Introduction

Fat tissue, like bone marrow, is derived from the mesenchyme and contains a stroma that is easily isolated, thus making it a potential source of regenerative cells [1, 2]. It has been shown that the lipoaspirate is naturally a rich source of mesenchymal and stromal cells, exhibiting stable growth and kinetic proliferation in specific culture media [1, 3]. When processed, these cells tend to concentrate in the stromal vascular fraction (SVF), which is the infranatant of the lipoaspirate [1]. When cultured in vitro, ASCs can differentiate into osteogenic, adipogenic, myogenic, and chondrogenic lineages when subjected to specific lineage factors [1]. On the other hand, the non-cultured heterogeneous population of cells, known as stromal cells (SCs) or SVF cells, contain ASCs, hematopoietic stem cells, progenitor cells, endothelial cells, red blood cells (RBCs), fibroblasts, lymphocytes, monocytes/macrophages, pericytes, and other cell types [4].

In clinical practice, adipose tissue-derived mesenchymal cells are generally not administered alone but as a constituent of the SVF, a heterogeneous mixture of cells resulting from aspirated adipose tissue processing [1]. Notably, the adipose tissue SVF has become the focus of mesenchymal cell research, regenerative medicine, and fat grafting, with new processing equipment and methods being developed worldwide. Indeed, the engineering associated with SVF cells represents an exciting field of research for different diseases, including degenerative, congenital, or traumatic conditions and bone, joint, and soft tissue defects. These cells are primarily used in plastic surgery to supplement fat grafts, improving graft retention and long-term results [5,6,7].

The SVF can be dissociated and isolated from the adipose tissue using enzymatic and nonenzymatic methods, manually, or in an automated closed system. The most used isolation protocol involves washing the lipoaspirate, digesting it with collagenase enzyme, centrifugation, and RBC lysis [1]. It should be pointed out that despite the frequent use of this technique, there are challenges and barriers associated with collagenase, making it currently only appropriate for in vitro and in vivo animal studies. This observation opens up the perspective for more innovative techniques that can be utilized in humans.

In 2009, the concept of nonenzymatic dissociation was proposed and investigated to circumvent these collagenase-related obstacles [8]. However, in 2013, Shah et al. (2013) observed that cells acquired from a nonenzymatic method proliferated more slowly in culture [9]. Several authors have demonstrated that the composition of cell populations recovered by simple centrifugation and other nonenzymatic methods contains more significant proportions of peripheral blood mononuclear cells (PBMCs) and substantially fewer progenitor cells [7, 9,10,11]. Despite this apparent drawback, unique devices have been created to separate and concentrate the ASC and SVF cells from the adipose tissue stroma. These systems circumvent the need for manual and external manipulation, consequently facilitating fat incorporation and improving grafting results [12, 13]. Currently, there are several devices at various stages of development for the isolation of SVF cells. Each device is distinct in terms of complexity, automation, cost, efficiency, and effectiveness [14].

Considering that an invention is a discovery or a new way of doing something, identifying innovation can transform the invention into a product. Some examples of innovation include using new devices in existing procedures, introducing new procedures that use new devices, and using existing devices in new procedures [15]. When an innovation changes the way people usually perform a procedure, it can become disruptive.

Technical innovation is essential for plastic surgeons because it stimulates basic and clinical research to develop novel procedures and treatments. Unique plastic surgery niches that have significantly benefited from innovations and inventions include fat transfer, microsurgery, muscle flaps, tissue expansion, craniofacial surgery, transplantation, liposuction, and laser technologies [16]. Indeed, innovation makes plastic surgery different and distinct and is vital for the specialty’s survival [15].

Even though innovation has constantly fueled the plastic surgery field, studies evaluating the innovative and potentially disruptive technologies in this specialty are scarce. Herein, we conducted a narrative review to inform plastic surgeons about emerging and potentially disruptive technologies involving adipose tissue-derived SVF cell-based therapies (i.e., ASCs and the SVF). We hope this article will open up new frontiers, develop ideas, and raise the awareness of physicians that want to utilize these cells in clinical practice [14].

Methods

Study design

This study consisted of a narrative literature review. Data collection was performed using works published from January 1993 to August 2020, in English, Portuguese, and Spanish, in the MEDLINE/PubMed databases. The search strategies to identify relevant studies describing innovative plastic surgery procedures and technological applications employed five search strategies, summarized in Table 1. Strategy #1 used the following terms or combinations of terms "mesenchymal stem cells"[MeSH Terms] OR ("mesenchymal"[All Fields] AND "stem"[All Fields] AND "cells"[All Fields]) OR "mesenchymal stem cells"[All Fields]; Strategy #2 used "stromal cells"[MeSH Terms] OR ("stromal"[All Fields] AND "cells"[All Fields]) OR "stromal cells"[All Fields]; Strategy #3 used (bodyweight [tw] OR obesity OR skinfold thickness [tw] OR bariatrics OR lipectomy OR lipoaspiration OR plastic surgery); Strategy #4 used Cloud computer OR Machine learning OR Big data OR Bussiness Intelligence Virtual reality OR Augmented reality OR Artificial intelligence OR Robotics; and Strategy #5 used diffusion of innovation OR inventions OR Knowledge Management OR information dissemination OR change management OR culturally appropriate technology OR patents OR patents as topic OR intellectual property OR tecnolog* OR economic development OR cost–benefit analysis. Additionally, we utilized a combination of Strategies #1 OR #2 AND #3 AND #4 OR #5.

