Microdermabrasion

Physicians and aestheticians use a variety of facial resurfacing techniques to improve the appearance of the skin by addressing skin quality, age spots, fine lines, and scarring. These techniques include traditional dermabrasion, chemical peeling, laser resurfacing, and microdermabrasion. All of these modalities exert their effects by resurfacing or ablating the skin at various depths and promoting skin healing. Microdermabrasion ablates the most superficial aspects of the skin using a high-pressure flow of crystals. The clinical results of these various skin-resurfacing techniques relate to the skin target depth desiring correction and the depth of ablation performed.

The concept of facial resurfacing dates back to 1500 BCE, when Egyptians reportedly used sandpaper to improve scars.[1] In the early 1900s, dermabrasion was developed to promote skin rejuvenation and was modified to its modern form in the 1950s. Dermabrasion involves ablating the skin to the level of the dermis to promote skin regrowth. Based on the power dental equipment design, the dermabrasion machine uses a wire brush with a diamond fraise to mechanically ablate the skin. Local anesthesia is required to perform the dermabrasion procedure. The technique is operator dependent, and, thus, highly skilled surgical technique is required to obtain optimal results. Reported complications include excessive bleeding, infection, scarring, and hypopigmentation. Furthermore, healing usually takes up to 7-10 days, owing to the deep dermal damage.

Microdermabrasion was designed by Marini and Lo Brutto in Italy in 1985 and addressed some of the adverse effects inherent with dermabrasion.[2] Advantages included ease of use, painless nature with minimal complications, minimal inconvenience to the patient, and proposed overall effectiveness. Monteleone presented the clinical effectiveness of this new technique at the Third Meeting of the Southern Italy Plastic Surgery Association in 1988.

The technique spread throughout Europe during the late 1980s and mid 1990s, until eventually Mattoli Engineering introduced microdermabrasion to the US market in 1996.[3] Microdermabrasion quickly gained widespread popularity, owing to its low risk, rapid recovery time for patients, and apparent results.

Most skin resurfacing techniques demonstrate clinical efficacy, but also carry the potential for serious complications such as pigmentary changes and scarring. These complications are related to the depth of skin abrasion obtained.

The advantages of microdermabrasion are also related to the superficial skin disruption that results from the procedure, which leads to an overall painless procedure that does not require anesthesia, is relatively simple and quick to perform, can be repeated at short intervals, and does not significantly interrupt the patient’s life. Since microdermabrasion produces a superficial ablation primarily limited to the epidermis, it is mainly effective for fine lines and superficial scars

Limitations of the procedure include a clinical effectiveness limited to more superficial issues and the need for multiple treatments to achieve clinical results. It is ineffective for deeper wrinkles or scars that extend beyond the epidermis.

The skin is functionally divided into two layers—the epidermis and the dermis.[4]

Epidermis

Overall, the epidermis replaces itself every 12-14 days. The epidermis is composed of epithelium and can be further subdivided into the following five layers:

• Stratum corneum
• Stratum lucidum
• Stratum granulosum
• Stratum spinosum
• Stratum basale

The most superficial later is the stratum corneum, which is composed of multiple layers of keratinocytes that are constantly sloughed and regenerated. This is the layer that provides the major barrier function and is targeted during mechanical exfoliation.

The stratum lucidum is found mainly in the palms of the hands and soles of the feet and contains a dense layer of keratin filaments that provides additional structural support. The stratum granulosum contains granules containing a lipid rich content that creates a waterproof barrier for the skin. The stratum spinosum is the thickest layer of the epidermis and contains multiple polygonal spiny cells rich in cytokeratin and with desmosomal intercellular connections. Langerhans cells, dendritic cells, and other immunomodulating cells are located throughout this layer.

The stratum basale contains one to three layers of cuboidal or columnar cells that are mitotically active and generate the cells that comprise the other layers of the epidermis. Melanocytes and Merkel cells can also be found within this layer.

See the image below.

Anatomy of the skin.

Dermis

The dermis forms two layers and is composed of connective tissue—the papillary layer and the reticular layer. The loose papillary layer is located beneath the epidermal stratum basale and contains the capillary network that supplies the skin and the nerve endings responsible for touch sensation. The thicker reticular layer is composed of densely packed collagen that is responsible for the skin’s tensile strength. This layer also serves as the foundation for hair follicles, sweat glands, and sebaceous glands, and it contains the dermal plexus.

