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  • 8/18/2019 A Review of Biocompatibility in Hernia Repair; Considerations in Vitro and in Vivo For

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    R E V I E W

    A review of biocompatibility in hernia repair; considerationsin vitro and in vivo for selecting the most appropriate repair

    material

    N. Bryan   • C. Battersby   •

    N. Smart   • J. Hunt

    Received: 14 January 2014/ Accepted: 29 August 2014 / Published online: 13 September 2014

     Springer-Verlag France 2014

    Abstract

    Purpose   Repair of hernia typically makes use of a pros-thetic material; synthetic or biologic in nature. Any mate-

    rial which enters the body is subject to interrogation by the

    inflammation and immune system in addition to numerous

    other cell families, the outcome of which ultimately

    determines the success of the repair. In this review, we

    discuss the fundamental biology which occurs in situ when

    a biomaterial associates with a tissue, compare and contrast

    the techniques available to predict this in vitro, and review

    how features of hernia repair materials specifically may

    manipulate tissue interrogation and integration. Finally, we

    conclude our article by examining how biocompatibility

    impacts surgical practise and how a better understanding of 

    the manner by which materials and tissues interact could

    benefit hernia repair.

     Materials and methods   A review of the literature was

    conducted using appropriate scientific search engines in

    addition to inclusion of findings from the groups’ primary

    research.

     Results   Using pre-clinical assays to anticipate the bio-

    compatibility of a medical device is critical; however, to

    maximise the scientific power of in vitro findings, we must

    carefully consider the in vivo niche of the cells with which

    we are working. Excessive in vitro culture or contact to

    non-self materials can add compounding complexity to

    studies involving leucocytes for instance; therefore, we

    must ensure careful and stringent assay design whendeveloping techniques for assaying pre-clinical biocom-

    patibility. Furthermore, many of the features associated

    with hernia repair material design specifically, included to

    enhance their mechanical or biodegradation characteristics,

    are inadvertently instructive to cells, and therefore,

    throughout the prototype stages of a materials develop-

    ment, regular biocompatibility assessment must be

    performed.

    Conclusion   The biocompatibility of a material is rate

    limiting in its ability to function as a medical device. The

    future of hernia repair materials will rely on close cohesion

    between the surgical and scientific communities to ensure

    the most robust biocompatibility assessment techniques,

    and models are utilised to predict the efficacy of a given

    material in a particular surgical application.

    Keywords   Biocompatibility   Inflammation   Leucocyte 

    Hernia   In vitro     In vivo

    Introduction

    Hernia repair often incorporates the use of an exogenous

    material to provide mechanical support to compromised

    tissue. Immediately after implanting this material, it is

    subject to contact with tissue and bodily fluids causing the

    physiologic processes which allow the body to decide the

    fate of the material to begin. Ideally, the material integrates

    with surrounding tissue acting as a scaffold for appropriate

    cells to colonise and provide permanent mechanical aug-

    mentation to damaged tissue. However, if this interrogation

    process is inappropriately stimulated, the material may

    become encapsulated, degraded and require removal with

    N. Bryan (&)    C. Battersby    J. Hunt

    Clinical Engineering (UKCTE), Institute of Ageing and Chronic

    Disease, Duncan Building Ground Floor, Daulby Street,

    Liverpool L69 3GA, UK 

    e-mail: [email protected] 

    N. Smart

    Exeter Health Science Research Unit, Royal Devon and Exeter

    Hospital, Barrak Road, Exeter, Devon EX2 5DW, UK 

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    Hernia (2015) 19:169–178

    DOI 10.1007/s10029-014-1307-8

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    morbidity to previously healthy interfacial tissues. There-

    fore, understanding the mechanisms at a cellular level with

    which a material interacts with tissue is paramount in

    predicting its efficacy and longevity in vivo. This review

    provides a concise overview of cellular considerations of 

    biocompatibility in addition to considering in detail how

    specific features of hernia materials may influence this

    process and the impact that this may have on surgicalpractise.

    Materials and methods

    A review of the literature was performed across a range of 

    scientific citation databases considering surgical and sci-

    entific research in the area of biocompatibility and hernia

    repair from over the previous three decades. In addition to

    this, findings from our research teams own work are also

    included.

