellulose; PVA: Poly(Vinyl Alcohol); N.I., Not Indicated.

Somatic cell therapy

Several literature studies conducted both in animal models and as clinical trials, have shown the enhanced healing of PU in response to cell therapy. Many of these animal studies have been based on a simple, non-invasive mouse model of PU developed by Stadler and colleagues.¹⁷ In this model, PUs are induced by three cycles of ischemia/reperfusion (I/R) using externally placed magnets that compress the skin on the back of the animal. Several research groups have confirmed that I/R can induce tissue damage associated with the pathogenesis of PU.¹⁸ Using this or similar models, several investigations have provided promising results for clinical translation.

Adipose tissue-derived stem cells (ASC) are one of the cell types assessed by several groups for their effectiveness in PU healing. Among these studies, we should mention that by Strong et al.,¹⁹ who examined the use of ASC to repair PU in young and elderly mice. These authors proposed that ASC promotes the healing of PU in both young and old mice via adipogenic differentiation and regeneration of the skin’s architecture.¹⁹ Other authors have also tested ASC to treat PU in young and older mice in the search for applications in patients. Bukowska et al.²⁰ observed that fresh and cryopreserved human ASC accelerated and enhanced PU healing in young mice of both sexes. However, in older mice, fresh ASC significantly improved wound closure compared to cryopreserved ASC.²⁰ This same group confirmed the safety and efficacy of human ASC isolated using the Icellator^®, a closed-system device designed for clinical practice that guaranteed a totally enclosed and automated process, being effective with minimal amount of adipose tissue.²¹ Until the publication of the work by Xiao et al.²² comparing the properties of diabetic and nondiabetic ASC for the treatment of PU, the characteristics and abilities of this kind of cells were controversial and the administration of ASC from diabetic patients had not been documented. Using a mouse model, they observed that diabetic ASC promoted PU healing and wound repair by modulating inflammation, promoting angiogenesis, enhancing collagen deposition, and increasing re-epithelization.²²

Besides ASC, other authors propose the use of (mesenchymal stem cells) MSC from other sources such as Wharton’s jelly (WJ-MSC) to treat skin injuries. Wharton’s jelly is the mucoid connective tissue enclosing the three umbilical vessels, or matrix, of the umbilical cord. Azari et al.²³ examined the effects of transplanted MSCs from such tissue on cutaneous wound healing. Using a goat skin incision model treated with WJ-MSC, these authors observed in their histopathological study that re-epithelialization was complete at 7 days in the treatment group, while in the control group, wound healing was still incomplete 12 days after incision.²³ In their search for the ideal cell type to regenerate PU, Yoon et al.,²⁴ based on the key role of fibroblasts in skin regeneration, examined the efficiency of these fibroblasts to treat PU in an animal model. To avoid the use of dermal fibroblasts obtained from a biopsy, as the supply of good quality cells from skin is limited, they used human embryonic stem cell-derived MSC differentiated into fibroblasts. These authors argued that these cells have ideal characteristics for wound healing and based on their results they concluded that they show promise as a commercial cell therapy source to treat PU.²⁴

Somatic cell therapy to treat PU has also been examined in clinical trials. Actually, there are two registered trials that used ASC: one is still active (NCT02375802), while the other (NCT02092870), which treated chronic wounds, including pressure ulcers, has been completed, although the results do not seem to have been published yet. Moreover, in a clinical trial performed by our group,²⁵ PUs were debrided and directly sutured through minimally invasive surgery following the application of autologous bone marrow mononuclear cells (BM-MNC) in patients with SCI (NCT01572376, EudraCT 2008-003015-12). According to our results, 86.36% of the PUs treated with these cells had fully healed after a mean time of 21 days. Moreover, during a mean follow-up duration of 19 months, none of the resolved PU recurred. We believe BM-MNC could be a good option to treat grade 4 PU in patients with SCI, avoiding major surgical intervention.

Wound healing is a complex process involving a cascade of events and various types of cells. This is why researchers have focused on somatic cell therapy to treat different skin conditions. The use of cells to treat PUs seems interesting. However, sometimes because of PU size and/or location, it is not possible to administer a cell suspension. In this context, tissue engineering, using scaffolds alone or combined with cells, provide some therapeutic advantages.

Tissue engineering

Tissue engineering seeks to develop strategies that will promote tissue and organ regeneration,²⁶ aiming at producing functional and living human tissue in vitro that can be implanted to replace damaged tissues and organs.²⁷ Moreover, classical tissue engineering approaches involve the extraction of cells from humans, their expansion and seeding in a biomaterial. These processes, sometimes, are followed by dynamic culture in order to increase cell growth.