Collagen is the main structural component of the dermis. Type I collagen constitutes 80% of dermal collagen and imparts tensile strength. Type III collagen comprises 15% of dermal collagen. It is found throughout the dermis and primarily serves to anchor the epidermis to the dermis. Elastic fibers comprise about 3% of the dermis and provide elasticity and resilience. Ground substance is an amorphous gel-like material that surrounds cells and imparts turgidity and resilience.

The dermis is bound tightly to the epidermis by the dermoepidermal junction, which is the interface between the epidermis and the papillary dermis. The basal layer keratinocytes and dermal fibroblasts produce the components that characterize the dermoepidermal junction, such as anchoring fibrils that bridge the basal epidermal layer and papillary dermis. Beneath the dermis lies the subcutaneous fat, which derives its blood supply from arteries and veins located in the fibrous septa.

Skin quality and appearance are affected by a variety of external and internal factors, which subsequently lead to skin conditions such as aging, photoaging, enlarged pores, and scarring. A distinction can be made between the changes in the skin that occur as a result of chronological aging and photoaging.

Chronological aging, or intrinsic aging, is the irreversible and progressive loss of homeostatic capacity and is assumed to be a genetically programmed phenomenon.

Photoaging results in changes in gene expression and results from exposure to ultraviolet radiation. Ultraviolet (UV) radiation consists of UVA, UVB, and UVC light.

Short-wavelength UVC light is the most damaging type of radiation; however, it is filtered by the atmosphere and does not reach the earth’s surface. Thus, UVB light and UVA light are mostly responsible for tanning and aging. Upon contact with the skin, some light is scattered and reflected by the stratum corneum, while other light is absorbed into the epidermis.

UVB light is shorter in wavelength (320-290 nm) and less able to penetrate the skin

It is mostly absorbed by the epidermis and only about 10% penetrates the deeper layers of the skin. This radiation is responsible for delayed tanning, burning, skin aging, and promoting the development of skin cancer.[5]

UVA light has a longer wavelength (400-320 nm) and is more able to penetrate the skin. UVA radiation accounts for about 95% of the UV radiation that reaches the earth’s surface. About 50% of UVA light penetrates the epidermis and reaches the papillary dermis. It is responsible for the immediate tanning effect, and it also contributes to skin aging.[5] Most of the immediate and long-term effects of photodamage occur in the epidermis and dermis.

The process of skin aging can be divided into intrinsic aging and photoaging.[6] Chronologic aging is skin damage resulting from the passage of time, whereas photoaging is damage resulting from exposure to UV radiation. These are two distinct processes. Clinically, naturally aged skin appears smooth, pale, and finely wrinkled. On the other hand, photoaged skin is coarsely wrinkled and associated with dyspigmentation and telangiectasias.[6] The most apparent differences between these two processes are evident in the dermis.

Chronological aging affects all the layers of the face including the bone, ligaments, fat, and skin in the upper middle and lower portions on the face.[7] Clinically, this results in bony resorption and reduction of facial skeletal support, atrophy of subcutaneous fat and volume loss, attenuation of the musculofibrous system, and gravity-induced soft-tissue redistribution. Alterations of the skin surface include thinning of the epidermis and dermis and loss of elasticity. Photoaging from chronic UV radiation exposure causes degenerative changes in the skin that are superimposed onto the normal chronological aging process.

As the skin ages, the structural stability of the dermis is compromised by the overall degradation of collagen.[7] Collagen is the major insoluble fibrous protein in connective tissue and is the major component of the dermis. Its major function is to provide tissue with the ability to withstand stretching. Type I and type III collagen predominate.[8]

In young, healthy skin, collagen is continuously degraded by matrix metalloproteinases and resynthesized by fibroblasts and other epithelial cells. The resulting long strands of collagen formed in the dermis impart the skin with its tensile strength and stability. UV radiation is known to induce the synthesis of matrix metalloproteinases and other collagen-degrading enzymes, shifting the balance toward collagen degradation and fragmentation. In addition, collagen synthesis slows down more in photoaged skin than in naturally aged skin.[8]

The aging process continues in a cycle of sustained collagen loss. Over time, this clinically results in thinning of the epidermis, wrinkling, loss of elasticity, dyschromia, and dermal atrophy. Histologically, the aging process first begins with hyperplastic response and increase in melanin production as it reacts to environmental insults. The process then shifts to skin atrophy and melanocyte dysfunction in the later stages of aging.[9] Histologic changes in an aging dermis include solar elastosis, degradation of elastin and collagen fibers, and increased ground substance. Furthermore, DNA damage from radiation increases the risk of squamous and basal cell carcinoma.