    Biocompatibility

    Tissue healing involves complementary cellular and path-

    ophysiologic processes which return compromised tissue to

    normal function [2, 7]. From the instant, a surgeon places a

    material in contact with tissue the processes that determine

    its fate, and ultimately the quality of the patient’s life

    commence. Material surfaces are coated with proteins from

    blood, interstitial fluids or exudates which commences

    cellular interrogation. A positive outcome is integration

    with surrounding anatomy in a mechanically stable manner

    having returned the tissue to normal homoeostasis. How-

    ever, should a material’s cellular interrogation decide it

    presents a risk to the function of the organism as a whole it

    will degraded via enzymes, small molecules and reactive

    oxygen species [12]. If destruction is not possible, mate-

    rials are isolated in a capsule of fibrous matrix where risk to

    surrounding tissue is contained. Broadly, the ability of a

    material to repress the negative and encourage the positive

    components of cell interrogation is termed

    biocompatibility.

    The interaction between material and tissue is dynamic.

    Throughout the proceeding hours to years (in the instance

    of permanent prostheses), the boundaries of its effects on

    the host are pushed as the body attempts to use it as a

    conduit to tissue regeneration, which may require its

    remodelling, cell infiltration or removal.

    We must disregard definitions that portray inflammation

    as intrinsically negative. A material that stimulates

    inflammation is a statement which informs us of inevita-

    bility due to lack of any true biologically inert material.

    Inflammation is the underlying process which repairs tis-

    sue. Without inflammation, wounds do not heal, materials

    do not integrate and infection prevails. A material is

    required to encourage constructive elements of inflamma-

    tion to support tissue remodelling through with appropriate

    debridement and disinfection as opposed to destruction of 

    material, and healthy tissue that occurs during exacerbated

    inflammation.

    The cells that decide material fate are a varied with an

    array of roles unified by a drive to return tissue to normalhomoeostasis. Primarily this involves leucocytes, but as

    time progresses, non-circulatory cells also play a role in

    material compliance. Within the first hours, a material is

    investigated by neutrophils; which secrete many destruc-

    tive proteins and small molecules capable of damaging

    both material and native tissue. Although historically

    thought of a cell whose role was purely destruction and

    decontamination, we now recognise neutrophils as media-

    tors of inflammation by signalling to attack and recon-

    naissance cells to orchestrate pro and anti-inflammatory

    processes [4].

    Over the proceeding days, neutrophils are accompaniedby macrophages and famed debridement through phago-

    cytosis, and these cells also signal and are arguably the

    most rate limiting cell throughout tissue healing [5].

    Additionally, these cells also present fragments of digested

    material using MHC molecules, which drives generation

    and persistence of long-term and targeted inflammation by

    lymphocytes.

    A site of tissue repair will require re-vascularisation

    meaning that a biomaterial must support migration of blood

    vessels through its bulk. This provides colonising cells with

    a mass transport network to prevent necrosis of neo-tissue.

    As cells enter the material and proliferate their nutrient and

    waste, import/export requirements require constant

    remodelling of neo-microvasculature throughout the graft;

    changing shape, size and direction to ensure that cells

    within receive support to keep the tissue healing and

    healthy.

    The interaction of cells with the periphery of the

    material must also be considered. Blood cells are rela-

    tively short lived, and as such, their presence at a

    material site is constantly turned over. Therefore, we

    must give thought to cells that remain throughout its

    existence and provide security and stability beyond the

    degradable suture used to secure it. A materials edge

    must permit cell infiltration such that matrix secretion

    knits the material with surrounding ECM. This is the

    role of fibroblasts; excessive fibroblast activation or

    proliferation results in encapsulation; enshrouding fibrous

    tissue preventing integration, colonisation and vasculari-

    sation. Therefore, the fibroblast cells must be managed to

    ensure the correct degree of matrix deposition to provide

    graft security, without fibrous encapsulation and material

    isolation.

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    Predicting biocompatibility in vitro and in vivo

    Biomaterial development should be guided by clinical

    feedback to researchers and companies, to define the ideal

    requirements for a particular style of repair.

    Any new material will be subject to direct and indi-

    rect contact cytotoxicity screening to assess cell vitality

    in response to a material using biochemical tests. Typi-cally, these report quantitatively (colorimetric or fluori-

    metric) in response to changes in cell metabolism or

    membrane integrity. These techniques quantify the pro-

    portion of live or dead cells after treatment with a

    material and can be tailored into deduce whether cell

    death was a consequence of apoptosis or necrosis.

    Cytotoxicity assays are run over short periods (hours) to

    deduce acute cytotoxicity or extended over days/weeks to

    consider modifications to a materials cytotoxicity as it

    degrades or is remodelled. The cells used for cytotoxicity

    screening can be tailored to a materials end use. Com-

    monly, an established cell line is used for initial cyto-toxicity screening which progresses to primary cells

    appropriate to the anatomy in which the material will be

    situated, as its development is refined.