Table 1 Search strategy using the MEDLINE/PubMed database
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Eligibility criteria

Review articles and articles that did not use human cells were not considered. The following eligibility criteria were used for the inclusion of articles:

  1. 1.

    Obtained adipose-derived cells (ADCs)

  2. 2.

    Obtained ASCs and SVF cells

  3. 3.

    Used plastic surgery procedures that remove and collect fat

  4. 4.

    Demonstrated potential for technological innovation in the processing or use of ASCs.

  5. 5.

    Original article or review

After selecting articles, an exploratory reading was carried out based on the titles and abstracts, and then a full reading of the selected article, contemplating the objectives of this study and interpretation of the text, was performed.

Results

The MEDLINE/PubMed database search was conducted on September 21, 2020, and retrieved 553 potential articles published between 1993 and 2020. Applying filters and reducing the timeframe only to include articles published from 2001 to 2020 reduced the sample to 296 articles. The organization chart in Fig. 1 shows the number of articles included and excluded from the study. Separating the 214 tentatively selected articles based on whether it was a human or mixed study, further subdividing these articles as plastic surgery-related or other, and then evaluating each article demonstrated that a total of 33 articles (17 human and 16 mixed) presented innovation in their area of research and development. Figure 2 displays the number and distribution of the identified articles. A summary of the selected articles for this study is presented in alphabetical order in Table 2.

Fig. 1
figure 1

Distribution of results from the MEDLINE/PubMed database. The organization chart shows the number of articles found in each research phase divided between inclusion and exclusion. The filters used were human, English, Portuguese, and Spanish

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Fig. 2
figure 2

Distribution of results included in the study. The organization chart shows the number of articles included in the study and later divided into human (used stem cells derived from adipose tissue) and mixed (used stem cells derived from adipose tissue associated with nonhuman models). The groups were subdivided to locate innovative plastic surgery procedures

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Table 2 Selected studies
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As shown in Table 3, the most used method for isolating ASCs was enzymatic dissociation (n = 21), followed by mixed methodologies (n = 3), using manual isolation by enzymatic and mechanical dissociation in the same work. Furthermore, one study used a mechanical technique, and six studies did not report the ASC isolation method. Concerning the preferred plastic surgery procedure, most of the studies used liposuction (n = 22) or abdominoplasty (n = 4). Four studies used more than one technique, and three did not report the surgical procedure (Table 3). Notably, the most observed innovative technologies were associated with scaffold development. Additionally, studies utilized and/or developed innovative equipment, such as a mechanical system for cell separation, 3D printers, bioreactors, and other procedures, for tissue collection, grafting, and therapeutic purposes (Table 3).

Table 3 Brief description of the general objective and conclusion of the selected articles
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Discussion

We are just now experiencing the beginning of a scientific revolution. Recent scientific advances have facilitated the sequencing of the human and other genomes and the creation of genetically modified plants and animals. The rapid growth in scientific research and development resulting from innovative and sometimes disruptive technologies has changed how we function at work and as a society.

In medicine, disruptive technologies are currently driven by biotechnology and digital processing. Indeed, bridging science and technology to convert signs and symptoms into information for Big Data and Machine Learning could improve health and society [17]. In plastic surgery, tissue manipulation, especially adipose tissue and adipose-derived cells (ADCs), has demonstrated regenerative potential, bringing innovation and technology to the daily life of plastic surgeons. Therefore, the field has been forced to embrace biomolecular medicine and tissue engineering, which sometimes requires the surgeon to bring the laboratory into the operating room.

Technology-based innovation leads to the development of new designs, materials and products, and/or procedures. It can include equipment, components, and/or processes that introduce novel techniques, describe new layouts, or improve existing procedures or methods. Indeed, innovative processes facilitate the efficient production of quality products at the lowest possible cost [18]. The main characteristic of innovation is its novelty and the possibility of different interpretations [19,20,21]. Several scholars have recognized that the criterion “novelty” cannot be the only criterion for innovation because inventions or ideas only become innovative as they are transformed and applied in practice [22,23,24]. In its simplest sense, innovation is an ongoing creative process that occurs with continuous research and development [25].

Another important aspect related to innovation is the creation of a niche. It involves a genuinely trivial change in the technology, which impacts production systems. The technical knowledge associated with this type of innovation is incremental, creating disruptive technology and opening up new market categories and applicability [26]. Thus, technological innovations and/or cutting-edge products in the cell and tissue biotechnology and bioengineering fields that directly involve plastic surgery bring plastic surgeons into this environment.

Plastic surgeons actively obtain adipose tissue in the form of lipoaspirate during liposuction, tissue block, or abdominoplasty procedures. Thus, innovations that significantly improve any physical equipment, technique, and organizational system, including information technology, hardware, and software, associated with these processes could change how plastic surgeons work, improving patient outcomes and quality of life.