The degree of skin wrinkling can be classified with the Glogau scale, ranging from type I to IV and corresponding to mild, moderate, advanced, and severe wrinkling, as follows[10] :

• Type I: No wrinkles and early photoaging such as mild pigmentary changes, no keratosis, and minimal wrinkles; use of minimal or no makeup
• Type II: Wrinkles in motion and early-to-moderate photoaging with early senile lentigines visible, keratosis that are palpable but not visible, and parallel smile lines beginning to appear lateral to the mouth; usually some foundation worn
• Type III: Wrinkles at rest and advanced photoaging, including obvious dyschromia, visible keratosis, and wrinkles without movement; always wears foundation
• Type IV: Only wrinkles and severe photoaging such as a yellow-gray skin tone, prior skin malignancies, and wrinkling throughout with no normal skin; photoaging precludes use of makeup because it cakes and cracks ^[11]

Microdermabrasion is most effective for superficial skin conditions, as it targets mainly the superficial skin layers of the skin. Superficial skin conditions include early photoaging, fine lines, superficial scarring, seborrheic skin, melasma, and reduction of pore size.[12] Deep scars, acne scars, and rhytides require a greater depth of skin injury for effective treatment.

Identifying the patient’s skin type and sun reactivity is necessary to define the risk of pigmentary changes with any procedure that wounds the skin, such a resurfacing with microdermabrasion. The Fitzpatrick classification classifies skin according to sun-reactivity and propensity for photodamage.[13]

Table. Fitzpatrick Skin Classification (Open Table in a new window)

Skin types I-III generally tolerate resurfacing procedures with minimal risk of pigmentary complications. Skin types IV-VI have a higher risk of pigmentary changes with deeper wounding, such as dyschromia, postinflammatory hyperpigmentation, and permanent hypopigmentation. Superficial wounding extends to the stratum granulosum or papillary dermis. Medium-depth wounding extends into the upper reticular dermis. Deep wounding extends to the midreticular dermis. Wounds that extend beyond this level can cause scarring, regardless of the skin type. Because microdermabrasion causes only superficial wounding, it does not carry the risks of pigmentary changes or scarring seen in other techniques, such as dermabrasion, chemical peels, or laser resurfacing.

Even patients with Fitzpatrick types IV-VI who may be more at risk of pigmentary changes with other resurfacing techniques can undergo conservative microdermabrasion with relative safety, when performed by an experienced provider.

Contraindications include current or recent use of isotretinoin (within 6 months of treatment) owing to the potential for hypertrophic scarring.[14, 15] Facial surgery within the past 2 months, radiation therapy to the head, or a history of hypertrophic scarring anywhere on the body are also contraindications.

Any bacterial or viral infections affecting the skin—such as impetigo, flat warts, and herpes simplex—are absolute contraindications and must be controlled with systemic therapy prior to undergoing microdermabrasion, owing to the risk of autoinoculation as the handpiece is moved from the affected region to unaffected skin.[16] Some authors propose that antiviral prophylaxis should be given to patients with a history of recurrent herpes labialis.[1]

Relative contraindications include the presence of rosacea and telangiectasias since these may be exacerbated by the treatment.[1]

Microdermabrasion can be performed by a physician, nurse, or licensed aesthetician. The most commonly treated area is the face, but it can also be used effectively on the neck, hands, and chest. The depth of the treatment depends on the strength of flow of the crystals generated by the crystal pressure and vacuum level, the rate the handpiece is moved against the skin, and the total number of passes over the treatment area. Deeper abrasion can be achieved with slower movement of the handpiece and more passes, which allows for longer contact between the crystals and the skin.