    In cell specific models, the goal may not solely be one of 

    confirming cytocompliance, in favour of deducing the

    influence of a material on the phenotype or function of a

    relevant cell population. To this end, we must ensure that

    in vivo culture platforms are designed with appropriate

    stringency to mirror the cells native 3D microenvironment

    as much as possible to obtain translatable data [6].

    A second stage in anticipating material biocompatibility

    is to subject it to contact with inflammatory cells and using

    molecular biology to deduce its influence on cell longevity,

    phenotype and function. Most commonly, these experi-

    ments utilise human peripheral blood mononuclear cells

    (PBMCs), extracted from whole blood via density gradient

    centrifugation. PBMCs contain the mononuclear fraction of 

    leucocytes; monocytes and lymphocytes; granulocytes are

    removed during density gradient centrifugation.

    The reason for utilising mononuclear leucocytes is that

    granulocytes rapidly become activated by the physical and

    chemical factors to which they are subjected outside of the

    cardiovascular system. When these cells are activated, they

    secrete cytokines and reactive oxygen species (ROS) which

    cause phenotypic changes across a cell population, making

    subtle, constructive leucocyte responses difficult to

    observe. ROS are an incredibly diverse class of signalling

    molecules capable of both pro- and anti-inflammatory

    processes, a number of which are detailed in Fig. 1.

    This highlights the limitations of considering inflam-

    mation in vitro. Although assays are regularly performed

    throughout biomaterials science which place leucocytes in

    contact with a test substance and record their response, we

    must treat this data as a baseline and avoid making direct

    translation to in vivo inflammatory physiology. This is

    because the ex vivo environment triggers phenotypic

    switches similar to invasion of the body with non-self 

    material. One must remember that cells recognise and

    activate in the presence of materials deemed foreign to

    healthy tissue, therefore, it should not be simply underes-

    timated that a tissue culture environment distorts leucocytefunction and influences data collection.

    Factors worthy of consideration include the physical and

    haemodynamic forces associated with venipuncture and

    centrifugation, and contact with xenoproteins in foetal calf 

    serum, a component of tissue culture media [11]. Impor-

    tantly, we must consider the influence of contact with tis-

    sue culture plastic vessels remembering that leucocytes are

    constantly suspended in blood. Therefore, a direct surface

    contact mirrors activation pathways such as contact with

    damaged tissues or activated endothelium in vascular

    walls. The compounding activating variables should be

    accepted as significant when extrapolating to wholeorganism physiology.

    Solutions have been developed which address and alle-

    viate components of this in vitro activating milleau,

    including non-adherent suspension culture systems using

    bioreactors, acoustic forces [13], chemically modified

    surfaces and serum-free media.

    Typically, the data output from in vitro studies of leu-

    cocyte activation should include several levels of molec-

    ular interrogation which support an ultimate assessment of 

    changes in cell function based on transcriptomic, proteomic

    and secretomic levels. We must take care to understand the

    implications and limitations of each stage of this workflow.

    Although PCR and microarray strategies provide insight

    into cellular behaviour, we must use these genomic/tran-

    scriptomic level analyses to provide support and robustness

    for protein and secretion data. Fundamentally, it is the

    protein level which allows a cell to perform its function,

    and therefore, whilst changes in gene expression provide a

    platform level of understanding into cell behaviour, it is

    pertinent that we do not cease our analyses at a nucleotide

    level and evaluate changes in proteins responsible for cell

    function in vivo.

    A further pitfall to studying material/leucocyte interac-

    tion in vivo is in using clean and pure populations of 

    PBMCs, we remove the synergy which happens as a result

    of material, cell and blood interaction. Biomaterial surfaces

    are rapidly coated with proteins from blood and tissue fluid

    which include immunomodulatory molecules such as

    immunoglobulin and compliment. Therefore, whenever

    possible, we must consider the inclusion of whole blood or

    human plasma in in vitro biocompatibility workflows.

    The authors have previously used a whole blood based

    assay to conclude a materials capacity to activate

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    leucocytes using chemiluminescent reporting of ROS pro-

    duction. Using whole blood adds granulocytes into theassay; therefore, we designed experiments to consider the

    initial phases of material interaction, running assays for

    90 min with appropriate controls to gather quantitative data

    before granulocyte degeneration [9,  10].

    After in vitro studies have concluded sufficiently that

    candidate materials do not possess inherent cytotoxicity

    nor are they intrinsically pro-inflammatory the next stage in

    predicting their biocompatibility at a whole organism level

    are in vivo animal models.