The present review revealed that the most frequently performed plastic surgery procedure in the selected articles was liposuction (n = 22) followed by an abdominoplasty (n = 4). Liposuction can also be combined with abdominoplasty or other procedures for body contouring, as in a few articles [27]. This observation is not entirely unexpected because liposuction and lipoaspirate are the best sources for obtaining ADCs. Additionally, liposuction is a mechanical maneuver that dissociates the adipose stroma from the ADCs, partially breaking the extracellular matrix (ECM) that strongly binds adipocytes to each other and maintains the intimacy of these cells in this compartment. In addition to its usefulness for purely aesthetic purposes, liposuction is an essential adjuvant in reconstructive surgery, during which the collected fat is reinjected (autologous) into other regions of the body, like the breasts, buttocks, or face [28]. Plastic surgeons cannot ignore that liposuction is a mechanical dissociative method that precedes chemical and/or nonenzymatic dissociation. Thus, liposuction-related innovations represent a niche for potentially disruptive technologies in the plastic surgery field. Abdominoplasty is used to restore the abdomen’s contour and shape, tighten the skin, correct rectus abdominis diastasis, and remove adiposity [27]. When used for obtaining ADCs, abdominoplasty produces a compact block of adipose tissue, making dissociation difficult, usually requiring an enzymatic and chemical treatment to obtain a liquefied material before starting dissociation.

Adipose tissue, like bone marrow, is derived from the mesenchyme and contains a stroma, thus making it a potential source of mesenchymal stem cells (MSC). Therefore, human lipoaspirate is an easy to isolate source of adult stem cells that exbibit stable growth and kinetic proliferation in specific culture media [1]. A previous in vitro study showed that ASCs and bone marrow MSCs can differentiate into osteogenic, adipogenic, myogenic, and chondrogenic lineages when subjected to specific lineage factors [29]. Due to the ease of collecting and processing ASCs, these cells have exciting potential in regenerative medicine. Indeed, fresh adipose tissue containing ASCs increases vascularization and graft take [5]. Moreover, ASCs have vast application potential in cell therapies and tissue engineering. For example, wound healing, reconstruction of bone defects in the calvaria, tissue regeneration with an anti-inflammatory effect, homeostasis, immunosuppression treatment in transplant-versus-host and transplant diseases, and neovascularization have all been reported following ASC administration [30]. Notably, to utilize ASCs for healing or reconstructive purposes, the adipose tissue must be processed, representing another plastic surgery niche that could produce innovative and perhaps disruptive technologies.

In the present review, we found that most of the selected studies used enzymatic dissociation, making it the most used method for adipose tissue processing and ASC isolation. However, most of the studies were conducted in the laboratory using in vitro or in vivo experimental animals. These ASC collection protocols require enzymatically digesting (i.e., chemical dissociation) the adipose tissue to separate the cellular components of that tissue. Zuk et al. (2001) introduced chemical dissociation as the gold standard method for isolating ASCs [1]. However, the methods for this procedure typically employ collagenase type I, limiting the therapeutic potential of ASCs in humans. Collagenase type I isolated from mammalian tissue components [31] increases the risk of patient contamination because some lots of this enzyme may contain infectious pathogens (e.g., prions), thus inhibiting the immediate use of autologous ASCs in fat grafting.

The US Food and Drug Administration (FDA) has set up contrasting regulatory pathways for “human cells, tissues, or cellular or tissue-based products” (HCT/P). An HCT/P that meets specific criteria of the regulatory requirements 21 CFR 1271 Sect. 361, including being minimally manipulated, intended for homologous use only, does not involve the combination of the cells or tissues with other substances; besides, water, crystalloids, or a sterilizing, preserving, or storage agents, are considered “361” and do not require FDA approval for release. On the other hand, other HCT/Ps that do not meet the regulatory requirements are classified as “351” and must be approved as a Biologics License Application (BLA). In most cases, mechanical and nonenzymatic dissociation methods that produce SVF or ADSCs are classified as “361,” while chemical dissociation methods are typically considered “351” because of the use of collagenase to digest the tissue.

After dissociation, ASCs are then collected in a pellet, known as SVF cells. The SVF contains a heterogeneous population of cells, including ASCs, endothelial progenitor cells, growth factor-producing cells, leukocytes, red blood cells (RBCs), macrophages, fibroblasts, and vascular smooth muscle cells [29]. Thus, innovations that provide alternatives or improvements to existing protocols for isolating these cells for human use would significantly impact the research and development (R&D) in the regeneration field and be of great interest to plastic surgeons.

As an alternative to chemical dissociation, mechanical methods, including topics related to the isolation of MSCs within the operating room, were reported publicly from 2006 to 2011. These methods rely on subjecting the tissue to disruptive dissociative forces, resulting in the release of adipocytes and ADCs from the ECM without the need for collagenase, minimizing contamination risks and providing a safer alternative to chemical dissociation, all within the operating room setting.

This protocol described by Lamblet is divided into two parts: shear force and pellet collection. The shear force portion involves collecting the lipoaspirated tissue in 50 mL syringes and subjecting it to mechanical dissociation via a Luer-Lock transpacer (Grams Medical, Costa Mesa, CA, USA) that provides a 0.5-inch channel firmly connected to another empty 50-mL syringe, allowing the connection between the two lumens of the syringes and facilitating material collection. The lipoaspirated tissue is subjected to shear forces that liberate the ADCs from the adipocytes and ECM from the tissue cluster when passed through the straight channel between the two syringes. The final product contains digested adipocytes, ADCs, and connective tissue that could be applied directly to the patient for regenerative purposes. In tissue augmentation, this product could be incorporated into the fresh lipoaspirate to be grafted, increasing retention.