For chronic wounds like PU, when skin self-regeneration takes place under adverse conditions due to the destruction of the dermis and underlying tissues,²⁸ the use of a supportive scaffold is necessary. This scaffold is designed to induce the generation of an artificial extracellular matrix which, when introduced in the wound bed, will attract both cells and growth factors supporting skin regeneration.²⁹ Scaffolds also allow for the infiltration of surrounding cells³⁰ and diffusion of nutrients and oxygen. To treat PU, tissue engineered scaffolds should be able to withstand pressure, shear and/or friction when used on an area such as the lower back.³¹ Over the years, numerous scaffolds have been designed for the treatment of PU either for use on their own or as carriers of various components delivered to the wound site. Treatment using different kinds of scaffolds is especially useful in patients who are not able to tolerate major or invasive flap surgery. These scaffolds can be naturally derived or synthetically manufactured,³² or may even be a combination of both natural and synthetic materials.³³ Natural materials have the beneficial properties of biocompatibility, biodegradation, bioactivity, cellular adhesion, low toxicity and low chronic inflammatory response. Examples of natural materials are collagen, gelatin, hyaluronic acid, chitosan, alginate, elastin and silk fibroin. In this context, it is also possible to find human body products including amniotic membrane, Wharton’s jelly (used directly or decellularized), or platelet-rich plasma.³⁴ In order to improve their mechanical properties, these biological products are usually combined with synthetic polymers.³⁵

Natural scaffolds

Many scaffolds based on natural materials have been developed to treat PU. In an article published by Ichioka et al.,³⁶ the use was described of a collagen matrix dermis substitute (Terudermis^®, Terumo Corp) in 9 patients with grade IV sacral PU.³⁶ Terudermis^® is an atelocollagen matrix with a silicone layer especially developed to treat deep wounds in which bone and/or tendon are exposed.³⁷ After cleaning and debridement, bony prominences were planed using an osteotome. Next, the artificial dermis was grafted and an alginate gel dressing was placed on the PU. After 84.1 ± 8.2 days of treatment, healthy granulation and coverage tissue along with complete epithelialization were observed. In control patients treated with conservative therapy (n = 6), these observations were made at 215.3 ± 38.4 days. This scaffold also provides an increase in vascular density, suggesting that the use of a collagen matrix promotes angiogenesis.

Chitosan (CHIT) is a natural polysaccharide composed of units of glucosamine that have positive charge at physiological pH, making CHIT adhesive and ensuring longer persistence at the application site.³⁸ This material also shows antimicrobial activity³⁹ and promotes homeostasis⁴⁰ and angiogenesis.⁴¹ CHIT scaffolds loaded with basic fibroblast growth factor (bFGF) contained in gelatin microparticles were developed and tested in an aged mouse model of PU induced with magnets. Animals were assigned to a control group, CHIT group and CHIT + bFGF group. Findings indicated that both the CHIT and CHIT-bFGF treatments significantly accelerated wound closure as compared to controls for up to 7 days, suggesting that CHIT is a good candidate for the treatment of chronic wounds. While bFGF induced significantly more angiogenesis, wound closure was not more rapid when compared to the CHIT-only treatment.⁴² The properties of CHIT promoting the healing of acute wounds have been also identified in previous studies.⁴³ These properties include its capacity to attract neutrophils in vivo⁴⁴ and in vitro.⁴⁵ Neutrophils are the major cell type involved in ischemia-reperfusion injury,⁴⁶ so they play a major role in the development of PU.⁴⁷ As a frequent component of CHIT scaffolds, some studies have assessed the properties of bFGF. This growth factor has been found to have angiogenic capacity⁴⁸ and this characteristic is combined with the protection activity exerted by CHIT in proteolytic environments, giving rise to an optimal situation in the setting of a marked inflammatory response as in PU. To prepare a sterile formulation of CHIT for use in a clinical context, Campani et al.⁴⁹ conducted a clinical pilot study in 20 patients with grade II PU treated with CHIT gel. After 30 days of treatment, significant reductions in PU dimensions were observed in most of the patients (over 90%); with complete healing in 20% of these cases, and an ulcer surface area reduced by more than 50% in 50% of patients. In response to treatment, no adverse reactions were produced. The main limitation of this study was that all the PUs treated were superficial.⁴⁹ Thus, these encouraging results need to be confirmed in higher severity PU. In another clinical study, the use of acylated CHIT was explored.⁵⁰ This formulation is especially recommended to treat chronic wounds with high exudate levels. Participants were 90 patients with chronic wounds, six of which were PU (no information on grade or size were provided). Half of the patients were assigned to the control group. Four weeks after treatment, the reduction in wound surface area was significantly greater in the treatment group. Moreover, the healing rate of infected wounds in the acylated chitosan group was significantly higher than in the control group (43% vs. 11.7%). Further observations were less pain, reduced wound depth and a lower mean exudate level in the CHIT group starting from the 4th week of treatment, meaning that chitosan’s effects are not immediate.