    Initially, implantation is performed to deduce inflam-

    matory characteristics of a material in vivo without the

    compounding variable of a healing or damaged tissue. Thisis typically a subcutaneous implant with a material placed

    adjacent to the dorsolumbar musculature in a rodent. This

    allows free movement of cells through the material,

    allowing inflammation to proceed and be monitored with-

    out compromise of the animal should the material fail

    mechanically. We must remember, however, particularly in

    device-specific models that animal models often present a

    material to different mechanical loading (quadrupedal vs.

    bipedal for instance), biochemistry, and differentiation

    degradation kinetics based on much shorter lifespan to

    humans. These factors should all be considered whendesigning and interpreting data from small animal series.

    After preliminary animal studies, materials are tested in

    a more anatomically correct model which describes the

    defect which it is designed to repair clinically, such as a

    full thickness abdominal wall excision model in the

    instance of ventral/incisional hernia. In tissue-specific

    models, material mechanical properties are also tested to

    comply with their target tissue. Furthermore, models

    should subject materials to specific populations of cells

    with which they would contact clinically, such as muscle

    cells and tissue-specific populations of leucocytes such as

    peritoneal macrophages in the instance of abdominal wallrepair.

    Classical analyses are performed by excising tissues

    after predetermined end points and performing histo-

    pathology in which the fixed, micron thickness sections

    are non-specifically stained to colour the cells and ECM

    to visualise using light microscopy. This enables path-

    ologically indexing using a numerical scale to semi-

    quantitatively elucidate material interaction with host

    physiology.

    Fig. 1   Leucocyte signalling is a vastly intricate process. The important signalling aspects of one class of leucocyte produced molecules, ROS,

    are detailed

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    This approach, whilst providing valuable baseline data,

    is suboptimal for elucidating finer points of cell/material

    interaction. Fundamentally, the system is subjective, based

    on the opinion and training of scoring pathologists, and

    negates of the importance of considering the function of 

    particular populations of cells at an implant in favour of 

    recording their mere presence.

    Although pathologists undoubtedly poses the skill todiscern between lymphocytes and neutrophils for instance

    based on cell morphology, the presence of these cells

    provides only basic insight into how a material interacts

    with tissue. Although greater numbers of neutrophils

    indicate stronger acute inflammation, we remain devoid of 

    detail concerning cell activation or signalling making true

    prediction of material fate difficult.

    Cells with microscopically similar morphology indicate

    vastly differing tissue consequences which only become

    apparent with more detailed molecular analyses such as

    CD4/CD8 T lymphocytes and classically versus alterna-

    tively activated macrophages.Therefore to quantify and critically, standardise bio-

    compatibility predictions, we must enhance pathologic

    interrogation beyond traditional histopathology. This

    should use validated SOPs to remove variability induced

    by human scored systems whilst adding value to informa-

    tion gained purely by tinctorial staining, providing a layer

    of molecular deduction of the roles and purpose of specific

    cells at an implant.

    The clearest alternative is immunohistochemistry, a

    process which utilises antibodies against antigens associ-

    ated with particular components of inflammation to visu-

    alise specific cells and tissue areas by visible light or

    fluorescent microscopy. Once cells or areas have been

    specifically allocated a spectral signature, human evalua-

    tion can be removed in favour of image analysis to count

    cells or calculate boundaries absolutely [8]. Using absolute

    quantification inter-sample standardisation is required

    when cell counting; a material may present with greater

    numbers of a particular cell simply because the sample is

    physically larger. Therefore, cells of interest should be

    normalised to a concurrent nuclear stain to deduce total cell

    number such as haematoxylin or Hoechst. This technique

    differentiates morphologically similar yet functionally

    distinct cells in addition to targeting surface markers

    associated with defined cell processes such as activation,

    phagocytosis, migration and apoptosis.

    Immunohistochemistry also targets secreted molecules

    which present in intercellular and interstitial spaces

    allowing signalling substances involved in inflammatory

    progression and remission to be observed, to real-time

    monitor inflammatory progression.

    Immunohistochemistry is, however, a relatively com-

    plex process made up of several sub-elements which also

    have potential to induce user mediated variation in the

    analytic procedure. There are numerous individual pro-

    cesses which can induce variation between practitioners

    including fixation, antigen retrieval, antibody concentra-

    tions/incubation incubation parameters, visualisation pro-

    tocol and even mounting and colorimetric/fluorimetric

    preservation. Therefore, although the technique undoubt-

    edly has the potential to add value beyond traditionalpathologic indexing, we must proceed with caution and

    ensure operating protocols are generated and followed to

    produce data which supports the rigour of inter-centre

    standardisation.

    A skilled histologist produces sections 4–6-lm thick,

    ensuring cells are sliced across their axis, presenting their

    internal cytoplasm. This allows observation of intracellular

    substances such as precursors to secrete factors and probing

    the phagolysosome. Depending on the tissue fixation, we

    may also probe a section at a nucleic acid level using

    nucleotide hybridisation in which a fluorescently conju-

    gated nucleotide sequence complementary to an mRNAsequence targets sequences within. Such techniques con-

    clude the gene expression in cells surrounding an implant,

    providing a window into their objectives as they decide the

    material fate.