The crystal pressure and vacuum level can be determined by testing an area of nonfacial skin, but it can also be dictated by patient tolerance, which may require an adjustment in the power settings. At the conclusion of the treatment, the entire handpiece must be sterilized to prevent the transmission of infectious particles to the next patient receiving treatment.[17]

A variety of microdermabraders are available on the market. Several components are common to all systems and include a pump that generates a high-pressure stream of aluminum oxide or salt crystals, a connecting tube that delivers the crystals to the handpiece, a handpiece, and a vacuum to remove the crystals and exfoliated skin. The crystals are then discarded. Handpieces are available in disposable and reusable types. The reusable handpieces must be resterilized after each use.

Microdermabrasion is most effective for superficial skin conditions such as early photoaging, fine lines, and very superficial scarring. It creates only a superficial depth of injury and does not carry the risks of pigmentary changes or scarring seen in other techniques such as dermabrasion, chemical peels, or laser resurfacing.

As a result, even patients with Fitzpatrick skin types IV-VI, who may be at risk for pigmentary complications with other resurfacing procedures, can undergo microdermabrasion with relative safety. Care must be taken to avoid deeper injury in these patients by not applying excessive pressure during the procedure.

This technique is also well suited for the neck, which is an area that carries the increased risk of scarring with the other resurfacing methods. Since the depth of injury with microdermabrasion is superficial, the neck area may be treated with relative safety as long as proper and careful technique is practiced. Overall, microdermabrasion involves little risk and rapid recovery.

Patient selection is paramount to ensure optimal results. When evaluating patients for any resurfacing procedure, it is important to address the patient’s concerns and expectations. It is necessary to distinguish skin-color changes due to photoaging versus chronological aging in order to ensure that the proper treatment is offered to the patient. Chronological aging is associated with sagging and loss of skin elasticity and can best be addressed with surgical rejuvenation procedures or other nonsurgical treatment modalities that are able to target collagen production or volumization of the aging face. While some tightening of the skin may occur with resurfacing procedures, facial contour is not significantly affected and issues such as jowling, midface ptosis, or neck laxity are not corrected. Patients must understand and be comfortable with this limitation.

Photoaging severity should be assessed, and skin should be classified according to Fitzpatrick skin type. The patient’s skin must also be evaluated for changes in texture, acne, or other scarring. Deep scars and rhytides must be distinguished from those that are superficial, as a greater depth of injury is required for effective treatment of the deeper lesions.

Patient lifestyle must also be addressed when gathering the history, as the amount of recovery time needed from work or social activities may affect the choice of resurfacing technique. Inquiries into upcoming social events should also be made in order to ensure adequate recovery time. Photoaging preventative measures should also be addressed, especially if the patient does not use sun protection daily.

Past and current use of medications and medical problems should be identified. Current use of isotretinoin, or use within the past 6 months to 1 year, is a contraindication to any resurfacing technique because of the increased risk of scarring. Inquiries should be made into the use of herbal remedies, as they may produce an anticoagulative effect with dermabrasion, chemical peels, or laser resurfacing.

Patients with a history of herpes simplex may require prophylaxis, as the procedure may cause reactivation of latent herpes simplex.[18, 19] Inquiries should also be made into any history of hypertrophic scarring or keloid formation in the patient and family members.

It is important to manage patient expectations and ensure that they are realistic. The goal is the improvement of overall skin quality by addressing changes of mild photoaging, fine lines, age spots, enlarged pores, and superficial scarring.

Results on brown spots and acne are variable.

Patients should be prepared for the number and frequency of treatments. A commitment to the series of treatments is important to ensure adequate results. Patients who do not commit to the full series are unlikely to see significant results and thus unlikely to be satisfied with the outcome. Patients should also be prepared for what microdermabrasion cannot accomplish, such as resolution of deep rhytides, deep scars, and pigmentary abnormalities. These issues are best treated with other modalities such as traditional dermabrasion, chemical peels, and laser resurfacing, which can achieve a greater depth of injury. These modalities increase the effectiveness but may also carry higher risks of complications and increased length of recovery.

Results achieved with microdermabrasion can be enhanced with medical therapies such as topical skin treatments. Adjuncts include tretinoin, alpha-hydroxyl acids, retinoic acid, and topical vitamin C. In patients treated for hyperpigmentation, the application of hydroquinone between treatments can be useful, along with the liberal use of sunscreen and moisturizers. The long-term benefits include reduction of sun damage and photoaging and improved skin moisture.