Hyaluronic Acid (HA) is a natural polymer present in the extracellular matrix (ECM) that plays a role in processes involved in wound healing such as cell migration, adhesion and proliferation. These processes facilitate the entry of cells to the wound site, contributing to ECM orientation.⁵¹ HA has a high viscosity and hydrophilic capacity that contributes to parenchyma architecture and porosity.⁵² It also plays a relevant role in angiogenesis and inflammatory events.⁵³ The efficacy of an HA-matrix has been reported to treat other kinds of ulcers such as diabetic foot ulcers⁵⁴ or leg ulcers.⁵⁵ To treat skin areas at risk of developing a PU, the use of a cross-linked HA scaffold has been described.⁵⁶ After careful disinfection of the skin in 15 participants, 30-50 ml of cross-linked HA were injected in the lateral side of erythematous edges under local anesthesia, and the zone sealed with Tegaderm™ dressing. Erythema disappeared 2–4 days after injection, and three months later, no PU was detected, suggesting a protective role of the scaffold able to prevent the appearance of PU. In other studies,⁵⁷ HA was used in combination with platelet-rich growth factor (PRGF) to treat chronic ulcers. This autologous derived blood product containing multiple growth factors with associated antimicrobial properties is obtained from fresh blood and is essential for tissue regeneration.⁵⁸ One of these studies included 100 patients with stage II–III PU, assigned to one of four study groups: standard care control group, receiving one dose of PRGF, two doses of PRGF, or two doses of PRGF plus HA. Complete treatment consisted of the steps PU debridement, cleaning with saline, and application of a liquid hydrogel and polyurethane dressing. After 36 days of treatment, PU surface area was significantly reduced in all groups compared to a control group, with greater reductions recorded in the patients given two doses of PRGF plus HA. These observations indicate that the incorporation of HA leads to a reduced PU surface area and increased likelihood of complete wound closure. Results, however, revealed no significant differences in wound healing between groups.⁵⁹

Silk is another natural biopolymer used in a great variety of tissue engineering and regenerative medicine applications. The biological properties (regeneration of collagens and dermal cells) and chemical composition of silk fibroin extracted from the cocoons of silkworms have been well characterized.⁶⁰ These properties include immunocompatibility,⁶¹ as well as excellent adhesiveness to the skin.⁶² In a PU animal model, Seo et al.⁶³ tested a β-cyclodextrin/polyethylenimine/silk fibroin xerogel.⁶³ This gel showed a high wound healing efficacy when used to treat PU created on the backs of mice. Exudates were confined to the wound area containing neutrophils and epithelial growth factor, promoting wound healing with no scar formation.