The second part of the protocol involves collecting the mechanically digested material into a pellet to isolate all the ADCs. The digested fat is washed with PBS and then centrifuged at 1100 × g. The infranatant is collected and washed one more time with PBS and centrifuged a second time at 1100 × g. The infranatant is collected and passed through a 100-nm filter that retains cells and debris greater than this diameter. The filtrate is collected and centrifuged a third time. The button of the centrifuge tube contains a pellet with all ADCs, including RBCs. At this point, it should be pointed out that this product could be used in patients by incorporating this pellet into the fresh adipose tissue to be grafted for autologous fat tissue augmentation purposes. Alternatively, ASCs can be isolated from this heterogeneous mixture of cells using a 100-nm filter followed by a 20-nm filter and RBC lysis buffer, as in the chemical dissociative method, and following the appropriate sorting and expansion protocols. However, despite not using collagenase in this final process, the presence of the RBC lysis buffer limits the use of this product to in vitro or in vivo laboratory animal studies.

Notably, the author presented the shear force part of the protocol at conferences since 2006 but was published and referred to as nanofat by different authors [32]. However, the nanofat study only described the mechanical part of this process and never developed the second part (Pellet collection), which was never presented publicly. This omission consequently generated confusion in the field, especially since the author did not disclose Lamblet’s protocol or that this was an alternative mechanical method to chemical dissociation, probably in an attempt to not correlate it with the protocol described above.

Since then, few protocols using mechanical dissociation for obtaining and isolating cells have been established compared to chemical methods [28]. From Romanov et al. 2005 to Gentile et al. 2015, the alternative methods to chemical dissociation were based on washing, vibration, shaking and filtration, or combined with centrifugation and were not even mentioned as mechanical methods. This approach is sufficient for isolating some progenitor cells but is not adequate when comparing the results to chemical methods.

Of the 33 articles identified in this review, only one used only mechanical dissociation [33], and three used a combination of mechanical and enzymatic dissociation techniques [31, 32, 34]. The lack of nonenzymatic methods and their previous credibility is partially attributed to nonenzymatic dissociation processes yielding significantly fewer nucleated cells than chemical methods [7, 11]. These methods generally involve washing and agitating the liposuction product or combining these processes followed by filtration or centrifugation to concentrate the SVF ADCs. It has been reported that the composition of cell populations recovered by simple centrifugation and other nonenzymatic methods contains higher amounts of peripheral blood mononuclear cells (PBMCs) and lower amounts of progenitor cells than obtained with chemical dissociation [7, 9, 10]. It has been suggested that ASCs are concentrated in small and medium adipose tissue vascular structures. Without enzymatic degradation of the ECM, many progenitor cells remain trapped in the vascular endothelial layers and connective tissue fragments of the liposuction product.

Indeed, immunohistochemical and immunofluorescent experiments revealed that ASCs and pericytes are primarily located in perivascular niches [35, 36]. Since there is no ECM disruption, some nonenzymatic isolation methods fail to release the progenitor cells from the perivascular spaces, leaving many desired cells trapped in larger tissue fragments and eventually discarded. Notably, it has been reported that reduced progenitor cell populations and CD34 + expression are the contributing factors for cultures requiring longer incubation times to reach 80–90% confluence [9]. Furthermore, these nonenzymatic dissociation studies are based on one or two methods using an apparatus or laboratory device used separately, without synergy or considering the intimacy between the ADCs and collected adipose tissue in human lipoaspirate (HLA).

The HLA comprises fat tissue clusters (FTCs), and ADCs are released during the liposuction maneuver, a mechanical dissociative force. The FTCs contain adipose tissue matrix, adipose cells, mature adipocytes, blood vessels, smooth vascular cells, and trapped ADCs, including adult stem cells (ASCs), growth factor producing cells (GFPC), endothelial progenitor cells, lymphocytes, macrophages, red blood cells (RBCs), fibroblasts, plasma, and derivatives.

Unlike chemical dissociation, the beauty of mechanical dissociative force, as in Lamblet’s protocol, is that when you apply shear stress forces to the FTC, it releases and at the same time retains substantial amounts of cytokines and GFPCs in the dissociated tissue, which are subsequently grafted along with the ADSCs and fresh collected tissue, which is essential for the graft to take in vivo. Interestingly, when grafted together, these released cells create an environment that promotes repair and regeneration through multiple mechanisms, known as the immunomodulatory response [30]. Notably, in the last year, several clinical trials evaluating the efficacy of ADCs as a COVID-19 therapy have also emerged.

It is important to point out that TGF-β has been shown to stimulate collagen production, and vitamins B1, B2, and B3 stimulate fibroblast collagen and glycosaminoglycan production, increase fibronectin synthesis, inhibit matrix degradation, and facilitate cell chemotaxis [30]. Additionally, PDGF stimulates collagenase glycosaminoglycan fibroblast production, angiogenesis, and wound contraction and facilitates cell chemotaxis. GM-CSF improves white blood cell function, activates neutrophils, eosinophils, and monocytes/macrophages, and stimulates the proliferation and differentiation of hematopoietic cell lines [30]. Furthermore, it is well-known that interleukins (ILs) regulate cell homeostasis. For example, IL-3 and IL-6 stimulate the proliferation and differentiation of hematopoietic cell lines, and IL-7 and IL-8 augment neutrophil functions, act as anti-inflammatory agents and stimulate wound healing [30]. These effects are easily detected by performing a complete blood count, platelets, and coagulogram 1 week after a fat graft is performed. However, the cells cannot be washed or separated after the mechanical dissociative process, as publicly described by Lamblet in 2011.