Kaushik and Keck found that microdermabrasion treatment removes approximately 23% of the stratum corneum, permitting greater penetration of transdermal agents.[20] In a study of 14 women with acne vulgaris, a split-face study showed that microdermabrasion with 40% pyruvic acid produced significantly less sebum secretion and increased stratum corneum hydration than treatment with 40% pyruvic acid alone.[21]  In a study of 30 female patients of Fitzpatrick skin type IV-V with melasma, microdermabrasion with 70% glycolic acid produced significantly superior results when compared to treatment with 70% glycolic acid alone.[22]

Chhatbar et al demonstrated that microdermabrasion can reduce the electrical resistance of skin, allowing for transcranial direct current stimulation (tDCS) treatment at higher amplitudes with less pain and pruritis.[23]

Patients should be instructed on proper skin care before the procedure. They should avoid waxing, electrolysis, and laser hair removal 1 week before procedure and avoid excessive sun exposure 2 weeks before procedure.[24]

No premedication is necessary, unless desired. Prior to the procedure, the skin is cleaned of all makeup and oil. No topical or local anesthetic is necessary, although its use is not precluded. Contact lenses are removed, and eye protection is placed to prevent injury from stray particles.

To ensure even exfoliation, the handpiece should make consistent contact with the treatment surface and an effective vacuum should be achieved. The operator uses the nondominant hand to stretch the treatment area taut in order to avoid excessive suction and abrasion in any one area, which could lead to pinpoint bleeding.

The vacuum pressure is controlled by a foot pedal, which allows the operator to vary the pressure on different treatment areas. The first pass is performed by gliding the handpiece over the treatment surface and allowing gentle suction of the skin. The second pass is made perpendicular to the first in order to avoid streaking. All subsequent passes should follow this alternating pattern. The desired endpoint is erythema. Typically, two passes per session are sufficient to treat the face.

Thicker skin, such as on the forehead, chin, and nose, can be treated more aggressively by adjusting the speed of the handpiece movement or increasing the number of passes. The thinner skin of the lower eyelids and the upper cheek can be treated with lower pressures. When treating the neck, it should be placed in extension to maximize the skin tension. The strokes should all be vertically oriented. This is different from the perpendicular approach that should be used on the face.

The face should be cleaned of any residual crystals in between passes. Any specific areas such as acne scars or age spots can be focally treated with additional passes. The treatment sessions typically last 30-40 minutes for the face and 20 minutes for the neck but can vary.

Once the treatment is completed, the area should be cleaned with a wet cloth to remove any residual crystals and to brush any residual crystals away from the eyes. Once dry, a moisturizer or ointment with sunscreen should be applied. Patients should avoid keratolytic agents such as retinoids, alpha-hydroxy acids, and benzoyl peroxide for 3 days following the procedure.[24] Continue the application of moisturizer or ointment postoperatively because exfoliation may occur.

The patient may experience a mild sunburnlike sensation for a few days, and photosensitivity may be increased during this time. Erythema usually resolves within hours of the treatment.

Effective microdermabrasion typically requires a series of 5-12 treatments, but this can vary according to the issue being treated. Initially, the frequency of treatments is weekly or biweekly and can be followed by monthly or biannual maintenance treatments.

Microdermabrasion is a relatively benign procedure, as it only produces superficial wounding of the skin, which is one of the major advantages of this resurfacing technique. Erythema is minimal and typically resolves within hours of the treatment, allowing for a rapid return to daily activities.

In the early years of its use, some reports of eye redness, photophobia, and epiphora after the procedure emerged. Physical examinations revealed conjunctival congestion, crystals adherent to the cornea, and superficial punctate keratopathy. These complications are virtually eliminated with the use of eye protection by both the patient and the operator, although corneal abrasion and eye irritation from stray crystals remains a theoretical risk.

Pulmonary fibrosis, tracheal papillomas, and laryngeal papillomas have been linked to aluminum oxide dust exposure.[24] Additionally, findings of aluminum in the brain senile plaques of Alzheimer disease patients have raised the question of the whether chronic exposure to aerosolized aluminum dust could place patients and operators at risk for cognitive impairment in the future.[24] The aluminum oxide crystals used for microdermabrasion are 100-120 μm and are much larger than the 24- to 50-μm particles used in dentistry. To date, the smaller particles used for dental air abrasion have not been found to pose a significant health hazard. Furthermore, the larger particles used in microdermabrasion are too heavy to become aerosolized and likely do not pose a risk to the respiratory system or impair cognition.