Finally, decellularized mammalian tissues, processed to remove cells while maintaining ECM components, could be an effective strategy to treat difficult-to-heal wounds, as has been reported by some investigators.^64,65 These matrices contain a framework of insoluble molecules like collagen, elastin or fibronectin, which promote cell attachment and proliferation, inducing re-epithelization and granulation tissue growth. Jeon et al.⁶⁶ used a paste-type acellular dermal matrix, derived from donated human skin, called CGPaste (CGBio Inc.) to treat chronic wounds (n = 7) at different sites (including three PU) where skin grafting or flap surgery cannot be easily performed. Briefly, after surgical debridement, the CGPaste was injected into the wound and then subjected to negative pressure wound therapy (NPWT). One of the three patients with PU showed the longest wound healing time (4 weeks), and in another patient, the PU failed to close. The authors concluded that CGPaste was effective in treating small, deep, chronic ulcerative wounds <10 × 10 mm and <10 mm in depth with granulation tissue-based wound beds.⁶⁶ Strauss et al.⁶⁷ designed a study (n = 49) to assess the use of an acellular fetal bovine scaffold (PriMatrix Dermal Repair Scaffold, TEI Biosciences Inc.) with a high percentage composition of type I and type III collagen to treat difficult-to-treat wounds of different etiology. Briefly, after wound debridement, the scaffold was applied following its rehydration in 0.9% sterile saline, fixed with staples and the wound site covered with petrolatum gauze. This treatment was used once in 11 patients who had a total of 14 PU at different sites. The wound healing rate was over 75% and the healing time was about 9–12 weeks. After application, the authors noted the rapid absorption of blood into the implant and stabilization of fibrin clot by collagen fibers. Subsequently, the scaffold sequestered cells including progenitor cells and activated platelets which release growth factors involved in cell migration, proliferation and angiogenesis.⁶⁷ Golla et al.⁶⁸ examined the use of a cryopreserved placental membrane containing viable cells (vCPM) (GrafixCORE, Osiris Therapeutics) after surgical debridement and prior to muscle flap closure, without the need for anchoring or suturing. This approach was used to treat four paraplegic patients with PU at different body sites. All patients showed complete wound closure after a mean period of 7 weeks, without complications or recurrence detected over one year of follow up.⁶⁸ Prior to this publication, Suzuki et al. had reported good clinical outcomes of the use of vCPM (n = 12) to manage non-healing wound with exposed bone, tendon and muscle. In all cases, complete wound closure was achieved without the need for a concomitant intervention.⁶⁹ In 2017, Dehgani et al. reported results obtained in a clinical trial comparing cryopreserved amniotic membrane (AM) with standard debridement and cleaning with phenytoin powder in patients (n = 24) with PU grade II and III. Complete healing was observed in 75% of patients in the AM group, and mean healing time was 20 days vs. 54 days in the standard care group.⁷⁰ In a later retrospective study, Garoufalis et al.⁷¹ treated 117 patients with six different types of non-healing wounds, including 16 PU, with dehydrated human amnion/chorion membrane (dHACM). In 12 patients, a minimum wound size reduction of 50% was observed within four weeks of treatment.⁷¹ The varying results obtained in these two studies could be explained by differences in wound severity and frequency of application. In a recent paper, results of the use of PURION processed dHACM (EpiFix, MiMedx Group Inc.) in 10 patients with grade II/III PU were reported. The allograft was placed on the wound bed, hydrated with saline (if necessary) and a dressing applied to cover the wound. Over the study period, 9 of the 10 ulcers were reduced in size and three completely healed. Collectively, these findings indicate that AM contains active growth factors (VEGF or PDGF, among others) and other biomolecules that help suppress inflammation and confer the allograft immune-privileged properties.⁷²

Synthetic scaffolds

Synthetic biomaterials such as polyvinyl alcohol, polyethylene glycol (PEG), polyurethane, poly L-lactic acid (PLLA), polyglycolic acid (PGA) and poly lactic-co-glycolic acid possess tailor-made properties allowing the manufacture of a scaffold with a defined shape and pore size, even in combination with natural materials.

To treat clinically challenging PU, a novel two-component hybrid dermal scaffold was designed by the Sharma group. The scaffold prepared from fibrin and alginate porous hydrogel in combination with a plasma-polymerized silicone membrane. This membrane binds firmly to the fibrin/alginate component adding strength to the scaffold without affecting its cellular properties. However, the article only describes how the scaffold is prepared and its properties, without testing its use in a preclinical animal model or in patients.⁷³ Moreover, Medprin Biotech GmbH has developed a synthetic 3D-printed scaffold made of PLLA and gelatin. The scaffold was designed to mimic human ECM microstructure, and is manufactured in two forms: a powder of particle diameter 400–800 microns that can be applied on its own or as a hydrogel in combination with platelet rich fibrin (PRF) or as a membrane 0.5 mm thick. After debridement and disinfection, the scaffolds were tested in 5 patients with stage III and IV PU. The powder form is mainly recommended for large deep wounds. When applied alone the scaffold showed good wound adherence and induced the formation of a fibrin layer, enhancing re-epithelialization and wound closure. When mixed with PRF to form a hydrogel, adherence and granulation tissue growth were improved as was re-epithelialization of the wound. The mean healing time was 32.2 days such that this type of treatment seems faster than conventional therapy.⁷⁴ However, as NPWT was used before surgery in some cases, it is difficult to discern whether this kind of scaffold is really more effective than standard treatments. Further work is needed in this area before any clear conclusions can be drawn.