Expanding a single-cell type requires first isolating, characterizing, and expanding it in a specific culture media. On the other hand, cells and cell aggregates produced with mechanical dissociation do not need to be isolated. Thus, if we use a mechanical maneuver that pays attention to the breakdown of the ECM and the release of this strong adhesion between adipocytes and the ADCs, a final count of nucleated cells is very similar or even higher than chemical dissociation will likely yield. Notably, the therapeutic potential has been reported to be optimized when cell conglomerates and/or aggregates are transferred together. When discussing cell conglomerates, we refer to all the cells in the collected tissue, including RBCs, sometimes considered contaminants in ADC transfer. Indeed, these cells are contaminants when attempting to isolate a single cell (i.e., ASCs) in the laboratory. However, when transferring the pool of cells, the RBCs can contribute to wound healing and, in some cases, compensate for blood loss in more extensive liposuction cases, where the RBCs are transferred to the third space, functioning as an auto hemotransfusion process. We hope that our patent-pending surgical device will offer the convenience and yield required to make this process a reality.

As mentioned, 25 articles employed liposuction as the plastic surgery technique, and 23 used enzymatic dissociation methods to isolate ASCs. Moreover, articles describing new devices (n = 2), techniques (n = 4), computational models (n = 1), tissue scaffolds (n = 21), and therapies and/or treatments (n = 5) were major areas of research and development identified in the present study. In the following paragraphs, these innovative studies and their possible clinical applicability are described. The potential applications of these inventions in terms of ASC procedures employed during plastic surgery are also discussed. While six articles did not report the cell isolation method (enzymatic or mechanical), innovation-related observations and possible clinical applicability are described. A summary of the characteristics and relevance of these studies are presented in Table 3. We will begin with innovative devices designed for ASC isolation.

Devices

Aronowitz et al. (2016) compared the performance of four SVF cell isolation systems [11]. The authors found that despite the innovation and technology of these devices, their clinical applicability still bumps into the regulatory barrier with collagenase (see above). Furthermore, the cost of the devices and associated disposables makes their large-scale clinical applicability unfeasible.

Güven et al. (2012) developed an automated procedure to isolate ASCs from adult human liposuctions in a closed clinical-grade device based on Sepax technology [37]. However, the Sepax system does not do all the processing in a closed system since the digestion is conducted outside the device with an adapted kit. Although the device's value should be lower compared to existing ones, the cost–benefit was not declared. In addition to devices, our search also identified alternative techniques that could improve downstream applications using ASCs.

Techniques

For example, in a different type of study, Vindigni et al. (2013) combined the great facility of ASCs to differentiate by applying an external mechanical stimulus that successfully creates a tendon-like structure reconstructed in vitro with a microcapillary network [38]. This study shows that physical factors influence the activity of ASCs, and mechanical stimuli, generated in vitro by bioreactors, can produce a transplantable vascularized tendon, representing an inexhaustible source of possibilities in tissue repair clinically.

Sesé et al. (2019) evaluated cell yields obtained from the generation of nanofat particles compared to traditional enzymatic dissociation methods [39]. Comparing the two methods showed that mechanical disaggregation provided a better cell inoculum than conventional enzymatic dissociation methods, using ten times less adipose tissue as starting material and providing a higher cell yield. The technology and products developed by Tulip Medical allow researchers and medical doctors to mechanically break down the adipose tissue into small fat particles, known as nanofat, immediately after the fat tissue is removed from the patient. The study compared chemical dissociation with mechanical but had two significant limitations. The first was that the authors used different fat tissue collection processes for each dissociation method. For more reliable and acceptable results, the same collection procedure should have been performed. The second concerns mechanical digestion, which discarded fluids after the liposuction process, eliminating a substantial percentage of the cell precipitate.

Vezzani et al. (2019) [34] performed cellular phenotyping and characterized mechanically fragmented human liposuction material. The authors found that mechanical dissociation of liposuctions resulted in the production of micro-fragmented adipose tissue (MAT), consisting of adipocyte aggregates with a microvascular network. They found that the mechanical micro-fragmentation process did not affect the perivascular cell compartment due to the presence of perivascular cells, both pericytes and adventitia, which were confirmed by flow cytometry. Furthermore, cytokines and angiogenic factors produced by both MAT and the SVF were more abundant in the MAT supernatants. Consequently, MAT digested with collagenase and placed in culture produced a secretome similar to conventional SVF. These results demonstrate that MAT is therapeutically beneficial and amenable to phenotypic and functional investigations. Additionally, the small size of the MAT clusters allows researchers to measure culture secretory activity, which is more challenging with larger adipose tissue pieces. It is important to point out that identifying specific cells in the fragmented lipoaspirate promoted the development of this innovative technology.

The study of Bi et al. (2018) compared mechanical dissociation (i.e., nanofat) with slight chemical dissociation (0.2 mg/mL collagenase type I for 15 min), referred to as Vivo nanofat [40]. The size of the transplanted nanofat graft was smaller than Vivo nanofat. The authors also observed that the number of viable adipocytes, colony formation, and MSCs expression was more remarkable in Vivo nanofat. However, the authors reduced the collagenase concentration and incubation period (0.2 mg/ml for 15 min) for the chemical dissociation compared to standard methods for isolating MSCs from adipose tissue (0.075% for 30 min). Notably, the experiments should have been limited to the animal model and not used in humans since even small concentrations of collagenase could harm the patient. Moreover, only the supernatant is collected after digestion, and the infranatant is apparently discarded. As previously mentioned, ASCs are concentrated in the SVF, which is the infranatant following enzymatic digestion.