Microdermabrasion produces a host of microscopic changes that account for its clinical effectiveness. Photodamage and aging lead to thickening of the stratum corneum, thinning of the epidermis overall, and lengthening of the rete ridges. Microdermabrasion treatment promotes thinning of the stratum corneum, thickening of the remaining epidermis, and flattening of the rete ridges.[1]

Photodamage also leads to pigmentary abnormalities such as hyperpigmentation and irregular melanization. Microdermabrasion has been found to cause redistribution of melanosomes and to decrease melanization.[25]

As skin ages, it loses its collagen and elastin, which leads to decreased elasticity and the development of fine rhytides.[1] Some studies have found elastin content to be increased in treated skin. Studies point to increased collagen in microdermabrasion-treated skin. Coimbra et al demonstrated that weekly microdermabrasion performed for 8 weeks resulted in thickening of the epidermis, increased collagen content, and newly deposited collagen with greater organization than that seen in control samples.[25]

Karimipour et al found that a single microdermabrasion treatment resulted in significant elevations of molecules that are well known as regulators of matrix-degrading enzymes (activator protein 1 [AP-1], nuclear factor kappa-light-chain-enhancer of activated B cells [NFKB], interleukin 1-beta (IL-1beta), and tumor necrosis factor-alpha [TNF-alpha]), such as matrix metalloproteinases.[26] These enzymes remove damaged collagen and help set the stage for new collagen replacement. Twenty percent of patients in this study also demonstrated type I collagen deposition. Even though the stratum corneum was not found to be significantly disrupted with the single microdermabrasion treatment, these findings suggest that even minimally invasive resurfacing techniques may have a significant effect on the dermis.

Other studies have found that microdermabrasion decreased transepidermal water loss and improved hydration in the 7 days following treatment, resulting in “supple and more hydrated looking skin.”[27] Microdermabrasion is also thought to regenerate the skin lipid barrier. This group also found that the pH of the skin was decreased and sebum secretion levels were increased 7 days following treatment, suggesting that the acidic pH may have created a more favorable environment to rebuild the lipid barrier of the stratum corneum.

In appropriately selected patients, microdermabrasion can be a very effective technique. Patients with photodamage, fine rhytides, age spots, enlarged pores, and certain very superficial acne scars can experience significant improvement in the quality and uniformity of the appearance of their skin. Improvement is accomplished with little downtime or disruption of daily living and minimal risk. Microdermabrasion is well suited for the patient with a busy lifestyle and superficial skin conditions. The number and frequency of treatments can be tailored to the individual patients and depending on the condition and desired result.

Microdermabrasion is not effective for deep wrinkles and scars or icepick acne scars because these lesions extend into the deeper layers of the dermis. It is also not effective for pigmentary problems such as melasma or postinflammatory hyperpigmentation, since these issues arise in the dermis. Patients with these problems are best treated with the more traditional resurfacing modalities, such as chemical peeling, dermabrasion, and laser resurfacing.

The role of microdermabrasion in the treatment of acne vulgaris is still unclear, as there are limited well-designed, randomized, and controlled clinical trials to clearly address this issue.[28] Patients with active erythematous papular and pustular acne should avoid microdermabrasion since it may lead to increased inflammation and erythema.[25] Comedonal acne can improve after a series of exfoliative microdermabrasion treatments.[25] . Microdermabrasion may enhance the absorption of topical medications and increase the penetration of light into the epidermis during use of phototherapy for mild-to-severe acne vulgaris.[29]

The improvement of fine rhytides with microdermabrasion has been inconsistent. Studies have demonstrated the improvement in fine rhytides after treatment as noted by lay observers; however, these same changes were not noted by experienced clinicians.[30] Some studies suggest that the immediate apparent improvement of fine lines may be secondary to transient posttreatment edema.[25] Neoplastic growths such as seborrheic keratosis, actinic keratosis, and milia have shown only occasional improvement with microdermabrasion and require numerous passes with the hand piece for partial or complete ablation. Other modalities are more efficient and efficacious.[25]

In the future, microdermabrasion may play a role in enhancing skin permeability for the purpose of the transdermal delivery of small hydrophilic molecules, insulin, vaccines, and other therapeutic molecules.[31]