    Explants can be dissociated and subject to gene probing

    using polymerase chain reaction (PCR) analyses. A par-

    ticularly useful modification to this, reverse transcriptase

    PCR utilises the enzyme reverse transcriptase to convert

    mRNA into DNA which enables PCR amplification. This is

    particularly interesting as elucidates genes which are

    expressed at a particular moment in time. It is now possible

    to characterise expression of many genes simultaneously in

    a single PCR meaning large data can be assembled rapidly.

    This is useful for deducing novel biomarkers of disease

    based on high throughput and cost effectiveness of PCR.

    However, we must ensure during any molecular biology

    transcripts of clinical interest have their translation to

    protein confirmed.

    Histopathology reports as a terminal procedure which

    does not help to dynamically predict biocompatibility.

    In vivo, the material will be excised when the animal is

    killed, clinically a material may only be available patho-

    logically when it failed and been removed/replaced.

    Therefore, we were not able to decipher molecular pro-

    cesses which preceded its failure which would provide a

    group of dynamic biomarkers to monitor material/tissue

    interaction throughout its integration/degradation profile.

    This insight would improve pharmaceutical management

    or refine surgical intervention to augment the process

    before clinical failure.

    Throughout animal studies, we must collection of bio-

    markers from experimental subjects in addition to histo-

    pathology. Most obviously, from this blood collection, we

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    can monitor populations of cells as they migrate to an

    implant or quantify substances in serum. We may also

    consider less invasive fluids specifically urine which can

    easily be collected from experimental animals.

    From biologic fluids, there are several techniques to

    monitor systemic molecules and rapidly screen for sub-

    stances of interest. Similarly to immunohistochemistry, this

    employs antibodies specific to particular molecules to flagthem up for luminescence/fluorescence detection. These

    may be ELISA systems in which antibodies tethered to a

    substrate bind molecules of interest, which allows quanti-

    fication with the addition of further antibodies conjugated

    to a reporter substance. Recently, these technologies have

    been advanced multiplex in which many molecules can be

    observed in a single assay using spectrally distinct reporter

    antibodies with detection apparatus capable of reading

    many emission spectra simultaneously.

    Material factors influencing biocompatibility

    Many factors influence biocompatibility of implanted

    materials. The physical properties of a material, either by

    design or as by-products of fabrication, inadvertently

    instruct cells and tissues. When considering biocompati-

    bility, we must remember material integration is syner-

    gistic between material and patient tissue; therefore, there

    are also patient-specific physiological variables which

    influence material interaction and modify biocompatibility

    at a personal level. This is highlighted in the Williams

    definition of biocompatibility ‘the ability of a material to

    perform with an appropriate host response in a specific

    application’ we must consider the specificity of an appli-

    cation to be personal given the heterogeneity of inter-

    individual tissue dynamics [1]. Therefore, generalisations

    regarding ubiquitous biocompatibility should be used with

    extreme caution until sufficient prospective data confirms

    this throughout a large range of subjects.

    The skill of the surgeon and appreciation for maintain-

    ing tissue vitality also contributes to the biocompatibility of 

    an implant. Excessive tissue manipulation causes damage

    and extends wound remission. Furthermore, surgical

    application of the material also influences its biological

    efficacy, such as material/tissue overlap or suture to

    material ratio. For this reason, deducing subtleties in

    material biocompatibility are difficult from retrospective

    analyses due to the variables introduced by lack of stand-

    ardised surgical application.

    Materials modify biocompatibility at the level of 

    chemical and biological composition and physical and

    mechanical properties. There is no such thing as an inert

    biomaterial, one which may be applied to a defect purely as

    a mechanical bridge without cell interaction, and it is

    essential to disregard this ideology. Fundamentally, we

    must consider the way in which biomacromolecules inter-

    act with a material surface as this gives a cell its first

    impression of an implant and begins the process of deter-

    mining acceptance or destruction.

    Cell surfaces are dynamic and continually adapt to the

    3D space to which they are subjected. As cells contact

    substrate membrane, fluidity allows it to flex and mould tothe surface, producing focal adhesions which feed back to

    the cellular cytoskeleton to change in shape and rigidity.

    Once intracellular cytoskeletal scaffolding is laid down, the

    cell produces ECM creating a more favourable environ-

    ment for colonisation as extracellular receptors recognise

    this environment is familiar using integrins and other rec-

    ognition molecules [2].