Computational model

Our search also identified a 3D computational fluid dynamics model based on a differential pressure laminar flow bioreactor prototype developed to examine the performance of constant changes in the culture environment [41]. This model characterizes the flow and pressure distribution within a perfusion bioreactor prototype, uniting biotechnology with computational Big Data, resulting in more precise control of the culture media. Next, we will discuss articles that reported innovative approaches for incorporating ASCs into tissue scaffolds, a theme observed in most of the selected articles.

Tissue scaffolds

In the study by Tang et al. (2019), the effect of monocytes/macrophages on the osteogenic differentiation of adipose tissue-derived mesenchymal stromal cells (MSCs) in three dimensional (3D) cocultures was evaluated [42]. The authors showed that monocytes and macrophage subtypes inhibit the osteogenic differentiation of ASCs in 3D PLGA/PCL structures. More specifically, the cocultured monocytes/macrophages decreased the expression of osteogenic markers such as ALP, BSP, and RUNX2. These results highlight the overlooked fact that inflammation can regulate osteoblasts from MSC-based bone constructs within the bone microenvironment. In this sense, the tight control of inflammation may be necessary to create an anabolic environment and improve cell-based bone construct performance.

McMaster et al. (2019) demonstrated that melt electrowriting (MEW) could be adapted for seeding multicellular spheroids [43]. The authors manufactured this scaffold in sheet form and produced spheroids containing 1000–2000 ASC aggregates inside each pore. The cultures are easy to handle and can be transferred to other sites for mixed implants containing living organic and inorganic elements, augmenting implant grip and adaptation. This purely innovative and applied technology was due to the utility of 3D printers and the new electro-engraving by fusion technique.

In the study by Pati et al. (2014), the authors developed a method of biological imprinting with laden cells incorporated into a decellularized extracellular matrix (dECM) [44]. The bioink provides a favorable microenvironment for 3D tissue growth. The ability to print analog tissue structures by providing living cells with the appropriate material in a defined and organized way, in the right place, in sufficient numbers, and in the right environment, is critical for many emerging technologies. The concept of tissue and organ printing or bioprinting is performed in a liquid medium, making it useful for in vivo tissue engineering experiments and in vitro experiments with drugs and tissue models and tumor growth.

In the study by Rath et al. (2016), stem cells from ASCs or bone marrow MSCs were tested for their ability to differentiate into highly porous 3D 45S5 Bioglass® scaffolds [45]. Interestingly, after five passages, ASCs differentiate into a bioactive glass, even without any means of differentiation. This technology, based on Bioglass scaffolds, opens up new possibilities for use in bone tissue engineering.

The study by Tong et al. (2018) was the first to evaluate the effect of breast epithelial cells on human ASCs in 3D culture [46]. This study revealed that ASCs form structures similar to acinar cells and exhibit characteristics of epithelial differentiation when stimulated by the HBL-100 mammary epithelial cell lineage in 3D. In the clinical context, the findings show that ASC characteristics are beneficial for cell-assisted lipotransfer for breast reconstruction since they can promote mammary gland growth. However, care must be taken when ASCs are cultured and expanded in vitro and then transplanted because little is known about the interactions between exogenous ASC and the breast epithelium.

An innovative 3D cell mass (3DCM) based on cell adhesion was described by Park et al. (2014). This study evaluated the therapeutic potential of 3DCMs composed of ASCs [47]. The 3DCM culture promoted efficient vascular stem cell differentiation. Additionally, 3DCM transplantation resulted in direct vascular regeneration of the injected cells and improved therapeutic efficacy. The authors also showed that 3D cell aggregates prevent cell apoptosis and promote cell stabilization. In ischemia models, stem cell spheroids improved therapeutic efficacy through enhanced cell viability and paracrine effects.

Other tissue scaffold-related articles described specific supplements that could serve as therapies or improve surgical outcomes. For example, the effects of exosomes derived from human ASC (ASC-Exos) on the regeneration of peripheral nerves in vitro and in vivo were investigated by Chen et al. (2019) [48]. The authors observed that human ASC-Exos promoted the axonal growth of neurons. This study represents a potential treatment for nerve regeneration and nerve tissue engineering.

In the study by Wang et al. (2018), collagen scaffolds and ASCs were combined for bone regeneration of the oral and calvaria mucosa using resveratrol (RSV) [49], which affects the differentiation of mesenchymal stem cells. The experiment proves to be innovative since it aimed to identify active substances like RSV that can activate the cellular potential of ASCs.

The study by Mou et al. (2019) showed that extracellular vesicles from adipose tissue-derived MCSs improve the vascularization of fat grafts and increase their retention rate [50]. The innovative aspect of this study is that the authors used adipose tissue MCSs and the extracellular vesicles, a by-product of the cell culture. Furthermore, using tissue engineering in which a Matrigel scaffold was colonized with umbilical vein endothelial cells supplemented with extracellular vesicles, the authors evaluated the angiogenic retention regenerative potentials of the transplanted adipose tissue. This complex but simultaneously simple and innovative process has great potential for clinical applicability.