Scaffolds combined with cells

Taking into account the strategies described above (tissue engineering and somatic cell therapy), some authors have combined these approaches in order to get stem cells seeded in supports of different natures to treat PU. In this context, Feldman and McCauley⁷⁵ assessed the use of three treatments: a biodegradable albumin scaffold, MSC and transforming growth factor-β3, alone or combined. In a rabbit PU model, these researchers examined the effects on wound healing of their approaches. One week after surgery, the group combining the three treatments showed a faster healing rate, mostly in terms of epithelialization. These authors, nevertheless, warned that more studies were needed to optimize the treatment strategy.⁷⁵ In the search for a suitable formula to heal wounds, Kilic Bektas et al.⁷⁶ prepared a bilayer scaffold consisting of a dermis of carboxymethyl cellulose hydrogel loaded with fibroblasts and an epidermis of collagen or chondroitin collagen containing keratinocytes. This scaffold was found in vitro to mimic natural skin tissue. The authors proposed its use to treat deep wounds and indicated that it could be a model for the study of the efficacy of bioactive agents on the skin.⁷⁶ In addition, two dogs with large non-healing skin lesions were treated with MSCs derived from Wharton’s Jelly seeded on a poly vinyl alcohol (PVA) hydrogel membrane. This formulation offered encouraging results both in humans and animal models.⁷⁷

Scaffolds combined with cells have been employed in patients with different skin disorders. Kuroyanagi et al.⁷⁸ used a dermal substitute composed of spongy collagen containing fibroblasts to treat 145 patients with various kinds of skin wounds including PU. Results were good or excellent in most cases and these authors proposed that this allogeneic dermal construct offers effective therapy for partial and/or full-thickness skin defects.⁷⁸ In a study by De Francesco et al.,⁷⁹ 30 patients with chronic ulcers were treated with dermal micrografts obtained from mechanical disaggregation of small pieces of skin tissue through a medical device called Rigeneracons^® (Human Brain Wave srl). These micrografts were found to improve the tissue repair process in venous, diabetic, pressure and post traumatic ulcers. These authors also reported better healing of ulcer site after micrograft application involving a reduction in wound size and increased granulation. Further, on the basis of clinical results, they tried to understand the mechanism of action of the micrografts in the regenerative process. De Francesco and colleagues complemented their work with in vitro experiments observing that the micrografts expressed MSC markers and formed a viable biocomplex with collagen sponge.⁷⁹

Among the different strategies used to expedite and promote wound healing, the method used by Falanga et al.⁸⁰ is particularly interesting. Good results were obtained using cultured autologous MSC applied up to four times to wounds using a fibrin polymer spray system in mice and humans.⁸⁰

Conclusion

Pressure ulcers are today still a major healthcare problem, especially for those patients with limited mobility and/or sensation such as SCI patients. Treatments are usually based on PU stage and patient characteristics, and span from simple skin wound care in mild cases to surgery in the more severe PU. Advanced therapy medicinal products are innovative complex biological products that are being examined in preclinical studies, and many are also being translated to the clinical setting as alternative therapeutic tools for several diseases. Among these diseases, PUs will especially benefit from these scientific advances, as for the more severe or complex injuries, standard of care treatment is not always successful. Taking in mind the characteristics of PUs, especially those with larger dimensions, where cell treatment may not be possible, tissue engineering approaches arise as the most promising option. Although several scaffolds, combined or not with cells, have been assayed, both in pre-clinical and clinical studies, its use in daily clinical practice have not been achieved. In this sense, further research is needed, with special attention to the development of new natural scaffolds, some of which could be obtained autologously, avoiding some safety and biocompatibility concerns.

Abbreviations

AM, Amniotic Membrane; ASC, Adipose Tissue-Derived Stem Cells; bFGF, basic Fibroblast Growth Factor; BM-MNCs, Bone Marrow-Mononuclear Cells; BM-MSCs, Bone Marrow Mesenchymal Stem Cells; CHIT, Chitosan; ECM, Extracellular Matrix; HA, Hyaluronic Acid; dHACM, Dehydrated Human Amnion/Chorion Membrane; NaCMC, Sodium Carboxymethyl Cellulose; N.I., Not indicated; NPWT, Negative Pressure Wound Therapy; PDGF, Platelet-Derived Growth Factor; PEG, Polyethylene Glycol; PGA, Polyglycolic Acid; PLLA, Poly (L-lactide) Acid; PRF, Platelet Rich Fibrin; PRGF, Platelet-Rich Growth Factor; PU, Pressure Ulcer; PVA, Poly (vinyl alcohol); SCI, Spinal Cord Injury; vCPM, Cryopreserved Placental Membrane containing Viable Cells; VEGF, Vascular Endothelial Growth Factor; WJ-MSCs, Wharton’s Jelly Mesenchymal Stem Cells.

Funding Statement

English Native Speaker Editing was supported by ISCIII (“PI19/01192”) co-funded by ERDF. Ministerio de Economía, Industria y Competitividad, Gobierno de España.

Disclaimer statements

Contributors None.

Conflicts of interest The authors declare no conflict of interest.