    If a surface environment is unfavourable to cells, the

    material is perpetually reliant on its sutures to retain its

    structural capacity as cells will never colonise and produce

    autogenous matrix to bind the device with surrounding

    tissues. The converse of this is a surface which signals cellsto hypersecrete matrix which results in fibrous encapsula-

    tion. We must be mindful of the interaction of cells with

    material surfaces and engineer implants to incorporate

    known topographic parameters with balanced adhesive and

    proliferative properties.

    The physical microenvironment of a biomaterial greatly

    influences cell response and supports interaction of the

    material with tissues as a whole. Topography at a micron

    scale, which is easily overlooked during materials fabri-

    cation as it may play a minor part in conferring mechanical

    properties, modifies cytoskeleton remodelling, matrix pro-

    duction and cells recruitment. Techniques such as atomic

    force microscopy (AFM) characterise material surfaces to

    quantify the roughness and randomness of their topography

    enabling the provision of optimal surface parameters for

    cell compliance.

    When a material contacts tissue fluids its surface is

    immediately coated with protein, so therefore when con-

    sidering surface topography in vitro, we allow this inevi-

    table fouling which may mask subtle topographic changes

    by providing a homogenous protein coat. On this basis, we

    should also consider a materials protein adsorption capac-

    ity when performing material evaluation and more specif-

    ically investigate materials possess preferential adsorption

    of certain specific proteins.

    Despite protein coating, it has been demonstrated that

    nanoscale modifications to material surfaces modify cell

    fate and function, particularly in the instance of adult stem

    cells, a pertinent cell in any healing tissue [15].

    The hydrophobicity/hydrophilicity surface must also be

    considered, as this will strongly influence protein adsorp-

    tion and subsequent cell interaction. Conversely, the field is

    continually seeking materials in which an anti-adhesive

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    face can be applied to the materials visceral side to inhibit

    intra-abdominal adhesions. Although a material which is

    intrinsically repulsive to tissue presents a tantalising sub-

    stance in herniorrhaphy, we must proceed with caution

    with anti-adhesive biomaterials and use our ability to repel

    brings surgical issues caused by compromised integration

    and poor mechanical stabilisation. There may indeed be

    more subtle pharmaceutical methods by which to manageintra-abdominal adhesions by targeting and suppressing

    migration and activation of inflammatory cells [3].

    Material integration will also benefit from colonisation,

    increasing and securing the bond between material and

    peripheral tissue by providing a meshwork of interfacial

    cells inside and outside the graft. Therefore, we must

    ensure that a material has sufficient porosity to allow cell

    penetration and migration. In the instance of large grafts,

    this should also allow the penetration of macroscopic tissue

    architecture such as microvasculature to provide the graft

    with mass transport capacity and preventing necrosis.

    Although understanding physical and chemical compo-nents of a surface is critical in anticipating its tissue

    interaction, we must also consider internal structure and

    chemistry.

    As material erodes, degrades or remodel’s its internal

    structure becomes visible to cells and tissues; therefore, in

    a material whose bulk is not homogenously distributed

    resulting in differing internal and external structures or

    chemistry, we must consider that this continually changing

    environment modifies tissue behaviour beyond a standard

    acute inflammatory interaction time frame, as with every

    new material parameters, as the device degrades comes a

    fresh wave of inflammatory investigation.

    In vitro, testing of material degradation is also the

    subject of debate. Various methods exist which include

    most basically incubating the material for extensive time

    courses in simulated body fluids and assessing changes in

    physical appearance and mechanical properties as time

    progresses. A further degree of complexity is induced by

    incubating with single enzymes such as collagenase or

    enzyme cocktails to mimic the tissue remodelling envi-

    ronment or the influence of microbial contamination

    secreting enzymes into the material field. Much like many

    in vitro tests, however, modelling degradation in vitro is

    challenging to directly extrapolate to a true in vivo tissue

    reaction as the actions of continuing waves of cells inter-

    rogating and potentially digesting the material, in con-

     junction with the huge and undefined array of 

    metabolically active proteins present in interstitial fluids, is

    difficult to recreate in vivo. In vitro, it is also difficult to

    create tests which appreciate the repeated and perhaps

    unpredictable influence of mechanical loading on a mate-

    rial and how this may synergise with the tissue environ-

    ment to modify the materials degradation kinetics.

    Therefore, although we can broadly predict a materials

    degradation profile based on in vitro tests, this is only

    absolutely quantified when a material progresses to an

    animal model.

    The rheometry of a device, its stiffness, rigidity and

    elasticity also influences cell function. Cells reside in

    specific niches in which they are accustomed to a set of 

    sheer parameters; therefore, subjection to physical forces towhich they are unaccustomed has the capacity to modify

    their response. We must also consider how this component

    of cell/material interaction may change over time if the

    mechanics of a device changes due to wear or repeated

    loading.