In the study by Lin et al. (2009), it was hypothesized that polyethersulfone (PES) surfaces modified with the fibronectin-based Arg-Gly-Asp (RGD) peptide sequence would increase ASC adhesion compared to the unmodified cells [51]. By evaluating ASC fixation and proliferation on PES surfaces modified with different fibronectin-derived peptide sequences and using cutting-edge biotechnology to evaluate RGD-treated surfaces to attach a more significant number of ASCs, the authors were able to develop a model that bridges bioreactor technology to in vitro 3D cell cultures and prospective studies of 3D tissues with fluid mechanics.

Hu et al. (2015) microencapsulated insulin growth factor-1 (IGF1) in poly(lactic-co-glycolic acid) scaffolds and implanted them into mice. The authors found that IGF1 positively regulates Axin2 and PPARc and simultaneously attenuates Wnt/b-catenin under adipogenic conditions, promoting a lineage bias towards CD31( −)/34( +)/146( −) cells at the expense of CD31( −)/34( +)/146( +) cells and leading to adipogenesis in vitro and in vivo [52]. This innovative study has direct implications related to obesity and adipose regeneration.

In the study by Kim et al. (2015), the effect of different sized silicon dioxide particles on the growth of and mitogen-activated protein kinase signaling in ASCs was verified [53]. This study demonstrated that only nanoparticles (NPs) with a 50–120 nm diameter are beneficial, while microparticles can induce apoptosis. Silica-based scaffolds and coated plates have hydrophilic properties, and it was also shown that only silica NPs entered cells, and some were clustered in vesicles, suggesting that silica NPs enter cells partly by endocytosis.

The effect of continuously expanding MSCs in parallel culture on tissue culture plastic (TCP) or polydimethylsiloxane gels (PDMS) of different elasticity was reported by Schellenberg et al. (2014) [54]. In this study, the authors showed that PDMS elasticity had no sustained effect on replicative senescence, intrinsic cell lineage impairment, or mDNA profiles of cells continuously cultured on these substrates.

Schellenberg et al. (2014) developed scaffolds of polyvinylidene fluoride (PVDF) non-tissue with round, trilobal, or snowflake fiber cross section and different fiber crimp patterns (10, 16, or 28 needles per inch) to evaluate the effect of the biomaterial’s 3D scaffold architecture on cell behavior and fate [55]. Due to their complex 3D conformation, they can provide interesting perspectives for some surgical interventions. For example, non-tissue PVDF scaffolds can be used as meshes in hernia repair. Notably, MSCs mediate and promote the wound healing process, supporting vascularization and differentiation into many cell types. The in vitro study of interactions between PVDF and MSCs could stimulate implant cellularization in vivo. The biotechnology used to fabricate non-tissue scaffolds with polymeric PVDF granules involves a completely labor-intensive and innovative method with three main processing steps: texturing, web forming, and gluing. It is plausible that additional innovations could be developed specifically for each step.

Li et al. (2018) decellularized adipose tissue and combined it, in variable concentrations, with a fraction of thiol-acrylate to produce hydrogels [56]. Incorporating bioactive molecules into a hydrogel system can increase the cell proliferation rate and improve the adipogenic differentiation performance of stromal stem cells. However, this approach suffers from high costs and can also cause cytotoxicity due to the light/heat used for curing (i.e., polymerization). The innovation associated with this study comes from applying polyethylene glycol (PEG) MEC without modifying it and encapsulating ASCs at various hydrogel/PEG/MEC concentrations. The authors reported that 1% is the ideal concentration for stimulating adipogenesis. Bioengineering and biotechnology were fundamental for fabricating ECM PEG hydrogel scaffolds and the encapsulation of the human ASCs.

In the study by Wang et al. (2010), the characteristics of a hybrid hydrogel framework composed of poloxamer 407 and the self-assembly oligopeptide EFK8 in vitro and in vivo were examined [57]. The hybrid hydrogel containing ASCs has great promise as a 3D scaffold for stem cell-based soft tissue engineering.

Interestingly, one article reported the identification of a new scaffolding material. Kiiskinen et al. (2019) demonstrated the potential use of wood-derived nanofibrillar cellulose (NFC) as a wound dressing and cell scaffolding material for ASCs. The authors developed an animal-derived component-free cell transplantation method for wound care [58]. This study proved to be innovative in developing a biotechnological dressing that allowed the adhesion of MSCs and did not interfere with their cell viability and function.

Nyberg et al. (2019) explored the bone formation potential of clinically relevant SVF cells obtained from human liposuction samples [59]. This study harvested the SVF, combined it with 3D-printed osteoinductive supports, and implanted it in the bone defect to stimulate regeneration in the same surgical procedure. This process reduces the number of operations required for the patient and eliminates the cost and practical limitations of handling ex vivo cells. Moreover, it leverages the regulatory feasibility of an intraoperative versus multi-operative procedure and enables administering the SVF intraoperatively.

Mineda et al. (2015) showed the therapeutic potential of stromal/stem cell microspheroids derived from human adipose tissue prepared by 3D culture in non-cross-linked hyaluronic acid gel [60]. The delivery system from cells to organs and tissues is promising, and the involved technology and subsequent innovation have been continuously improved. Furthermore, hyaluronic acid, which is already being used on a large scale in plastic surgery and other specialties as fillers, is a perfect candidate for this purpose.

In the study by Haskett et al. (2018), the tissue-engineered vascular graft (TEVG) models were seeded with SVF for 8 weeks in vivo [61]. The approach shows that bioengineering combined with surgery and methods for obtaining and isolating SVF from adipose tissue allows its later use in the laboratory and immediate use in vivo.

Krawiec et al. (2017) evaluated the use of the SVF from adipose tissue as a vascular graft developed based on stem cells in producing a urethane polyester scaffold that had been previously shown to be effective with other cell types [33]. This study demonstrated the possibility of using freshly obtained stromal SVFs for seeding polyester urethane scaffolds rather than adipose tissue-derived stem cells that required culturing for 2–3 days. It brought innovation, demonstrated clinical applicability of a tissue bioengineering technology, and opened the door to new studies in vascular graft fabrication in tissue engineering with these polyester urethane scaffolds.

Therapies/treatments

Beyond describing and discussing some innovative and exciting emerging technologies in the field, other articles provided insights into novel therapies and treatments. For example, the immunomodulatory effect of purified human perivascular stromal cells (PSCs) implanted during bone formation was reported by Meyers et al. (2018) [34]. The study demonstrated that differentiated cells from the perivascular region of the adipose stroma have a different phenotypic differentiation from the total cells of the SVF. These observations are particularly relevant to bone induction and differentiation and open up several lines of research investigating PSC versus SVF bone differentiation.

In another study by Meyers et al. (2018), high WISP-1 expression was observed in human PSCs in vivo after purification and transplantation into a bone defect [62]. Previously, WISP-1 was defined for its role as a bone matrix protein and its upregulation of osteoblastogenic differentiation into other types of osteoblastic cells. However, this study demonstrated a new role for WISP-1 signaling in stromal progenitor cells in the perivascular niche of human adipose tissue. Moreover, the authors implicated WISP-1 in promoting angiogenesis, especially tumor-associated angiogenesis. This study complemented the previous one and built upon it by showing the relationship of perivascular adipose stromal cells with the WISP-1e activity of the osteogenic marker, implicating these cells as progenitors of osteogenic activity and opening up several research paths with these differentiated cells that do not need expansion.

In the study by Chen et al. (2019), a new method which is immobilization method by self-assembly of biotinylated MSC-EVs on the surface of biotin-doped polypyrrole titanium (Bio-Ppy-Ti) was reported to improve in vitro and in vivo biofunction that could be applied to bone regeneration [63]. In this study, human fat ASC-derived exosomes were stably anchored in a scaffold produced by the electropolymerization of a biotin-doped polypyrrole film on titanium EV-Bio-Ppy-Ti and incorporated into the differentiation process of cultured osteoblasts. The authors assessed osteoinduction and found it to be 185 times greater than the control cells [48]. This technology is particularly relevant to individuals requiring metallic titanium implants.

Tawonsawatruk et al. (2016) investigated the capacity of human bone marrow-derived MSCs and human adipose tissue-derived pericytes percutaneously administered to the bone fracture gap to prevent pseudarthrosis [64]. In this study, innovation and technology came together to create a method for measuring bone healing of non-atrophic fracture union, from mathematical methods for measuring bone callus to the use of Micro-CT to assess bone density and thickness of trabeculae, histological evaluations and mechanical tests to determine final load and tension and ending with radioisotope cell tracking tests to track and display the contribution of transplanted cells to repair and remodeling. Thus, it shows the preparation and scientific rigor in detecting functional evidence using state-of-the-art technology from different technological sectors. It also brought an immense contribution to clinical applicability.

A novel physical approach was also identified to produce a cell-free aqueous extract of human adipose tissue [65]. The therapeutic potential of fat extract (FE) was investigated in the hindlimb of nude mice following ischemia. This study is extraordinarily innovative and uses a different mechanical dissociation from those previously demonstrated. The in vitro and in vivo experiments showed that endothelial cells assume a tubular formation and are induced with and without FE when combined with Matrigel scaffold formation. A significant advantage of this technology is that no immunological rejection should occur when using cell-free FE, meaning that FE could be marketed as an “off-the-shelf” product for treating ischemic disorders. This study, in particular, supports the idea that no fluid should be discarded from the chemical or mechanical dissociation of adipose tissue without fully understanding its properties since it is possible that these “waste products” contain valuable material with regenerative potential. The innovative adipose tissue dissociative methods and all of the currently available information could attenuate the waste of cells and derivatives and generate different products from the human lipoaspirate.

Based on the results of this review article, it is clear that biotechnology, Big Data, and technological disruption have entered the plastic surgeon’s world and have started to change our way of thinking, guiding us to a horizon of new ideas and innovation.

Conclusions

Innovation and technology are linked to plastic surgery and procedures that use adipose tissue as a source of ASCs and the SVF. Of the 33 selected articles, the isolation of ASCs was primarily performed using an enzymatic dissociation method. It was also determined that liposuction was the most performed plastic surgery procedure, while most innovative technologies were tissue scaffold related. These reoccurring themes possess enormous potential in innovative areas and introduce plastic surgeons to regenerative medicine, transforming them from a supporting role to the leading actor in this scenario of disruptive technological innovation. We conclude that these experimental research and development areas could facilitate utilizing ASCs for in vivo human plastic surgery and regenerative medicine applications.