    In the instance of hernia, there are many compounding

    engineering features which influence the biocompatibility

    of a prostheses, this is particularly challenging in the

    instance of biologics [14].

    Biologic meshes are subject to proprietary processing

    steps which are partially or completely undisclosed, this

    modifies the inflammatory and mechanical characteristicsof the tissue to render it suitable as a medical device. A

    number of these processes result in material changes at the

    level to instruct cell fate.

    Starting material may influence biocompatibility by

    utilising a range of source species and tissues which it

    would be naive to assume contain identical structural and

    mechanical features and induce the same cell responses.

    Proprietary grafts exist which are xenogeneic (porcine or

    bovine) or allogenic (cadaveric) derived from a range of 

    tissues including dermis, small intestinal submucosa and

    pericardium. These materials vary in degradation profile,

    continually unmasking epitopes to inflammatory cells as

    degeneration modifies material.

    When materials are decellularised, it is critical that cells

    and cellular remnants are completely removed or cell pattern

    recognition receptors will flag the graft as non-self resulting

    in destruction. The goal therefore is to ensure the cellular

    component is removed, meaning the remaining matrix con-

    tains only proteins (largely collagen) that are conserved

    across individuals and species and provides a 3D environ-

    ment for cells to colonise, without epitopes for the immune

    system to differentiate graft from autogenous ECM.

    In some instances, this core protein structure is modified

    using cross-linking to join the collagen molecules within

    the decellularised tissue. Cross-linking increases resistance

    to bacterial proteases, which increases its persistence in

    infected fields. It would be reasonable to assume changes

    induced by cross-linking modify inflammatory character-

    istics by presenting cells with a protein conformed in a

    manner that they are not accustomed. However, studies by

    our own group both in vitro and in vivo have generally

    demonstrated that cross-linking does not significantly

    influence cell response in dermis derived biomaterials [30].

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    A final variable is sterilisation, with many techniques

    employed including gamma rays, electron beams and eth-

    ylene oxide.

    The surgical consequences of unsatisfactory

    biocompatibility

    In the light of progress in the scientific understanding of 

    material/tissue interaction, and advances surgical applica-

    tion, the ideal characteristics of hernia repair materials are

    perpetually refined. The qualities described by Cumberland

    and Scales, and Hamer-Hodges and Scott, are as follows:

    non-carcinogenic, chemically inert, resistant to mechanical

    strain, sterile, unresponsive to body and tissue fluids, able

    to limit foreign-body reaction, modifiable to defect size,

    and non-allergenic [16]. Required characteristics applied

    specifically in the instance of biologic meshes include

    resistance to infection, anti-adhesive, and the capacity to

    act like native tissue [17]. Additionally, an ideal materialshould be associated with very little surgical morbidity,

    such as seroma, easy to handle in open and laparoscopic

    instances, and cost effective. Due to significant advances in

    synthetic and composite meshes, a vast array of products

    for repair of uncomplicated hernias is available [18].

    Despite this panel of ideal material properties and the

    apparent ease at which a material can be engineered to

    modify its mechanical characteristics, synthetic meshes are

    not without risk, even in clean, uncontaminated operations,

    which highlights the need for a more thorough under-

    standing at a material science level of how tissues and

    material are interacting. Often the consequences of syn-

    thetic mesh, failure can be catastrophic. This includes

    erosion, adhesion, fistulation or encapsulation, which are

    incredibly debilitating requiring challenging surgeries to

    correct with potential for life modifying long-term out-

    comes such as stoma as a consequence of bowel resection

    during adhesiolysis [31, 32].

    Undoubtedly, synthetics improve the quality of life of 

    many people each day, however at present, we can say that

    these materials are not ideal in every patient, and a more

    thorough understanding of how the biological and cellular

    origins of inter-patient differences in response is required

    as a matter of urgency to take the field forward.

    Identifying the ideal repair material for complicated

    hernias remains a challenge. Although classification of her-

    nia complexity is debated and has been the subject of a

    number of consensus meetings [19, 20], it is accepted that

    complex hernias include those in which there is contami-

    nated or clean–contaminated tissue, exposed bowel or fis-

    tula, and significant loss of domain such that low- or tension-

    free repair is not possible. The use of synthetic meshes for

    complex hernias has been associated with infection,

    recurrence and significant adhesions to intra-abdominal

    organs, leading to obstruction, erosion and fistula formation

    [21]. Infected synthetic meshes are difficult to treat, fre-

    quently require surgery to explant, and may leave the patient

    with a colonised defect requiring prolonged treatment [17].

    For synthetic mesh repair in the presence of contamination or

    infection, several authorities recommend a two-stage pro-

    cedure in which contamination is managed surgically in thefirst stage, with the hernia repaired 6–12 months later [22].

    Biologic meshes are promoted for repair of complex

    hernias, as these meshes become incorporated into the

    wound, acting as a scaffold for tissue repair leading to a

    strong, well-healed, vascularised wound [22]. Due to the

    nature of biologics, the adhesions associated with synthetic

    mesh should not occur and vascularisation allows delivery

    of immune cells and antibiotics [17]. Retrospective con-

    sideration of biologic mesh efficacy is compromised due to

    the compounding variables induced throughout their

    application: the variety of available products, the tech-

    niques used to repair hernias (onlay, inlay, sublay, com-ponent separation) and the complex nature of hernias being

    repaired [23,  24]. To date, no data from randomised con-

    trolled trials specifically designed to investigate biologic

    meshes is available [21].

    Besides the previously mentioned issues surrounding

    classification of, the classification of ventral hernia repair

    and subsequent outcome reporting is also the subject of 

    debate [24]. Current literature is based on single centre

    case series, (and reviews of those series), with several

    series reporting a variety of hernia complexity, a range of 

    surgical techniques, a variety of mesh types, and use of 

    biologic mesh in both clean and contaminated wounds [21–

    23, 25, 26]. The recently launched European hernia registry

    is a development designed to bring clarity to reporting of 

    ventral hernia repairs and outcomes, and should provide

    robust, prospective data [24,   27]. Although overt recur-

    rence of the hernia might be considered a failure, one

    author, reporting a series of ventral hernia repairs using a

    variety of biologic meshes in contaminated or clean–con-

    taminated tissue, reported a 31.3 % rate of hernia recur-

    rence at 21.7 months, but described the majority of 

    recurrences as asymptomatic, with only 17.5 % requiring

    further surgery. The same study showed a recurrence-free

    survival of 92 % at 1 year, 77 % at 2 years and 51 % at

    3 years—a trend that questions the durability of such

    repairs [22]. Another study, describing repairs of ventral

    hernias secondary to emergency or trauma laparotomy,

    reported a 100 % recurrence with AlloDerm at 1 year, and

    a 31 % recurrence rate with FlexHDAs, all recurrences

    progressed to further surgery [26]. A recent review of 17

    retrospective series showed that recurrence depended upon

    wound class, with an overall recurrence rate of 13.8 %. The

    recurrence rates were 2.9 % in clean or clean/contaminated

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    cases, 19.4 % in complicated hernias, and 23.1 % in con-

    taminated or dirty wounds. The rate of recurrences

    requiring further surgery is not clear from that review [23].

    Associated surgical and wound morbidity is well docu-

    mented in ventral hernia repairs, with a review of 25 series

    showing a 46.3 % complication rate, wound infections being

    most common in 15.9 % of all cases [23]. It is interesting to

    note that only 4.9 % of infected implants required removal tomanage infection; the remainder were salvageable via to

    non-operative management. Although not all series report

    seroma formation, that problem is commonly documented

    following biologic mesh repair. The review of 25 series

    shows an overall seroma rate of 14.2 % [23]; however, it is

    likely that the surgical technique, mesh, and wound class all

    influence seroma risk. A study describing Strattice to repair

    contaminated or infected ventral hernias documented ser-

    oma in 28 % of cases [28].

    The future of biocompatibility—a surgeon’s perspective

    The current literature relating to biologic meshes is not

    sufficient to support evidence-based decision-making in

    ventral hernia repair, as it is composed of relatively small

    series, often with limited follow-up period. Moreover, the

    series describe significant variations in technique, using a

    range of biologic meshes. Interpreting data from such

    series is compounded by the fact that the nature of com-

    plicated or contaminated ventral hernias is also a signifi-

    cant source of variability. The only valid conclusion that

    can be supported from current literature is that it seems safe

    to use biologics in contaminated fields; however, the

    recurrence and morbidity rates remain high, especially in

    challenging cases. As Belyansky and Heniford [29] point

    out in response to the work of Kissane and Itani [21], large

    prospective trials would be challenging and expensive,

    considering the number of variables that have to be con-

    sidered; however, from a surgical point of view, estab-

    lishing the role of biologic meshes in this complex field

    must be driven by evidence from prospective trials, in

    combination with development of biologic materials.

    Conflict of interest   NB declares conflict of interest not directly

    related to the submitted work.CB declares no conflict of interest.

    NJS declares conflict of interest not directly related to the sub-

    mitted work.

    JH declares conflict of interest not directly related to the submitted

    work.

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